ALICE—An advanced reflectometer for static and

REVIEW OF SCIENTIFIC INSTRUMENTS 86, 063902 (2015)

ALICE—An advanced reflectometer for static and dynamic experiments in magnetism at synchrotron radiation facilities R. Abrudan,1,2 F. Brüssing,1 R. Salikhov,1 J. Meermann,1 I. Radu,2 H. Ryll,2 F. Radu,2 and H. Zabel1 1 2

Institute for Condensed Matter Physics, Ruhr-Universität Bochum, 44780 Bochum, Germany Helmholtz-Zentrum-Berlin for Materials and Energy, 12489 Berlin, Germany

(Received 3 December 2014; accepted 14 May 2015; published online 4 June 2015) We report on significant developments of a high vacuum reflectometer (diffractometer) and spectrometer for soft x-ray synchrotron experiments which allows conducting a wide range of static and dynamic experiments. Although the chamber named ALICE was designed for the analysis of magnetic hetero- and nanostructures via resonant magnetic x-ray scattering, the instrument is not limited to this technique. The versatility of the instrument was testified by a series of pilot experiments. Static measurements involve the possibility to use scattering and spectroscopy synchrotron based techniques (photon-in photon-out, photon-in electron-out, and coherent scattering). Dynamic experiments require either laser or magnetic field pulses to excite the spin system followed by x-ray probe in the time domain from nano- to femtosecond delay times. In this temporal range, the demagnetization/remagnetization dynamics and magnetization precession in a number of magnetic materials (metals, alloys, and magnetic multilayers) can be probed in an element specific manner. We demonstrate here the capabilities of the system to host a variety of experiments, featuring ALICE as one of the most versatile and demanded instruments at the Helmholtz Center in Berlin-BESSY II synchrotron center in Berlin, Germany. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4921716] I. INTRODUCTION

The availability of highly brilliant synchrotron light sources with tunable photon energy and variable polarization renders possible investigations of spin structures and spin dynamics in magnetic materials. By tuning the photon energy to a specific absorption edge, not only much larger sensitivity is achieved as compared to non-resonant investigations but it is also possible to scrutinize the magnetism of different elements within the same material separately. This opens the possibility to study magnetic properties of heterogeneous materials, such as multilayers and superlattices of magnetic metals, alloys, or compounds with different elemental stoichiometries, which are of great interest for both, current and future technological applications and for many areas in fundamental research. Different techniques can be combined for gaining deeper insights, such as x-ray resonant magnetic scattering (XRMS), x-ray resonant absorption spectroscopy (XAS) in terms of magnetic circular dichroism (XMCD) or magnetic linear dichroism (XMLD), photon correlation spectroscopy (PCS), and various pump-probe techniques for dynamic studies. The possibility of integrating more than one experimental method within the same sample environment is considered today as a necessary condition for a successful scientific output. We have developed a diffractometer/reflectometer chamber (ALICE) for the use at synchrotron storage rings like the one at BESSY II of the Helmholtz Center in Berlin (HZB) to fulfill at least some of the ambitious conditions mentioned above. The basic chamber design and first experimental results were discussed in an earlier publication by Grabis et al.1 In the meantime, following recent technical developments, we have extended the functionalities of the chamber by implementing a series of

upgrades in a modular manner as concerns mechanical parts as well as electronics of the detection system. These upgrades have led to a highly versatile and flexible experimental setup, enlarging the spectroscopic capabilities of the main chamber and making it more suitable for conducting sophisticated experiments at modern synchrotron storage rings. Similar instruments operating at other sources have been discussed at different places.2–5 For a more complete overview on the different uses of synchrotron radiation for the investigation of magnetism, we refer the reader to reviews by Dürr et al.,6 Srajer et al.,7 Macke et al.,8 and Fink et al.9 II. GENERAL DESCRIPTION OF THE EXPERIMENTAL SETUP

The ALICE chamber was built as a diffractometer/ reflectometer for XRMS applications1 and is in operation since December 2002. It combines a two-circle goniometer with an accessible range of 175◦ in 2θ with a flow cryostat reaching temperatures as low as 4 K. A magnetic field of maximum ±7.1 kOe is available with a yoke that can rotate freely within the horizontal scattering plane. The chamber is mounted on a support frame and can thus be moved to various beam lines (undulator or dipole based) within the experimental hall, depending on the experimental requirements. A. Main chamber

Due to its high absorption cross section of the radiation in air, any experiment which requires soft x-rays has to be conducted under vacuum conditions. In the present case, this is realized by a two-chamber system: a main chamber

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having a moderate pressure of typically 3 × 10−7 mbar housing the sample stage, detector arm and monitor system, and a small chamber used for differential pumping. Both are connected through a flexible bellow with a pinhole of 2 mm diameter. A valve mounted in-between can be closed in order to give the possibility to easily vent the main chamber, e.g., for sample change. Samples can be changed manually through a large load-lock window, while the pressure in the differential pumping station holds the vacuum requirements for the storage ring. In this combination, the valve upstream the beamline can be opened relatively quickly after a sample change (1-2 h). The differential pumping system serves for the necessary pressure of ≤5 × 10−9 mbar towards the beamline, while in the sample region, the pressure is still in the 10−7 mbar range. This system has high flexibility with respect to sample exchange and does not result in long baking downtime of the chamber in order to re-establish the required base pressure for connecting to the main storage ring vacuum system.1 B. Sample environment and detectors

signals (TEY, FY, and XRMS) also at low temperatures. Each holder accommodates one or two IRD-Si diodes13 for FY signal recording. Figure 1(a) shows the holder, which can be used to record all of the mentioned signals. The sample is oriented vertically, perpendicular to the scattering plane and the beam impinging onto the sample surface is then reflected and can be recorded (XRMS signal). For a better understanding of the geometry, we refer the reader to Sec. II C. The sample current or the drain current (TEY signal) is read by using a Keithley Ammeter.14 FY is measured reading the current from an AXUV 100GX type diode positioned below the sample. This sample holder accommodates only one sample at a time. Figure 1(b) shows a sample holder used to record only two of the above mentioned signals (TEY and FY) and for experiments where the geometry of the incident beam does not play a decisive role. The samples sit at specific and fixed angles with respect to the scattering plane (typically 30◦), suitable for XMCD experiments. The sample holder can accommodate up to 8 samples, each of them is exposed to the incoming x-ray beam by rotating the sample holder about the Z-axis.

The sample holders are mounted on a cold-finger of a Janis flow cryostat10 that can be run with both liquid nitrogen (LN2) or liquid helium (LHe), where in the latter case, sample temperatures down to 10 K can be reached. Lower temperatures can be achieved using a proper gold (Au) or copper (Cu) shielding of the sample holder. The whole cryostat is fixed on a motorized XYZ-table and can be moved in the horizontal (scattering) plane with a resolution of 1 µm. The cryostat can be translated vertically, i.e., perpendicular to the scattering plane (Z direction) by approximately 8 cm with a smallest step size of 0.01 mm. This is important for accessing several samples mounted on the sample holder. The motorization of the stage is performed by using high resolution stepper motors from VG-Scienta11 together with a motor controller from Huber company using control boards from Phytron GmbH.12 Different sample holders can be accommodated on the sample manipulator. The mounting of the samples on the holders is done using copper (Cu) clamps or different adhesives, like Cu or carbon (C) tape or silver (Ag) paint. In the most common situation, the samples are fixed vertically, perpendicular to the scattering plane and can rotate in the x-ray beam from normal to grazing incidence. Use of different signal channels is provided, including Total Electron Yield (TEY), Fluorescence Yield (FY), photo-diodes for reflected signal (XRMS), and avalanche photodiode (APD) detector for photon-out detection. Depending on the sample holder used, different detector signals can also be measured simultaneously. International Radiation Detectors (IRDs, type AXUV 100GX and UVG-20B series) are used to cover the complete photon spectral range from 0.0124 nm to 1100 nm and from 140 nm to 1100 nm, respectively. They can also be used for detection of low energy electrons and ions because of their 6 nm oxide window and 100% internal quantum efficiency. As an example, Fig. 1 shows two types of sample holders which are being used most commonly. Their figure of merit is the possibility to record more then one of the above mentioned

