Temperature and thickness dependence of the magnetization reversal in DyFe2/YFe2 exchange-coupled superlattices
K. Dumesnil, S. Fernandez, A. Avisou, C. Dufour, A. Rogalev+, F. Wilhelm+, E. Snoeck°
Institut Jean Lamour (UMR CNRS 7198), Université H. Poincaré - Nancy I, BP 239, 54506 Vandoeuvre les Nancy cedex, France +
European Synchrotron Radiation Facility, BP 220, 38043 Grenoble cedex, France
° CEMES, BP 94347, 31055 Toulouse cedex 4, France
PACS: 75.60.Jk, 75.25.+z, 75.70.-i, 71.20.Eh Corresponding author:
Karine DUMESNIL Institut Jean Lamour (UMR CNRS 7198) Université Henri Poincaré – Nancy I BP 239, 54506 Vandoeuvre les Nancy cedex FRANCE
[email protected]
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Abstract [DyFe2/YFe2] superlattices have been grown by Molecular Beam Epitaxy. The magnetic properties of this hard/soft composite system, the components of which are exchange-coupled at the interfaces, have been investigated in the 10K-300K temperature range, with a specific attention paid to the influence of the soft and hard materials thicknesses. In order to unravel the very rich magnetization reversal processes, conventional susceptibility and magnetization measurements have been combined with element selective X-ray Magnetic Circular Dichroïsm analysis. The superlattice with thin individual thicknesses ([1nm DyFe2/4nm YFe2]70) reverses as a unique giant ferrimagnetic block in which the exchange-favored antiparallel arrangement between net magnetization is maintained under magnetic field. In the superlattices with rather thick individual thicknesses ([10nm DyFe2/13nm YFe2]18 and [10nm DyFe2/20nm YFe2]13), the expected exchange spring behavior is observed: the soft YFe2 layers reverse for positive bias fields, followed by the irreversible switch of the hard DyFe2 layers. In the case of intermediate thickness for the individual DyFe2 layers ([3nm DyFe2/12nm YFe2]22, [5nm DyFe2/20nm YFe2]13, [7nm DyFe2/28nm YFe2]10), the magnetization reversal process strongly depends on temperature. In particular, an unusual magnetization reversal process occurs in the high temperature range where it becomes easier to reverse the hard DyFe2 layers for positive fields, while keeping the dominant YFe2 magnetization along the field direction.
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1.
INTRODUCTION
Exchange-coupled systems combine two magnetic materials that interact with each other at their interfaces via the exchange coupling mechanism. This interaction tends to strongly modify the magnetic behavior of each material compared to their individual properties, as for example the coercivity, the anisotropy… Moreover, the interface exchange coupling is at the origin of interesting phenomena that have been generating an intense research activity for several years: the spring magnet behavior and the exchange bias effect. Among the numerous systems under investigation in this field, the REFe2 (RE=Rare Earth) Laves phase superlattices are of potential interest for applications in permanent magnets [1] or magnetic sensors [2], as well as for the theoretical understanding of the magnetic coupling in these materials. They are among the few single crystalline superlattices that exhibit spring magnet behaviour [3-5], most of the other systems being either textured polycrystalline [6-7] or amorphous [8] materials, or consisting of randomly oriented magnetically hard grains embedded in a soft matrix [9]. The Laves phase superlattices constitute therefore a model system for the study of magnetic springs, of exchange bias phenomena, and of the magnetization reversal process in exchange-coupled systems. This latter process is still a debated issue, as proved by the number of studies reported on magnetization reversal of inplane- [7] and perpendicularly-biased systems [10-11]. In the Laves phases, there are three magnetic interactions that couple the moments of the rare earth and transition metal sites (figure 1). For DyFe2, these are (i) the transition metaltransition metal interaction (JFe-Fe ≈ 3.10-6 erg/cm), (ii) the rare earth-transition metal interaction (JDy-Fe ≈ -0.45.10-6 erg/cm) and (iii) the rare earth-rare earth interaction (JDy-Dy ≈ 0.13.10-6 erg/cm) [12]. DyFe2 is thus a ferrimagnet with a resultant magnetization (4.6 μB/f.u. 3
at 300K) along the dominant Dy subnet magnetization. Because of the shape of the 4f electrons cloud of Dy, DyFe2 is a hard magnetic material. YFe2 is also ferrimagnetic, but the yttrium site only having a small induced moment [13], the iron contribution is the dominant one (2.79 μB/f.u. at 300K). Its anisotropy, which also originates from iron moments, is weak and YFe2 is a soft magnetic material. Finally, due to the strong exchange interaction between iron moments, the magnetic coupling at the DyFe2/YFe2 interfaces occurs mainly through the ferromagnetic coupling between iron spins, which results in a global antiparallel coupling between the net magnetizations of the DyFe2 and YFe2 layers (figure 1). DyFe2/YFe2 superlattices are thus composite systems of hard and soft magnetic layers, antiferromagnetically coupled at their interfaces (from the net magnetization point of view). The simplest scenario to describe the magnetization reversal in such a hard/soft coupled system is usually the formation of exchange springs in the soft material, before the irreversible switch of the hard material [4]. Such a reversal mechanism, referred to as “Soft First” (SF) in the following, has been previously observed [3, 5] in DyFe2/YFe2 superlattices. However, some studies, based on the chemical selectivity of resonant soft x-ray magneto-optical Kerr effect [6, 10], have shown that this oversimplified scenario of the magnetization reversal in coupled systems is likely too crude a description. In the case of DyFe2/YFe2 superlattices with thin DyFe2 layers, we have observed an unusual magnetization reversal process [14], where the magnetization reversal first affects the hard DyFe2 layers (“Hard First” (HF) mechanism), whereas the YFe2 magnetization remains aligned with the applied field direction. This HF mechanism occurs in the high temperature range due to the decrease of anisotropy energy, while the reversal is still governed by the classical SF mechanism at low temperature [15-16]. This experimental result has been qualitatively reproduced using micromagnetic simulations [17]. On the other hand, Bentall et al. [12] showed by neutron scattering that canted states 4
could occur in superlattices when the total net moment of the DyFe2 layer is very close to the total moment of the YFe2 layers.
The aim of this paper is to give an overview of the wide variety of magnetic behaviors observed in DyFe2/YFe2 superlattices, as a function of the individual layers thicknesses and temperature. Details on the sample growth and on their structural characteristics are first given in section II, while the results concerning the magnetization reversal are reported in section III. The mechanisms have been mainly unraveled by the combined analysis of magnetization and susceptibility measurements that average over the entire magnetic system, and of X-ray Magnetic Circular Dichroïsm (XMCD) experiments that permits to measure the two compounds separately. The combination of these complementary techniques has shown that three distinct magnetization reversal processes can occur in these systems, depending on the layers’ thicknesses and on temperature: (1) Rigid magnetic Block (RB) (2) Soft layers First (SF) and (3) Hard layers First (HF). The RB process means that both layers reverse simultaneously, while SF and HF processes imply that the complete magnetization reversal in the superlattice occurs through the succession of two or more steps. RB behavior is observed in the case of thin layers, while the SF process, i.e. the classical exchange spring phenomenon, is observed in the case of thicker layers. The HF reversal phenomenon appears for relatively thin DyFe2 layers, as a high temperature regime following the low temperature SF process. The transition between these two regimes is shifted towards lower temperatures when decreasing the individual DyFe2 thickness.
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2.