FIG. 1. Two different sample holders are screw-mounted on the cold finger of a LHe flow cryostat. Top panel: sample holder for reflectivity measurements. The sample is fixed in a vertical position and a silicon diode is mounted on the bottom of the cap. Aside from recording the reflected x-ray beam, the sample holder allows to acquire simultaneously TEY and FY signals. Bottom panel: this sample holder is designed for XMCD experiments and can accommodate up to 8 samples sitting on a truncated pyramid at an angle of 30◦ with respect to the horizontal incoming x-ray beam. Two silicon diodes are placed on a Y shaped holder, each one monitoring two of the pyramidal faces. The sample holder is rotatable by 360◦ to access all samples. The XMCD signal can be recorded either by TEY and/or by FY. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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For both sample holders, there is a separative sapphire plate that serves a double purpose. For one, it assures a good thermal conduction between sample plate and the rest of the sample holder and second, it decouples electrically the sample plate from ground. Both sample holders are surrounded by a protecting copper cap, which also accommodates the fluorescence diodes. For low temperature experiments, the cap acts like a small cryopump for residual gases close to the sample (more in Sec. III A). The APD diodes together with the whole detection system associated with the dynamic experiments carried out are described in detail in Sec. IV A. C. Scattering unit

An IRD silicon photodiode (model UVG-20B) is used as detector for the reflected x-ray intensity, which is shielded inside a metal box against stray light. This is located further away from the sample (20 cm) on a metallic arm rotating about the chamber’s rotation center. With a slit in front of the detector, the angular resolution can be set manually before starting the experiment. Figure 2 presents, as an example, two rocking curves done in reflection geometry for a laterally patterned sample and for different detector slit openings. The satellite peaks surrounding the specular reflected intensity are due to the lateral periodicity of the pattern. It is clearly visible that the resolving power of these satellite peaks increases with decreasing the slit size.15 The typical resolution of the instrument is ∆θ = 0.0576◦ calculated for an incoming beam size of 100 µm, a detector slit size of 100 µm, theta incident angle of 30◦, and a beam horizontal divergence of 6 mrad. The photocurrent is delivered with a shielded Kapton® isolated Cu cable by an electrical feed-through to a Keithley Ammeter (K6514, K6517A, K6517B).14 The sample and detector are held by two concentrically mounted differentially pumped 100 mm bore rotating platforms, driven by two worm-gear-connected stepper motors outside of the vacuum chamber. Sample and detector rotate independently, allowing longitudinal θ − 2θ scans and transverse θ scans of the sample (rocking scans). The angular resolution for sample and detector rotation is 0.005◦. Maximum rotation of the sample is ±180◦ and for the detector ±175◦. For a perfectly aligned setup, the vertical axis of the sample has to coincide with the rotation axis of the detector and also to intersect the beam direction. The estimated sphere of confusion for the instrument is approximately 10 µm in diameter.

FIG. 2. Reflected intensity during a θ scan (rocking scan) from a sample with 1 µm broad stripes and 3 µm periodicity for two different detector slit settings. For the smaller slit size, a better angular resolution is clearly visible. Reprinted with permission from S. Buschhorn, “Element resolved magnetization dynamics,” Ph.D. thesis (Ruhr University Bochum, 2011). Copyright 2011 Ruhr University Bochum.

D. Monitor

E. Computer control and data recording

To discriminate against beam intensity fluctuations in time and/or space, the experimental setup provides several beam monitors. In the most simple case, two ammeters are used to detect the reflected intensity and the monitor (I0) signal, the latter one being recorded either as drain current of the last refocussing mirror of the beamline, or from a special monitor holder. The monitor holder is mounted on a XYZ manipulator upstream the sample (see Fig. 3(a)) and is designed to accommodate a gold grid as well as several x-ray transparent membranes with different deposited metals (Cu, Au, Ta, Al, Pd, Pt, Ag). Users have the possibility to

The computer control of all motors and data acquisition from all detectors are carried out with the control program SPEC,16 a specific software environment for reflectometers or diffractometers, which is a standard UNIX® command-line environment. A unix-based computer, external components (motors and controllers) of the chamber, and detectors communicate via the GPIB (General Purpose Interface Bus) interface. Within the SPEC software, all necessary parameters are automatically logged within the same file. To vary the photon energy and polarization (depending on the beamline), a serial connection (wired) or an Experimental Physics and

choose the proper monitor in order to avoid that any resonant edges from the monitor material overlap with the sample resonance energies. Since the electron beam in the storage ring has a certain lifetime, the x-ray intensity decays in time during long measurement scans. Normalization of the signal is imperative. Also, during normalization process, any artifacts from the beamline optics visible in the intensity signal can be removed. Modern synchrotron sources (including BESSY II) have implemented a “top-up mode” such that beam lifetime and refilling of the storage ring are no longer an issue. Nevertheless, beam monitoring remains mandatory because of remaining beam variations in space and time. The monitor holder can also accommodate another type of monitor (Fig. 3(b)) consisting of a dual diode system disposed in a geometry as to accommodate a variable opening angle in between the diodes. These two diodes can be read separately and thus two I0 signals are compared and used for normalization. At the same time, the size of the beam is well defined and monitored. This device serves not only as a monitor but also as a slit for beam chopping. It can be used for absorption or transmission spectroscopy measurements where the size of the incoming beam impinging on the sample surface plays an important role for defining the required energy resolution. In case of XRMS experiments, this slit defines the scattering volume.

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FIG. 3. (a) The monitor holder is mounted on a fully rotatable and shiftable manipulator and can accommodate a number of different monitors (meshes, wires, or transparent membranes). (b) Alternative dual diode monitor. The full or a part of the direct incident beam passes through the monitor and subsequently hits the sample surface. The drain current from the exposed monitor is used for normalizing spectroscopy measurements.

Industrial Control System (EPICS) connection (wireless)17 is required. The storage ring current is provided via the TCP/IP port or EPICS variable read out. A multitude of scan types can be easily adapted for different experimental purposes. For instance, it is possible to run energy scans where the monochromator (and undulator) is changed with a finite energy step (via serial communication) called stopand-go scans or moving continuously the energy (via EPICS communication) called fly mode scans. In the stop-and-go mode, for each point in the scan, one has the possibility to execute a particular protocol for the magnetic field, for instance, switching the magnetic field from positive to negative saturation or to remanence. This drastically improves the quality of the data with respect of the background. Multiple averages are also possible for increasing the signal to noise ratio of recorded spectra. In the so-called fly mode scans, the energy is changed with a constant defined speed and the recording is done continuously at each energy sweep step of 0.1–0.2 eV. The result is a very fast recorded energy scan (within 2-3 min), but the final data need interpolation in order to be useful for further analysis. Also in this case, the magnetic field can be switched at different points in the energy scan. The quality of the recorded XAS spectra is usually very high with respect to the signal-to-noise ratio as well as with respect to the energy resolution. Spectroscopic artifacts, a very important issue when dealing with absorption signals, are clearly absent as can be seen from Fig. 4. Here, we show the first XAS spectra recorded in fly mode at BESSY II for the Ni L 3,2 edges for two opposite circular polarizations σ ± of the incoming synchrotron light. The spectra were taken at normal incidence geometry, while the magnetization is aligned in the sample plane. Thus, according to the selection rule for the absorption cross section, no XMCD dichroism is expected to be seen from the difference σ + − σ −. The existence of

an XMCD signal could come from a small misalignment of the sample with respect to its normal direction or from measurement artifacts (energy shifts, saturation of the detector diodes, etc.). In the new top-up operation mode of the HZB BESSY II storage ring, electron injection occurs approximately every 100 s.18 For removing injection artifacts (noise or intensity variations) from the recorded spectra, an inhibitor of the injection time is required. The delay time during the injections is dynamically set and varies all the time depending on the lifetime of the electrons in the storage ring. This is the reason why a prediction of the exact injection time, taking into