SAMPLE GROWTH AND STRUCTURAL ANALYSIS
2.1. Sample Growth Bulk DyFe2 and YFe2 compounds both crystallize in the same C15 Laves Phase structure, made up of a face centered cubic diamond lattice of RE atoms and tetrahedrons of iron atoms. Eight REFe2 units are contained in the cubic unit cell. These compounds also have close bulk lattice parameters of 0.732nm and 0.763nm respectively at room temperature [18]. The superlattices have been grown by Molecular Beam Epitaxy in a chamber with a base pressure around 4x10-11 Torr. The growth process is based on the one originally developed in our group for the epitaxial growth of REFe2 thin films [19]: a (11 2 0) orientated sapphire substrate is first covered at 700°C by 50nm of (110) niobium as a chemical buffer, followed by deposition of a 2nm iron seed layer at 600°C. The role of this thin iron deposit is to interact with the niobium interface to create a surface alloy with a two dimensional rectangular lattice that enables the epitaxial growth of (110)REFe2. The REFe2 compounds are then alternatively grown by co-deposition of iron and yttrium or dysprosium. The deposition temperature used for the superlattice growth is 500°C for the first YFe2 layer and is then reduced to 400°C for the following layers, in order to avoid interdiffusion between both compounds. The samples are usually covered with 100nm of Nb to protect them from further oxidation by air. However, a part of the sample’s area has been systematically left uncovered, which permits to significantly reduce the signal to background ratio for XMCD experiments. The growth process is monitored in-situ by RHEED diffraction that permits to check the quality of the crystal growth with a rather smooth surface and to determine the epitaxial
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relationships between (110)Nb and (110)REFe2: [001]DyFe2//[001]YFe2//[001]Nb and
[1 1 0] DyFe2// [1 1 0] YFe2// [1 1 0] Nb [3]. The experimental results reported in this paper have been obtained from a set of six [t1 nm DyFe2/t2 nm YFe2]N superlattices (N is the number of bilayer repeats), the structures of which are gathered in table I, along with some of their structural properties. In these samples, the nominal ratio t2/t1 is close to 4, except for the samples with the thickest DyFe2 layers (samples E and F). From the magnetic point of view, superlattices SLA to SLD are dominated by the YFe2 magnetization over the entire temperature range, and the superlattice SLE is dominated by the DyFe2 magnetization over the entire temperature range. In the superlattice SLF, the DyFe2 magnetization is dominant at low temperature and the YFe2 magnetization is dominant at 300K. The table II gathers the expected magnetization values (in μB/at.) for the six superlattices for several temperatures and for two different magnetic configurations: (i) the Fully Aligned configuration (F.Al.) when both net magnetization point towards the field (neglecting the interface domain walls thickness) (ii) the Y-ferrimagnetic (Y-Ferri), when the ferrimagnetic arrangement between net magnetizations is orientated with the YFe2 magnetization pointing towards the field. This Y-Ferri value is negative (positive) when the DyFe2 (YFe2) magnetic contribution is dominant.
2.2 High Resolution Transmission Electron Microscopy
Cross section views of the samples have been observed using the CM30 microscope of the CEMES in Toulouse (France), operating at 300 kV whose point resolution is 0.19 nm. To achieve cross-sectional specimens for TEM studies, the samples were cut in two pieces that are glued face to face then thinned by mechanical grinding and ion-milling using a PIPS system to the electron transparency. Figure 2 shows a typical image obtained through several bilayers of 7
superlattice SLD with a bilayer repeat distance of 33.1 nm. These results confirm the crystal quality and the periodic stacking of the layers. The interfaces between the compounds are especially abrupt and smooth. It is interesting to note that the picture made at lower magnificence exhibits however a periodic roughness of the stacking, with a very long wavelength of about 0.2μm that seems to smoothen during the growth of the superlattice. The origin of this modulation might be due to the presence of a periodic lattice of defaults, but this point still remains to be clarified.
2.3 X-ray scattering
X-ray scattering experiments have been performed with a PANalytical X’Pert Pro MRD diffractometer, using and hybride monochromator delivering an intense parallel and monochromatic incident beam. Measurements have been performed with the wavevector transfer varying longitudinally through the (220) Bragg reflection. The structural characteristics extracted from these are gathered in table I. The bilayer thickness is in reasonable agreement with the nominal value. The coherence length along the growth direction ξ is in the 40nm range with a slight tendency to increase for superlattices with the thickest
individual layers. The mosaic spread, measured from the so-called rocking curve through the (220) Bragg reflection, is approximately of 2°, which is a satisfactory value taking into account the complex growth process with the initial deposit of the iron seed layer. The lattice spacing measured along the [110] growth direction has been reported in figure 3 versus the ratio t2/t1. The actual values for t1 and t2 have been deduced from the combined Xray scattering experiments (yielding the bilayer thickness) and X-ray fluorescence analysis (yielding the atomic ratio between Dy and Y elements). The bold curve corresponds to the calculated
average
lattice
spacing
along 8
[110],
assuming
bulk
lattice
constants
(aDyFe2=0.7324nm and aYFe2=0.7346nm) in the individual layers. Because of the small lattice mismatch between both compounds, these values are very close to those obtained by minimizing the elastic energy in the case of a perfect matching of in-plane DyFe2 and YFe2 lattice constants. The measured average lattice spacing increases as expected with the ratio t2/t1, but it is definitively smaller than the calculated value; it is necessary to consider a reduction of the lattice constants by approximately –0.2% for both compounds to get a proper agreement between calculated and measured values. This contraction of the entire system along the growth direction likely finds its origin in the difference between the thermal expansion coefficients for the sapphire substrate and for the deposited compounds. The former (8.6x10-6 K-1) being smaller than the latter (16x10-6 K-1), the sapphire contracts less than REFe2 during the cooling process from the growth temperature to room temperature. By clamping effect to the substrate, the superlattice is consequently submitted to an in-plane extension which results in an out-of-plane contraction. This effect has been already observed in the case of REFe2 films and bilayers [20]. The out-of-plane contraction reported for the films is twice larger, of the order of –0.4%, because of the higher deposition temperature.