FIG. 4. Example of XAS of Ni in the energy range of their respective L 3 and L 2 absorption edges, recorded in the continuous mode (fly mode) at BESSY II of the HZB in Berlin. The undulator and monochromator run continuously and simultaneously while the intensities are recorded with a specific frequency. The speed of reading is limited by the speed of the EPICS data server transfer (∼1000 reads/s) and the reading time of the ammeters (set by the user). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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account the beam intensity, decay, and lifetime, is needed and this is done by reading an EPICS variable provided by the control operator. When the injection mode is switched on, our software inhibits reading and writing of data during injection time. Waiting time before and after injection can be set by computer control and is typically between 1 and 3 s. III. STATIC EXPERIMENTS A. X-ray spectroscopy 1. Description of the technique

In the x-ray absorption process, a core electron is excited to an empty state and, as such, x-ray absorption spectroscopy probes the unoccupied part of the electronic structure of the system. An important aspect of the usefulness of XAS is that one is able to obtain element-specific information. In addition, if two or more inequivalent types of the same atom are present, the spectral shape is a linear combination of all the individual sites.19,20 There are different detection methods that can be used for XAS as follows: a. Transmission radiation: Transmission radiation experiments are the classical approach also used in optical spectroscopy and known as the Lambert-Beer law. In this case, the intensity of the x-rays is measured before and after passing through a sample (less then 0.2 µm for soft x-rays), and the percentage of transmitted x-rays is determined.21,22 b. Total electron yield detection—TEY: In TEY detection, the measured signal is the drain current appearing in the sample during x-ray absorption to balance photon emitted electrons. Due to the possibility of extracting quantitative data, TEY is the preferred method in most experiments in the soft x-ray regime. c. Fluorescence yield detection—FY: In FY mode, the detector is a photodiode measuring the x-ray fluorescence signal from the sample when exposed to the x-ray beam. The results are qualitative23,24 and with respect to TEY, which is more surface sensitive (∼5–6 nm), FY signals probe a deeper volume up to ∼30 nm below the sample surface. 2. Experimental details and results

Technically, for TEY, one needs to keep the sample electrically isolated with respect to the rest of the chamber and at the same time have a good thermal conduction through the sample holder. Generally, this is done using a two block sample holder, sandwiching a sapphire (Al2O3) plate for electrical isolation and good thermal conductivity. Since the pressure in the chamber is rather high (10−6-10−7 mbar), at low temperature, a residual gas condensation may occur at the sample surface. This would make the use of the TEY signal almost impossible because of its surface sensitivity. This problem can be avoided by covering the sample holder with a metallic shield in thermal contact with the cryostat, which acts as a condensation absorber (cryopump) close to the sample. The cylindrical shield has a horizontal slit parallel to the scattering plane making the sample accessible for both incoming and reflected x-ray beam (a cut in the scattering

FIG. 5. XRMS, TEY, and FY signals measured simultaneously for a Ni film at an incidence angle of 25◦. The first signal has a high depth/interface sensitivity, TEY and FY are, respectively, more surface and bulk sensitive.

plane), see Fig. 1. With this arrangement, scattering and spectroscopy can be performed simultaneously. Another special holder designed only for spectroscopic use can accommodate up to eight different samples, as shown in Fig. 1(b). TEY and FY signals can be measured simultaneously making use of two IRD diodes situated at the bottom of the holder cap in addition to an ammeter for measuring the drain current. The samples on the holder are inclined by 30◦ with respect to the scattering plane for optimized magnetic sensitivity. An example for a simultaneous detection of reflected signal (XRMS), absorption signal (TEY), and fluorescence signal (FY) from the Ni L 3,2 edges is shown in Fig. 5. A proper analysis of the XMRS signal can depict the magnetic profile in films and multilayers, which is described in more detail in the next paragraph. The TEY signal is more surface sensitive; thus, the quality of the sample is important. The measured TEY spectra can easily be used for applying the so called sum rules to extract the orbital and spin moments.25–27 On the other hand, the FY signal is more bulk-sensitive and can be used for experiments where surface and bulk are expected to exhibit different magnetic or structural properties. As already mentioned before, FY is more recognized as a method for studying bulk-like systems. The FY signal is in most of the cases and geometries accompanied by the so-called self-absorption effect caused by the absorption of the exiting photons going into the sample as well as the generated ones going out of the sample. This effect can distort the spectrum in a way that it appears as if the FY signal was saturated, referred to as saturation effect.28 For this reason, unlike TEY, the fluorescence yield cannot be used for obtaining quantitative results (e.g., for XMCD sum rules). Nevertheless, the FY is a useful tool to study spectral peak changes, for instance, in case of core-shell nanoparticles or buried layers which are not accessible by TEY.29,30 B. X-ray resonant magnetic reflectivity and scattering 1. General description and background

X-ray reflectivity (XRR) is a standard tool for nondestructive structural analysis of thin films, multilayers, and interfaces. While not yielding direct imaging information,

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XRR is capable of collecting quantitative and global statistical information on layer thicknesses, interface roughness, and structural correlations in the parallel and perpendicular directions with respect to the film plane.31–33 Conventional x-ray scattering techniques are sensitive to the electron density profile parallel to the scattering vector, i.e., to structural information only. For a long time, polarized neutron reflectometry (PNR) was the only tool yielding similar information on magnetic structures, including the magnetization profile in thin films and multilayers, magnetic correlations, and magnetic domain distributions.34–36 However, in the mid1990s, it was realized that the XMCD effect, first demonstrated by van der Laan et al.37 and Schütz et al.,38 can also be used for scattering experiments.27,39–41 This is particularly true for the soft x-ray energy regime (500 eV - 1500 eV), which covers the magnetically dichroic L 3,2 absorption edges of the transition metals and the M5,4 edges of the rare earth elements. Soft X-ray Resonant Magnetic Scattering (SXRMS or short XRMS) is by now established as a powerful tool for the detailed analysis of a wide range of magnetic thin films, multilayers, hetero-, and nanostructures. For review works on this topic, we refer the reader to Refs. 6, 8, 9, and 42–45. 2. Experimental details and results

Within the frame of XRMS experiments, different types of scans can be performed. The most typical ones are very similar to the non-resonant counterparts, such as longitudinal scans, transverse scans, rocking curves, and off-specular scans. The main advantage is that the energy and the polarization of the incident radiation are well controlled and the magnetic sample has to be in a well defined magnetic state known from hysteresis measurements. As a representative example, the specular reflectivity curve from a magnetic CoFeB/MgO multilayer is reproduced in Fig. 6(a).46 Reflectivity scans (θ − 2θ scans) were recorded by tuning the photon energy

to the Co L 3 resonance edge while magnetizing the sample in positive and negative saturating magnetic fields. This type of measurement is sensitive to the out-of-plane structural and magnetic correlation while probing the component of the magnetization vector in the scattering plane. The magnetic sensitivity in XRMS experiments corresponds to the one probed by longitudinal magneto-optic Kerr effect measurements, i.e., maximum sensitivity is obtained when the wavevector of the incident beam ⃗k i is parallel to the magne⃗ while the scattering vector Q ⃗ = ⃗k f − ⃗k i is tization vector M, oriented perpendicular to the film plane (k f is the wavevector of the reflected beam). The top panel is a plot of the average sum intensity (I + + I −)/2 of the reflected intensity as a function of scattering vector for right circularly polarized incident x-rays, while the sample is switched between positive (I +) and negative magnetic saturation (I −). The averaged reflected intensity is a measure of the charge scattering. The lower panel is a plot of the difference divided by the sum (I + − I −)/(I + + I −), which is due to the magnetic state of the sample. However, this normalized asymmetry is not entirely magnetic in origin as it would be for PNR, but a convolution of charge and magnetic cross section. From fitting the asymmetry, the magnetization profile in the multilayer can be gained,46 as shown in Fig. 6(b). The magnetization amplitudes close to the cap layer and to the substrate are reduced compared to those inside the multilayer system. The schematics in the left inset of the figure present the connection between the magnetic profile extracted from the reflectivity curves and the multilayer structure. The measured XRMS scans shown in Fig. 6(a) were simulated with the REFTOOL47 program, in which the reflectivity intensities are calculated following the universal approach to magneto-optics proposed by Zak et al.48 where multilayers with arbitrary ⃗ are considered. magnetization direction M XRMS scans can be performed with alternating left and right circular polarized incident x-rays while keeping the magnetic field constant,49 or, vice versa, by using only

FIG. 6. (a) X-ray reflectivity curve together with fitting line (upper panel) and the respective asymmetry (lower panel) after measuring with circular polarized x-rays tuned to the Co L 3 resonance edge and keeping the sample in positive and negative saturation magnetic fields. (b) Magnetic profile across the sample after data analysis together with a sketch of the multilayered structure. In this way, one can distinguish the magnetization profile in each separate layer or interface. Reprinted with permission from J. Appl. Phys. 108, 063922 (2010). Copyright 2010 AIP Publishing LLC. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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one polarization and switching the magnetic field from positive to negative saturation. With the ALICE chamber, both experimental procedures are possible. Changing the x-ray polarization is normally an option of the beamline operation (undulator or dipole). Changing the running parameters of the beam line takes time and introduces instabilities. Thus, changing the magnetic field is easier and more frequently used. The electromagnet can be rotated synchronously together with the sample, such that the magnetic field is always oriented parallel to the sample surface. In addition to longitudinal scans, transverse scans can be performed, either for the study of structural and magnetic correlations parallel to the film plane or for the investigation of patterned samples with periodic arrays of magnetic islands, which produce satellite peaks on either side of the specular ridge (Q x = 0) as shown in Figs. 2 and 9(b). As a result, XRMS techniques are the main source of information when studying magnetic multilayers and interfaces, from which magnetic depth profiles and lateral correlations of charge and magnetization can be extracted. C. Reciprocal space imaging

The ALICE experimental station also offers the possibility to conduct experiments based on imaging the reciprocal space with an area detector replacing a single photodiode. This way, one can record and image simultaneously a large fraction of the Q-space. As area detector, a CCD camera is used. We made use of a Princeton Instruments camera, model PI-MTE for soft x-ray and vacuum compatible. The CCD chip size is 2048 × 2048 pixels with a pixel size of 15 µm. In our geometry, the camera is situated at 25–30 cm from the sample.

It can be mounted in the forward direction for detecting small angle scattering of x-ray transparent samples, or at 45◦ with respect to the direct beam for imaging the reciprocal space at higher Q-values in reflection geometry. These two angles, coinciding with the scattering plane, are provided by flanges at the chamber to which the CCD camera can be attached. A free rotation of the CCD camera inside of the chamber would be desirable but is presently not possible. If used in the forward direction, a beamstop blinds the central part of the image thus allowing the use of the full dynamic range of the CCD for image recording. We first discuss resonant magnetic small angle scattering in transmission geometry and continue later with a discussion of reciprocal space imaging at higher diffraction angles. 1. Resonant magnetic small angle x-ray scattering (SAXS)

SAXS is a scattering method in transmission geometry, in contrast to the reflection geometry used for x-ray reflectivity measurements. The schematics of the scattering geometry are depicted in Fig. 7(a). Correspondingly, the scattering vector Q in case of SAXS lies in the film plane, probing in-plane charge correlations. Because of the transmission geometry, the sample must be x-ray transparent. Therefore, samples for this scattering method are usually deposited on thin Si3N4 or Al membranes. When using incident circularly polarized x-rays and tuning the x-ray energy to any element with absorption edges that show dichroic effects, Resonant Magnetic SAXS (RMSAXS) probes the in-plane magnetic correlation on a nanoscale, i.e., RMSAXS becomes sensitive to lateral magnetic domain fluctuations. Furthermore, one should keep

FIG. 7. (a) Schematics of the measurement geometry. (b) Resonant SAXS in transmission geometry from PdFe films on SiN membranes recorded at the Fe L 3 edge at low temperatures for an Fe composition of 7.2%. The speckle pattern recorded (magnified area) together with the autocorrelation of the speckle pattern (c). The data were recorded at low temperature below the Currie point of the PdFe alloy. Reprinted with permission from Ewerlin et al., J. Phys.: Condens. Matt. 25, 266001 (2013). Copyright 2013 Institute of Physics Publishing. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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in mind the selection rule for magnetic x-ray scattering, ⃗ Since ⃗k i is oriented which contains the scalar product ⃗k i · M. perpendicular to the sample surface, RMSAXS is sensitive to the out-of-plane component of the magnetization vector. In short, RMSAXS probes in-plane magnetic domains with outof-plane magnetic anisotropy, revealing ensemble averaged domain structures and reversal properties of perpendicular magnetic media including domain size, domain correlation length, domain wall width, and anisotropy of the domain pattern. A particular fruitful field for RMSAXS investigations is magnetic stripe domains in Co/Pt and Co/Pd multilayers with perpendicular anisotropy and the way they change along a hysteresis loop.50–52 An example for a RMSAXS pattern recorded with a CCD camera attached to the ALICE chamber is shown in Fig. 7(b) for a PdFe alloy film.53 The energy of the incoming circularly polarized x-ray beam was tuned to the L 3 absorption edge of Fe. The small angle scattering ring seen in Fig. 7(b) is due to magnetic domains with a perpendicular magnetization component. As it is of magnetic origin, it vanishes in high fields or by changing the x-ray energy to off-resonance. The dashed line indicates the mean radius of the SAXS ring, corresponding to a domain size of about 100 nm. Further examples of RMSAXS from Co/Pt multilayers are displayed in Fig. 8 and will be discussed further below.

2. Coherent x-ray resonant magnetic small angle x-ray scattering

By increasing the longitudinal and transverse coherence length of the incident circular polarized soft x-ray beam, the isotropic and continuous first order ring in a SAXS pattern will decompose into a speckle pattern. This speckle pattern is the consequence of multiple interferences of the scattered coherent light beam, which is in direct relation with the exact spatial correlation of the scattering centers within the coherence volume of the illuminated sample. As the correlation distances vary irregularly in the sample, so do the speckles. The longitudinal coherence length is defined by ξl = λ 2/(2∆λ), where λ is the soft x-ray wavelength. The longitudinal coherence length is a property of the beam line, consisting of undulator and grating monochromator and is typically 1.5-2 µm. The transverse coherence can be enhanced by using a very small defining pinhole in the incident beam. The transverse coherence length can be quantified as ξ t = zλ/(2πd), where z is the distance between pinhole and sample and d is the diameter of the pinhole. From this, we estimate a transverse coherence length ξ t ≈ 4 µm for a pinhole of 5 µm. With such a partially coherent beam, the smooth small angle scattering ring decomposes into an ensemble