3.
MAGNETIZATION REVERSAL
The magnetization reversal process in the DyFe2/YFe2 superlattices has been studied by combining SQUID magnetization measurements and XMCD experiments. The former technique enables to record the net macroscopic magnetization of the whole sample as a function of the applied magnetic field, while the second one has been chosen for the unique capability to isolate the magnetic response of each compound in the superlattice. Monitoring the XMCD signal at the Dy L3 (2p-5d transitions) and Y L3 (2p-4d transitions) absorption 9
edges as a function of applied magnetic field, we were actually able to record element-specific, and thus compound-selective, hysteresis loops. The XMCD spectra at these edges are related to the induced magnetic moments of the Dy 5d and Y 4d states, whereas the magnetism of the compounds is dominated by Dy 4f and Fe 3d states. The polarization of the Dy 5d and Y 4d shells is, however, proportional to the total magnetization in the DyFe2 and YFe2 layers respectively. Thus the field dependence of the XMCD signals is directly related to the field dependence of the magnetization of the specified compound, projected onto the direction of the incident X-rays (i.e. of the external magnetic field).
3.1 Experimental conditions
The SQUID measurements have been performed with a conventional Quantum Design magnetometer that provides a vertical magnetic field ranging between -7T and +7T. The diamagnetic contribution from the sapphire substrate has been removed and the measurements collected in emu have been converted into μB/at., taking account of all chemical species in the system. All the experiments presented in the following sections have been performed with the magnetic field applied along the REFe2 in-plane [1 1 0] direction. In bulk DyFe2, the easy magnetization axis given by the crystal field interaction is at any temperature [21]. In DyFe2 films, the strains in the lattice give rise to a supplementary magnetoelastic contribution that drives a change of the easy magnetization axis when the temperature increases [22] : it evolves from the bulk directions towards the directions that are close to the inplane [1 1 0] direction. In the YFe2 films, the easy magnetization axis does not change with temperature and remains close to the [1 1 1] in-plane direction, in agreement with the bulk easy directions [22]. In the superlattices, no analysis of the easy magnetization axis in each compound separately has been carried out. Nevertheless, for each temperature studied, the 10
magnetization measurements have been performed with the magnetic field applied both along [001] and [1 1 0] in-plane directions. The results obtained below 100K are very similar for both directions, while the [1 1 0] direction is obviously easier above 100K. Therefore, the results presented in the following are those collected for the field applied along [1 1 0] . AC susceptibility measurements were performed using a PPMS Quantum design instrument, with the DC and AC magnetic field both applied along the [1 1 0] in-plane direction. The susceptibility measurements permit to bring new insight into the reversal mechanism since the magnetic moments aligned perpendicular to the applied DC and AC fields exhibit a much stronger response to the oscillating field (transverse susceptibility) than the moments aligned parallel to the fields (longitudinal susceptibility). An increase in the magnetic susceptibility is thus the signature of an increasing amount of transverse moments. XMCD experiments were performed on the beamline ID12 of the ESRF. The applied magnetic field was parallel to the direction of the incident X-ray beam. In order to apply the magnetic field as close as possible to the in-plane [1 1 0] direction, a specific sample holder was used to permit grazing incidence geometry, with the incident beam and the magnetic field at 8° from the in-plane [1 1 0] direction. The XMCD spectra were recorded by flipping the helicity of incoming X-rays and keeping the direction of the magnetic field fixed. For the experiments at the Y L3 edge, we used the fundamental harmonic of a hybrid electromagnetic helical undulator (EMPHU) [23], which allowed us to flip the helicity of the X-rays at every energy point of the scan. For experiments at the Dy L3 edge, the second harmonic of the HELIOS-II undulator has been used and the helicity of incoming photons was changed after each consecutive scan. In both cases the degree of circular polarization of the monochromatic X-ray beam was estimated to be in excess of 85%. The spectra were recorded in the total fluorescence detection mode,