FIG. 8. Schematics and results for holographic imaging of magnetic domains in Co/Pt multilayers with perpendicular anisotropy. (a) Schematics of the small angle scattering with the sample between the pole of an electromagnet. (b) Schematics of the sample design. The Co/Pt multilayer is deposited on the backside of SiN membrane. On the front side, the membrane is etched by an ion beam to allow the main x-ray beam to penetrate through the sample. A small reference hole to the side serves for the holographic imaging. (c) Local magnetic hysteresis loops of the Co/Pt multilayer with perpendicular anisotropy. (d) and (f) MFM image of the magnetic domains after in-plane and out-of-plane sample demagnetization, e.g., corresponding resonant magnetic small angle x-ray speckle pattern, for in-plane and out-of-plane demagnetized sample, taken with a coherent incident beam. (h) Domain imaging by holographic Fourier transform of the speckle pattern. Reprinted with permission from Günther et al., Appl. Phys. Lett. 93, 072505 (2008). Copyright 2008 AIP Publishing LLC. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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of speckles. These speckles usually change with applied magnetic field, most strongly close to the coercive field, and they vanish in a single domain state, i.e., at saturation.54 For the already mentioned PdFe alloy film, a speckle pattern can indeed be observed, as shown in Fig. 7(b).53 3. X-ray photon correlation spectroscopy (XPCS)

If the configuration of the scattering centers changes in time, also the speckle pattern will change. The fluctuating speckle intensity is a direct fingerprint of dynamical processes in the sample. Its analysis, termed XPCS,55,56 shows characteristic features that allow to distinguish between different relaxation processes on the millisecond to second time scale. For correlating speckles, an autocorrelation function is taken which is given by g(Q, τ) =

⟨I(Q,T)I(Q,T + τ)⟩ ⟨I(Q,T)⟩2

= 1 + ξ t · F(Q, τ)2, (1)

where I(T + τ) is the intensity for a given pixel at a time increment T + τ, ξ t describes the transverse beam coherence, and F(Q,T) is the intermediate scattering function at the scattering vector Q and as a function of relaxation time τ. Below the Curie temperature Tc , the domain structure in ferromagnets is pretty stable in time, but close to Tc , domain fluctuations prevail, which are reflected in a time dependent autocorrelation function. The typical speckle pattern of Pd1−xFex alloy films is shown in Fig. 7(b). For investigating domain fluctuations, a series of images was taken with an exposure time of 1 sec at a temperature of T = 150 K, which is close to Tc = 163 K for this particular alloy concentration of x = 7.2%. Figure 7(c) shows the correlation function versus time measured at T = 150 K, which is surprisingly flat, indicative for a rather static system over time spans of hours.53 By increasing the temperature closer to Tc , stronger fluctuations are expected, as was indeed observed for Ho and Dy at their respective Néel temperatures.57,58 4. Lensless imaging of magnetic domains by x-ray spectro-holography

After having established a speckle pattern in SAXS geometry, the next step is recording spectro-holograms, which provide real space images of magnetic domain structures.59,60 For this, a reference beam has to cross and interfere with the scattered beam from the sample before reaching the CCD-detector. A reference beam is taken from the incident beam by passing through a tiny pinhole (∼50 nm) close to the sample aperture. The in-plane reference hole and sample aperture then define a lensless Fourier transform holographic geometry, providing a hologram of the object in the detector. The object image is retrieved by a Fourier transform of the scattered intensity. Figure 8(a) shows the experimental setup where the sample is situated in between magnet poles and the CCD camera collects the transmitted x-ray beam through the sample. The sample (Fig. 8(b)) has to be manufactured beforehand by lithographic means in order to allow x-ray transparency only in two regions (object and reference). The reference hole is extended through both sample and substrate, whereas the object hole penetrates up

to the magnetic sample.59,60 Hysteresis loops measured on the Co/Pt sample showing the magnetic characteristics of a multilayer sample are presented in Fig. 8(c). Magnetic domain structure measured with Magnetic Force Microscopy (MFM) for different sample demagnetization states in-plane (Fig. 8(d)) and out-of plane (Fig. 8(f)) and their corresponding resonant magnetic x-ray speckle pattern (Figs. 8(e) and 8(g)) are also shown. The magnetic domain structure measured by holographic imaging (Fig. 8(h)) is presented in the lower panel of Fig. 8 resulting from a Fourier transformation of the holographic image. X-ray spectro-holographic imaging provides a good lateral resolution (≈20-30 nm) and gives the possibility to image magnetic configurations in an applied magnetic field as well. Lensless imaging of magnetic domains was performed with the ALICE chamber in the early stage of development, and nowadays, it has become a common technique used at a number of specialized instruments.61,62 5. Magnetic imaging of reciprocal space at high angles

For magnetic imaging of the reciprocal space at higher angles, the CCD-camera has to be remounted accordingly. The reciprocal space imaging is then carried out in reflection instead of transmission geometry. Laterally patterned samples are very good candidates for x-ray resonant magnetic reciprocal space imaging at higher Q-values. Due to the lateral periodicity, many Bragg reflections are generated, which can be well resolved by soft magnetic x-ray scattering. As an example we show in Fig. 9, the reciprocal space map of an array of Co/Cr/FeCr trilayers with island size of 0.2 × 3 µm and periodicities of 0.8 µm (horizontal) and 5 µm (vertical).63 A scanning electron microscopy picture of the pattern is shown as an inset in Fig. 9(a). Each island has spin valve character, meaning that if used as a magnetoresistive device, it could be switched between low and high resistance for parallel and antiparallel alignment of the Co and FeCr layer magnetization, respectively. The CCD-frames capture reciprocal space images taken at the Co L 3 edge for three different temperatures (50 K, 150 K, and 300 K) and for different fields at positive and negative coercivity ±Hc and in saturation. These images allow a direct comparison of the spin valve array under different conditions and whether lateral correlations are present or whether their switching is uncorrelated. Finally, it should be mentioned that reciprocal space images also allow to analyze the diffuse resonant magnetic scattering, which is due to short range magnetic correlations. Random alignment of the spin valve patterns can be clearly distinguished from ordered parallel or antiparallel alignments by the diffuse magnetic scattering. 6. Further scattering modes

Typical scattering experiments, i.e., photon-in, photonout detection, are not only used for reflectivity experiments as discussed above but also for measuring the magnetic hysteresis for each element in an alloy or for each element in a magnetic heterostructure. For measuring the element

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FIG. 9. (a) Schematics of the experimental setup. The x-ray beam (green) is hitting the sample (red) which is sitting in between the pole shoes of the magnet and its reflected beam illuminates the CCD chip (blue) located at 45◦ with respect to the incident beam direction. The magnetic field direction is horizontal, parallel to the scattering plane. (b) Acquired images of the patterned sample for different temperatures and different magnetic fields. The images are taken for a photon energy tuned to the Co L 3 edge.

specific magnetic hysteresis, the sample and detector angles are set to the highest asymmetry and the magnetic field is scanned while keeping all other settings constant. An example is shown in Fig. 10(c) of the magnetic hysteresis of Co and Cr in a Cr/Co/Cr heterostructure measured at the respective L 3 edges of Co and Cr.64 The measurements not only confirm the ferromagnetic state of Co with a coercivity of 120 mT but also the magnetic polarization of Cr at the Co/Cr interface in the direction antiparallel to Co. Furthermore, an energy scan can be performed at fixed scattering angle and fixed magnetic field. Energy scans are usually performed in absorption geometry as detailed below. In reflection geometry, energy scans can also be very useful, although the sum rules cannot be applied. In a reflection geometry, while scanning the energy, the scattering vector also changes length. Thus, it is impossible to keep everything constant and only change the energy. Therefore, energy scans in reflection geometry are convoluted with structural interference terms and the typical line shape at the L 3 and L 2 edges responds to this interference. However, if the film is very thin, reflectivity oscillations are widely separated and one can choose an angle where the reflectivity between oscillation minima is relatively flat. Under those circumstances, energy scans in reflection geometry resemble XMCD scans, as shown in Fig. 10(a).

IV. DYNAMIC EXPERIMENTS A. LASER (pump) x-ray (probe) experiments

A fast way of manipulating or even controlling the magnetic order parameter is achieved by using femtosecond

FIG. 10. (a) Energy scans measured in reflection geometry for a Cr/Co/Cr trilayer sample at 15◦ and different magnetic fields. (b) The dichroism calculated by making the difference between the two mentioned energy scans. (c) Hysteresis loop measured for Co and Cr at energies where the maximum dichroic signal occurs. Reprinted with permission from F. Brüssing, “Magnetization reversal of laterally structured multilayers,” Ph.D. thesis (Ruhr University Bochum, 2013) and F. Brüssing, Phys. Rev. B 88, 094431 (2013). Copyright 2013 Ruhr University Bochum and 2013 American Physical Society.

(fs) laser pulses as an ultrashort, external stimulus.65 When combined with x-ray probing techniques, such as XMCD66 or XMRS,67,68 this technique (i.e., laser pump/x-ray probe) becomes a very powerful investigating tool being able to monitor in real time the dynamics of spins on their characteristic time and length scales. In the following, we describe timeresolved x-ray techniques and their corresponding technical implementation with the ALICE chamber. 1. Experimental details

Here, we describe the design, development, and implementation of a laser pump x-ray probe setup whose feasibility is demonstrated for transmission, reflection, and scattering geometry measurements. The combined laser and synchrotron experiment requires that the repetition rate of both light sources should match. An amplified Ti–Sapphire laser system (Coherent, RegA9050) provided by the Max Born Institute (MBI, Berlin) is used as pump source, producing pulses with a width of 70 fs at a repetition rate of maximum 208.3 kHz. The maximum pulse energy at the fundamental wavelength of 800 nm is 6 µJ focussed to a spot of less then 150 µm

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FIG. 11. (a) Scheme of the laser implementation with the ALICE chamber and the measurement geometry. The x-ray beam enters from the right side into the chamber in the single bunch mode with a frequency of 1.25 MHz. The laser beam enters from the left side and passes through a delay stage, before reflected via the mirrors M1 and M2 into the chamber. The area shaded in blue is enlarged in panel (b). (b) Sample environment and the available detection modes. The sample is placed in transmission geometry between the poles of an electromagnet. The pinhole can be moved in the x-ray beam position to overlap both beams. HZB refers to the synchrotron source, and MBI designates the fs-laser facility. (c) General overview of the pump-probe synchronization between the laser and the x-rays. (d) Laser and x-ray spot sizes measured in order to ensure a proper spatial overlap.

diameter. The x-ray beam has a spot-size on the sample of approximately 90 µm at the x-ray photon energy tuned for the Co L 3 edge. The x-ray pulse length in hybrid or single bunch mode is ≈50 ps. A schematic description of the measurement geometry is presented in Fig. 11(a). The laser beam enters the chamber through a window flange and is reflected by an in-vacuum mirror along the direction of the incoming x-ray beam. The sample sits in transmission geometry in

between the pole shoes of a magnet assuring a magnetic field perpendicular to the sample surface (Fig. 11(b)). A set of avalanche photodiodes is mounted on a mobile arm revolving around the sample and detecting the transmitted (or reflected) beam from the sample. Different types of APDs are used here, which allow also single photon counting and thus guarantee high sensitivity in scattering experiments. The APDs used in this work are the series S2383 from Hamamatsu.69 This series

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is known for high sensitivity, fast response, and a low noise at reduced applied voltages. Nominal breakdown voltage is approximately 150 V. The obscured APD is covered with an Al membrane that blocks the laser light (or laser reflections within the chamber) and transmits the x-ray probing beam. This diode is used for acquiring data in transmission or in small angle scattering geometry. One or two open APDs can be used to monitor the relative positions of the laser and the x-ray pulses in the time domain. A MPPC (Multi-Pixel Photon Counter) diode array from Hamamatsu69 is used for reflectivity or small angle scattering measurements where the intensity of the reflected/scattered signal diminishes strongly (Fig. 12(b)). The MPPC used in this work is the model s10362 and provides a 1 × 1 mm effective photosensitive area with a pixel pitch of 25 µm and 1600 pixels. The signals were amplified by a Hamamatsu high speed amplifier model C5594 with a frequency bandwidth between 50 kHz and 1.5 GHz. A scheme of the time overlap (frequency synchronization) for both the laser and x-ray bunches is presented in Fig. 11(c). The laser pulse can be scanned in time across the x-ray pulse either electronically using a delay pulse generator (Stanford Research Systems70 model DG535) or optically using a delay stage. The time instance when both the laser and x-ray pulses temporally overlap is denoted as time zero (t 0). The relative temporal position of the x-ray and laser pulses is monitored with a fast Waverunner 2 GHz oscilloscope from LeCroy71 that allows a determination of zero delay with a 100 ps accuracy. Fine adjustments of the zero delay, and the actual pump-probe measurements, are done by varying only the optical path of the laser using the delay stage. For the spatial overlap, we have used a fluorescent plate with a 500 µm pinhole in the same plane as the sample. In this

manner, the x-ray beam and the laser beam can be overlapped by passing them both through the pinhole. The x-ray and laser spot sizes are measured with a CCD camera located outside the vacuum chamber, having both beams impinging on the fluorescent plate. To ensure the quality and robustness of the spatial laser/x-ray overlap, a careful characterization of their spatial profiles in focus has to be performed. One such example is shown in Fig. 11(d). The final adjustment for spatial overlapping is done by maximizing the so-called DC-heating effect by spatially moving the laser beam across the x-ray beam (and keeping the laser pulses at negative delays, t ≪ t 0). Three different detection possibilities, lockin or boxcar techniques as well as a single-photon counting detection, have been implemented with the APD detectors. For time-resolved XMCD performed in transmission, the xray intensity levels are relatively high, and therefore timeintegrating techniques like lock-in and boxcar detection can be used even for low laser repetition rates below 20 kHz. In the double-modulation detection scheme, we use two lock-in amplifiers: one triggered at the clock frequency of the x-ray probing beam (here, 1.25 MHz) and the second one triggered at the laser repetition rate (variable between 208.3 kHz down to single-shot). The output signal of the (large bandwidth) first lock-in is fed into the second one. In this scheme, the first lock-in measures the un-pumped magnetic signal (i.e., the magnetic state before laser excitation) while the second lockin measures only the laser-induced magnetization changes. One typical result showing the time evolution of the pumped and un-pumped XMCD signals upon fs laser excitation is displayed in Fig. 12(a). For this case, the external magnetic field was reversed at each delay step in order to assure the saturation state for the respective sample. The boxcar detection

FIG. 12. (a) Principle of the double lock-in modulation technique using two lock-ins (left) and the demagnetization curve measured in transmission geometry for a Co/Pt multilayer sample (right). (b) Principle of boxcar measurement technique (left) and the demagnetization curves measured in transmission geometry for a FeGdCo sample for two saturation fields. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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is used to recover fast, repetitive, analog signals. A time gate of predetermined width is precisely positioned relative to an external trigger to coincide with the signal pulse of interest. The gated integrator amplifies and integrates the signal within the gate time window, minimizing the noise of the measured transient signal. The boxcar provides differential gated and integrated signals, which allows to obtain the pumped and un-pumped signal as seen in the graph of Fig. 12(b). For this case, the external magnetic field is switched from positive to negative at each point in the delay scan. The double lock-in and boxcar detection techniques can easily be implemented and used for measurements done in transmission geometry, where the signals are relatively large. For small-angle scattering experiments, the scattering signal is decreased by orders of magnitude compared to transmission XMCD detection. Here, another type of detection using a Time Correlated Single Photon Counting (TCSPC) system (PicoHarp 30072) yields much better results. This type of detection can also be used in reflection geometry. The device makes possible large data integration of different events with certain timing and allows high repetition sampling rates (10 kHz - 208 kHz). In-house software development at HZB73

made possible to gate the interesting part of this time domain in a way that only the events in the gate will be counted and added in a specific integration time (which can vary from minimum 1 s for transmission experiments up to 40 s for scattering experiments). As an example, the measured signal is sketched in Fig. 13(a) (left panel). The time pattern of the x-ray beam can clearly be distinguished for the case of a hybrid mode operation of the synchrotron. Lower green bands represent the quasi-continuous part of the x-rays with a certain intensity (multi-bunch) separated by the clearing gaps (gaps with no xray intensity except the single bunch one). The intense single bunches used in this experiment can be clearly seen in the middle of the gaps at the so-called clearing gaps. Likewise the previous examples, the laser beam is then synchronized to one of these single bunches and can be moved across the x-ray bunches using the same delay stage. The software permits to record up to four channels, each channel being fed with one of the x-ray bunches seen in the full spectrum. In this way, one can easily count synchronously different events for each point in the delay scan. In the present case, the signals represent the pumped bunch (blue), the bunch prior (red), and the bunch after the pumped one (yellow).