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which is not sensitive to the external applied magnetic field, at least in the range of interest (+7T to –7T). Typical XMCD spectra at both Dy and Y edges are shown in figure 4, together with the normalized X-ray absorption spectra. It was verified that the shape of the spectra does not change with field and especially exhibits a maximum for the same energy value. To record the element-selective hysteresis curves, the energy of the incident X-ray photons was thus tuned to the maximum of the XMCD signal either at the Dy L3 edge (7.796 keV) or at the Y L3 edge (2.078 keV). The amplitude of the dichroïc signal at each value of the applied field was measured by flipping the helicity of the X-ray beam. Let us remind that Dy moments are parallel to the net DyFe2 magnetization, while Y moments are opposite to the YFe2 magnetization: for a same orientation of the net magnetizations, this yields opposite signs at the Dy and Y L3 edges. In a sake of clarity and in order to simplify the discussion, the XMCD loops measured at the Y L3 edge have been systematically multiply by a factor (-1). In the following, for both edges, a positive (negative) XMCD signal reflects a positive (negative) longitudinal component of the magnetization along the direction of applied magnetic field.
3.2 Reversal as a Rigid magnetic Block (RB) Figure 5 shows the variation of the macroscopic magnetization versus external field at 100K and 200K (top panel) and the separate hysteresis loops recorded by XMCD at the Dy and Y L3 edges at 100K (middle panel) and at 200K (bottom panel) for the superlattice SLA. For both temperatures, the XMCD signals measured at both edges exhibit opposite signs and change sign for approximately the same magnetic field, which proves that the net magnetizations in DyFe2 and YFe2 layers are always antiparallel in the -7T/+7T field range. For +7T, the dominant YFe2 magnetization points towards the external field and the DyFe2 one is opposite 12
(configuration Y-Ferri), as requested by the exchange interactions, and as sketched in the figure. In decreasing the magnetic field, this configuration is maintained until both magnetization reverse simultaneously for a coercive field Hc that is close to -3.5T at 100K and -0.7T at 200K. The magnetic configuration at -7T is then the symmetric of the initial one, with all the Fe moments pointing along the applied magnetic field. The reversal proceeds via a unique step and the equilibrium ferrimagnetic arrangement is maintained throughout this process. The resultant magnetization measured by SQUID is in reasonable agreement with this scenario, since the magnetization value of the whole superlattice in the ferrimagnetic state should be close to 0.29μB/at. at 200K and 0.21μB/at. at 100K (table II). Note that the vertical shift of the XMCD loops at 100K has no physical origin since it does not appear in the SQUID measurements. It has been attributed to a stability problem occurring during XMCD experiments. In this sample with particularly thin individual layers, the external magnetic field does not permit to break the ferrimagnetic configuration resulting from exchange: the Zeeman cost due to the DyFe2 magnetization opposite to the external field is smaller than the cost of interface magnetic walls that would result from breaking the ferrimagnetic configuration. In other words, the YFe2 layers are too thin to enable the development of magnetic walls with a reasonable energy cost. The equilibrium configuration under field corresponds to the ferrimagnetic arrangement between net magnetizations, with the dominant YFe2 magnetization pointing along the ±7T external field. It is also interesting to notice the strong coercive field in this system (-3.5T at 100K). This value can be compared to the -2T coercive field for 5nm thick DyFe2 films at 100K [24] and to the -10-2 T coercive field for 100nm thick YFe2 layers at low temperature. In SLA, for negative field above -3.2T, the YFe2 magnetization remains opposite to the field, although this 13
compound is intrinsically very soft. The magnetic field required to reverse this rigid magnetic block is even larger than the one to overcome the magnetocrystalline anisotropy in thin DyFe2 layers, because of the smaller magnetization in this ferrimagnetic block.
3.3 Reversal of the Soft layers First (SF), i.e. exchange spring behavior Figure 6 shows the macroscopic magnetization (top panel) and the XMCD signal at the Dy edge (bottom panel) measured at 100K and 200K as a function of applied magnetic field for the superlattice SLE. This sample is a typical example of what is observed in the case of rather thick individual layers (the DyFe2 layers are 10 times thicker than in the superlattice SLA described in the previous paragraph). A common feature with SLA is the shape of XMCD loops measured at the Dy absorption edge. They are almost square and exhibit a strong coercive field of -3.2T and -1.6T at 100K and 200K respectively, characteristic of a hard magnetic compound. The sign of the XMCD signal is however opposite, which means that the DyFe2 net magnetization points toward the applied field for the +6T and -6T extreme values. From the macroscopic magnetization value measured at +7T and the magnetization reversal described below, one can conclude that the magnetic configurations at +7T and -7T are close to those sketched in (a) and (d). In this superlattice, contrary to SLA, the applied magnetic field permits to break the ferrimagnetic arrangement: the gain in Zeeman energy obtained by aligning the net DyFe2 magnetization along the field overcomes the cost of interface magnetic walls. Given the respective anisotropy constants in DyFe2 and YFe2 compounds, the interface walls are most likely located in the YFe2 layers where their cost is expected to be the smallest. When reducing the applied magnetic field, the macroscopic magnetization decreases for positive field while the Dy XMCD signal remains constant. This first step of the magnetization 14
reversal (b) is therefore attributed to expansion of interface walls inside the YFe2 layers. The much harder DyFe2, the magnetization of which is also dominant in this superlattice, remains aligned with the field and the YFe2 magnetization reverses for a positive field, driven by the interface exchange coupling. The YFe2 reversal is biased by a positive field of approximately +1.5T for both temperatures, as expected from an interface exchange-coupled system with a negative interface exchange coupling [25-26]. The complete reversal of the YFe2 magnetization results in what has been called the Dy-Ferri arrangement (c). The amplitude of this first magnetization reduction does not depend on temperature, as expected from the slow variation of YFe2 magnetization in this temperature range. The minor loop performed in this +7T/-1T field range (inset figure 6, filled symbols) shows that the YFe2 reversal is completely reversible, as expected from expansion and compression of exchange springs. Moreover, the magnetic susceptibility (inset figure 6, empty symbols) exhibits a pronounced maximum around 1.6T, as a result of the expansion of the domain wall: when the domain wall becomes wider, the amount of magnetization having a component perpendicular to the field direction increases, which increases the susceptibility. The second step in the magnetization reversal (at –3.2T at 100K) corresponds to the abrupt reversal of the DyFe2 magnetization, as proved by the XMCD results. The configuration (c) is stabilized by exchange interactions until the negative applied field becomes large enough, for the Zeeman gain obtained by reversing the DyFe2 magnetization to overcome both the DyFe2 magnetocrystalline anisotropy and the cost of interface walls. The ferrimagnetic configuration is then broken again and the interface walls compress in the YFe2 layers when the applied field further decreases (d). The amplitude of this second reversal step decreases by approximately 16% between 100K and 200K, in agreement with the reduction of the DyFe2 magnetization, from 6.39μB/f.u. to 5.4μB/f.u.. 15
The coercive field in this sample is significantly larger than in an individual 10nm DyFe2 film [24], by approximately a factor of 2 at 100K. This strong hardening is the logical result of the negative interface exchange coupling that tends to maintain the DyFe2 magnetization opposite to the YFe2 one. In summary, this superlattice behaves as an archetype example of a hard/soft composite system with negative interface exchange coupling: The soft phase reverses first (SF) under positive field and in a perfectly reversible process, whereas the hard phase irreversibly switches for a strong negative field. Let us recall that in this [10.2nm DyFe2 /11.9nm YFe2]18 superlattice, the whole magnetization is dominated by the DyFe2 contribution. The Dy-ferrimagnetic arrangement in (c) exhibits a smaller Zeeman energy than the opposite ferrimagnetic configuration. This is however not the case in the example presented in figure 7 (SLF superlattice): at 100K, both DyFe2 and YFe2 net magnetization are almost equal and the ferrimagnetic arrangement stabilized after the first reversal step (sketch (c) in figure 6) results in approximately zero net moment. At 300K, the YFe2 magnetization is dominant on its turn and still experiences the first magnetization reversal. The Dy-ferrimagnetic configuration at low positive field is thus a metastable state achieved by the easier magnetization reversal in YFe2. The energy barrier between the initial configuration and the lowest energy Y-ferrimagnetic configuration is too high. This leads to the unusual crossover of the loop branches and to a negative remanent state, as it is enlightened in the inset of figure 7. G.J. Bowden et al. [27] have used a Stoner-Wohlfart model, adapted to include magnetic exchange springs, to reproduce hysteresis loops in DyFe2/YFe2 superlattices. Considering that the magnetic moments remain in the growth plane, this model has proved reasonably successful in describing the SF reversal mechanism at low temperature in three systems: a 16
DyFe2 magnetically dominated, an almost magnetically compensated, and a YFe2 magnetially dominated. The calculation especially reproduces positive or negative coercivities, a phenomenon that is observed in varying temperature in the present study.