FIG. 13. (a) Principle of counting with PicoHarp technique (left) and measured delay scans on three different single bunches for a FeGd alloy for two different magnetic fields in transmission geometry (right). (b) Small angle scattering scans across the magnetic satellite peak measured as a function of the laser repetition rate used as trigger (up) and the Co L 3 demagnetization curve measured in transmission geometry (down). (c) Dynamic detector scans across the magnetic satellite measured on different x-ray bunches; inset is showing a typical detector scan of the main transmitted intensity together with its magnetic satellites due to the lateral periodicity. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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2. Selected results

First results on Co/Pt multilayers, which exhibit perpendicular anisotropy, are shown in the top panel of Fig. 12(a). The measurements were performed in transmission geometry at 300 K and for a laser fluency of 1.4 mJ/cm2. Here, a double modulation lock-in technique described earlier was used. Together with the static XMCD signal (the blue solid line), the pump-induced magnetization dynamics (red open circles) is clearly visible in the graph of Fig. 12(a) (right panel). A fast drop of the saturation magnetization on the time scale of 50 ps (here limited by the x-ray pulse length of 50 ps) and a slow recovery over 500 ps is observed. The offset between the pumped and un-pumped XMCD curves is due to cumulative heating effects—the so-called DC heating effect— as the sample is not fully recovering its initial magnetic state between two laser pumping events. The total measurement time for these scans was about 15 min. Figure 12(b) (right panel) shows the demagnetization measurements in transmission geometry for the intermetallic compound FeGdCo. The signal is measured with the boxcar technique by subsequently flipping the external magnetic field for each pump-probe delay. The XMCD signal was normalized at ±1. As for the case of double-modulation technique, a large degree of demagnetization is observed occurring on a timescale limited by the x-ray pulse duration followed by a slow relaxation back to the initial state. Better signal-to-noise ratios can be achieved by using a boxcargated detection and performing a baseline subtraction of the dynamic signal, i.e., trigger the boxcar at twice the frequency of the excitation event to obtain a “pumped” and an “unpumped” signal instance and by recording the difference of these signal instances. The same type of time-resolved XMCD scans measured this time in a single photon counting mode (with the PicoHarp device) is presented in Fig. 13(a) (right) upon flipping the magnetic field. As mentioned before, the technique allows recording up to four different channels. In this present case, three subsequent x-ray bunches were recorded, the middle bunch (blue curve) being the pumped one. For this bunch, a full demagnetization is observed. The red curve shows the signal recorded on the bunch prior to the pumped one demonstrating that the sample fully relaxes before the arrival of the next pump pulse. The same flat XMCD signal can be seen from the bunch coming after the pumped one (i.e., at a positive delay of 800 ns), but at only 50% of the initial XMCD value. This reveals a relatively slow relaxation of the FeGd sample. All the measured curves can be fitted with a double exponential function convoluted with the time resolution of the actual experiment taking into account two processes governed by two different time constants: first is a rapid drop of the signal and second a recovery of the initial one. As mentioned before, the PicoHarp technique allows to measure small signals making possible experiments in small angle scattering geometry. In this geometry, one can access the magnetic domain structure of the sample facilitating studies of the average domain size distribution, in the present case of CoPt multilayers. In the upper panel of Fig. 13(b), detector scans over the magnetic satellite in small angle scattering

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geometry are presented. This proves the feasibility of the technique even for the medium-to-low repetition rates of the laser; here, the repetition rate of the laser is used to trigger the photon counter, which results in larger counting rates for higher repetition rates of the laser. It is important to mention here that the possibility to change the laser repetition rate is imperative for achieving a balance between a sufficiently large scattering intensity and sample damage due to the laser heating. In the lower panel of Fig. 13(b) is shown a XMCD delay scan measured in transmission at Co L 3 edge showing 50% demagnetization of the sample. The time for maximum demagnetization is used in the following scattering measurements where “dynamic” detector scans are performed across the magnetic domain satellites. In comparison, the inset of Fig. 13(c) represents a “static” detector scan across the transmitted beam for different applied magnetic fields where the magnetic contribution is marked on the graph. Figure 13(c) shows the magnetic peak evolution measured with PicoHarp in small angle scattering geometry on three different x-ray bunches, as explained before. The red and the yellow represent the prior to pump and after pump signals, whereas the blue curve shows the pumped signal. The decrease of the peak intensity for the pumped bunch (blue) can be correlated with the demagnetization level measured in transmission mode. The data shown here demonstrate the feasibility of time-resolved diffuse magnetic scattering with soft x-rays. B. Magnetic field pulse (pump)-x-ray (probe) experiments

In Sec. IV A, we discussed the technical realization of time resolved x-ray resonant magnetic absorption and reflection experiments for the study of demagnetization processes induced by ultrashort laser pulses. These experiments are aimed for the analysis of transient states after heat pulse excitations, including quenching and recovery of the modulus of the magnetization. Another transient state can be excited that changes the direction of the magnetization after field pulse excitation, followed by a damped precessional motion about the new field direction.74–78 Implementation of time-resolved XRMS (TR-XRMS) for probing the transverse magnetization dynamics has the advantages of being element specific and sensitive to layer thickness and interfaces, which is particularly advantageous for investigations of magnetic heterostructures. It should also be mentioned that similar methods have been developed for studying the magnetization dynamics on the picosecond time scale, including the use of microwave excitations phase-locked with photon bunch frequency. This allows investigations of element specific magnetic moment dynamics at different microwave frequencies using time-resolved79,80 and time averaged81 modes. Here, we give a brief description and an overview of possible applications of TR-XRMS investigations. 1. Experimental details

For pump-probe experiments, a stripline geometry is used to generate a magnetic field pulse at the sample position.

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The stripline consists of a conducting wire on which the sample is deposited. Fast rise-time current pulses are delivered from a pulse generator and fed through the stripline that generates a pulsed Oersted field Hpulse perpendicular to the stripe direction. Time resolution is mostly determined by the finite photon bunch length of about 50 ps and by the rise time of the current pulse. A schematic view of our experimental setup is presented in Fig. 14(a). The sample atop the stripline is aligned perpendicular to the scattering plane of the circular polarized x-ray beam. A pair of Helmholtz coils is used for providing a static bias field Hbias in order to saturate the magnetization of the sample parallel to the stripline direction (i.e., perpendicular to the scattering plane). In reflection geometry, the magnetization is probed in the direction parallel to the scattering plane. Thus, by varying the delay time between current pulses and photon bunches, the evolution of the transverse magnetization as a response to the step-pulse magnetic field excitation is recorded. After a fast rise-time of about 0.3–0.4 ns, the induced Oersted field Hpulse defines together with the bias field a new equilibrium direction parallel to Heff , see Fig. 14(b). As a source of step-field pulses, we use commercial square wave pulse generator AvTech model AVMP-2A-B.82 At the selected pulsewidth and amplitude of 10 ns and 10 V, respectively, the rise time of pulses is below 225 ps (measured value) with

35 ps jitter. The rise time at the sample position increases to the value of 300–400 ps depending on the sample quality. Thus, magnetic systems with characteristic relaxation rates above 1 ns can be probed in our experiment. Such relaxation rates are typical in most cases for 3d magnetic metals and their alloys. The damped spin precession about Heff is probed by an x-ray beam of 100 µm spot size at the angle of incidence of 7◦-15◦. The width of the stripline was selected to be 350 µm as the best compromise between the induced fieldpulse amplitude and signal-to-noise ratio, which decreases significantly with narrowing the stripline due to x-ray beam and sample position instabilities. Proper alignment with fine adjustments of sample position with respect to the incident x-ray beam and adjustment of photon energy is essential for this experiment. More details can be found in Refs. 75 and 83. The TR-XRMS experiments are performed during single bunch operation mode with a repetition frequency of the photon bunches of 1.25 MHz. The pulse generator is triggered with the same frequency using the photon bunch clock provided by the synchrotron radiation facility. The length of a single photon pulse in the single bunch mode is about 50 ps, which is comparable with the pulse generator jitter; therefore, the time resolution of our experimental setup is limited to about 50 ps. The free precessional frequency of permalloy (Py or Ni81Fe19) layers at an effective field of 35 Oe does

FIG. 14. (a) Time resolved XRMS setup with the ALICE chamber. The sample (stripline) is sitting in a vertical magnetic field (Hbias). A current pulse is passed through the sample creating a small field (Hpulse) perpendicular to the stripline axis. (b) Magnetization dynamics in a quasi-ideal sample. Magnetization is precessing along the effective field direction and finally is getting dumped. The experiment shown here is a stroboscopic technique to record different instants of the magnetization during its precession. (c) Time-domain magnetization dynamics of Py layer in Co/Cu(40 nm)/Py for different values of the bias fields. Solid lines represent the fits of data using the damped sinusoidal function described in the text. (d) Hysteresis loop (inset) of the Co/Cu(40 nm)/Py sample, measured by SQUID magnetometry. Colored dots indicate the position on the hysteresis loop, where delay scans for different relative magnetization orientations were measured. Reprinted with permission from Salikhov et al., Appl. Phys. Lett. 99, 092509 (2011) and Salikhov et al., Phys. Rev. B 86, 144422 (2012). Copyright 2011 AIP Publishing LLC and 2012 American Physical Society. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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not exceed 1.5 GHz. Therefore, the delay time between the current pulses and photon bunches is scanned in increments of τ = 100 ps, which is higher than the resolution limit but sufficient for the experiment. In order to improve the signal to noise ratio and reduce the effect of long term time drifts, delay scans are performed with the use of a lock-in amplifier (Signal Recovery 726584) capable to extract signals from the photodiode detector with best sensitivity of 100 fA. Using a TTL reference signal to gate the pulse generator, we modulate the pulse switching with a frequency of 1 Hz and integrate the modulated signal with a time constant of the lock-in amplifier gated to 10 s. With this improvement, we significantly increased the sensitivity allowing to detect magnetization dynamics in systems with low concentration of impurities, such as Gd magnetization in permalloy diluted to 9% Gd.85 2. Selected results

Performing delay scans, we obtain the free precessional magnetization decay, i.e., a ferromagnetic resonance signal in the time domain. An example of measured delay scans for a Py layer at the photon energy of Fe L 3 resonance edge and for different values of the bias fields is presented in Fig. 14(c). ⃗ in an effective field H ⃗ eff is The motion of magnetization M described by the Landau–Lifschitz (LL) equation, ⃗ dM ⃗ × [M ⃗ ×H ⃗ eff ]), ⃗ ×H ⃗ eff ] + λ ( M = −γ[ M dt ⃗ s2 M

(2)

⃗ s is where γ is the absolute value of the gyromagnetic ratio, M the saturation magnetization (magnetization per unit volume), and λ is the LL damping parameter. As shown in Ref. 86, the solution of the LL equation for a step pulse excitation, where the magnetization angle (φ) oscillates about the new equilibrium angle φ0 (see Fig. 14(b)) can be written as φ(t) = φ0 + β0 exp[−t/τ] sin(ω pt + Φ),

(3)

where τ is the decay time τ = 2/λ and ω p is the angular frequency of the free precession (ω p = 2π f p ). The fitting parameters β0 and Φ were introduced to take into account the presence of a finite rise time of the pulse field. By fitting experimental data points in 14(c) (solid lines represent the fit), one obtains ferromagnetic resonance parameters: magnetization precessional frequency f p and damping parameter λ. In a limit of small damping (α ≪ 1), the dimensionless Gilbert damping parameter α, obtained from the FMR experiments, is related to the LL damping as α = λ/γ Ms .87 The ability of TR-XRMS to probe element specific free precessional magnetization dynamics allows studies of artificially prepared complex structures with more than one magnetic material. We have studied the free precessional dynamics of the Fe magnetic moments in Ni81Fe19 alloy films (Py),75 in spin valve type trilayers such as Co/Cu/Py, Co2MnGe/V/Py and Co2MnGe/Au/Py,77,78,88 or Py/Cu and Py/AlO multilayer structures.76 An interesting effect has been found by studying magnetization precessional dynamics of Py layers in the Co/Cu/Py spin valves having two different configurations of mutual orientation of magnetization in Py

and Co layers: parallel (P) and antiparallel (AP). The magnetic hysteresis loop of the Co/Cu/Py sample is shown in the inset of Fig. 14(d). The spin valve character of the hysteresis is clearly visible with extended plateaus where the magnetization vectors in the adjacent ferromagnetic layers are antiparallel, in contrast to parallel alignments in saturation. Comparison of TR-XRMS scans measured at Fe L 3 absorption edge (Fig. 14(d)) reveals that in AP configuration, the LL damping parameter of Py is by about 30% bigger than in the P state. This has been taken as a clear signature for a spin pumping effect.77 In case of Co2MnGe/Au/Py trilayers, evidence was found for the excitation of two magnon scattering, which is expressed in sudden shift of the precessional phase without a change of frequency. The two magnon scattering in the Py film is only observed when the Co2MnGe Heusler layer is in a domain state, generating inhomogeneous stray fields from the domain walls.78,88 Moreover, TR-XRMS was used to study the dynamics in Py/Cu and Py/AlOx multilayer systems as well as simple alloys with different stoichiometries (FexNi1−x) and for rare-earth (Gd, Dy) doped permalloy layers.85

V. SUMMARY

We have presented here an experimental station, named ALICE, which can be used for a plethora of different experiments in the field of nanomagnetism investigated by synchrotron radiation in a static as well as in a dynamic fashion. Modular upgrades of an already existing vacuum chamber as well as possible experiments related with these upgrades are presented. This proves the full flexibility and versatility of the system now in use at the synchrotron source BESSY II of the HZB Center Berlin and available through beam time application for the users community. ACKNOWLEDGMENTS

We would like to acknowledge all collaborators, who have helped with their demands and ideas to steadily improve the performance of the ALICE chamber at BESSY II and to fulfill many more tasks than originally designed for. In particular, we would to thank Alexei Nefedov, Stefan Buschhorn, Björgvin Hjörvarsson, Stefan Eisebitt, Olav Hellwig, Sergio Valencia Molina, and many others for their essential input. Karsten Holldack, Rolf Mitzner, Niko Pontius, Christian Stamm, Loïc Le Guyader (HZB), Marc Vrakking, Martin Weinelt, and Rainer Schumann (MBI) are acknowledged for their support during the implementation of the time resolved techniques. Beamline support offered by Torsten Kachel (PM3), Willy Mahler, Birgit Zada (UE56/2 PGM1), and Kai Godehusen (UE52 SGM) is also acknowledged. The operation of the chamber is funded by the BMBF under Contract No. 05K10PC2. 1J.

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