In these superlattices (SLE and SLF), the hard character of the DyFe2 layers thus makes the system reverse in following the soft first (SF) process over the whole temperature range, even for a dominant soft phase. This feature, associated to the negative interface exchange coupling, may lead to an unusual temperature variation of the coercivity and remanent values that change sign in increasing temperature. This is especially the case when the dominant contribution changes in temperature.
3.4 Temperature dependent reversal process Let us now examine the behavior that is observed in the superlattices with DyFe2 layers of ‘intermediate’ thicknesses. The magnetization reversal process will be first described in details in the superlattice SLC that constitutes a typical example. Then the influence of the layer thicknesses is presented, via the cases of superlattices SLD and SLB that exhibit the same thickness ratio as SLC, but respectively thicker and thinner layers.
3.4.1 Typical behavior in superlattice SLC Figure 8 presents the macroscopic magnetization (top panels) and XMCD signals (bottom panels) recorded at both the Dy and Y absorption edges, as a function of applied magnetic field and at four different temperatures (100K, 165K, 200K and 250K) for the superlattice SLC. A first glance at these results reveals a significant temperature dependence of the magnetization reversal process, in contrast to the behaviors reported so far. At 100K, the system exhibits a 17
negative coercivity and a negative remanent state, while both the coercivity and the remanent states are positive at 200K. Moreover, the magnetization reversal occurs in two separate steps at 100K, while three successive steps are clearly visible at 200K and 250K. These different behaviors can be understood by analyzing the corresponding XMCD results. At 100K, the reversal mechanism is qualitatively very similar to the one described in SLE and SLF and corresponds to the same scenario (sketches in figure 6). The coercive field for which the DyFe2 layers reverse reaches almost -6T at 100K. This is higher than in SLE and SLF at the same temperature (-4T), because of thinner DyFe2 layers and in agreement with the large increase of the coercive field reported for DyFe2 films when reducing the thickness [24]. It is interesting to note that the formation of interface magnetic walls at Hc is visible on the Y XMCD loop: the slight reduction of the Y signal around -6T is due to the YFe2 moments involved into the walls and that are thus no longer aligned with the applied field. The magnetization reversal observed at higher temperature (200K and 250K) turns out to be radically different from the low temperature SF regime. It occurs via three successive steps occurring between various magnetic configurations, sketched on the figure. Starting as previously from a configuration where the net magnetizations in both compounds are aligned with the external field (a), the first stage of magnetization reversal affects both compounds, as evidenced by the reduction of the XMCD signals, at both the Dy and Y edges. This process ends in the complete reversal of the DyFe2 magnetization around 1T at 200K, as proved by the XMCD Dy signal opposite to the one measured at +7T. This process is called ‘Hard First’ (HF), in comparison to the SF reversal process occurring at lower temperature. This complete reversal of the DyFe2 layers induces the annihilation of interface magnetic walls (increase of the XMCD signal at the Y edge) and leads to the ferrimagnetic configuration (b). The second stage of the magnetization reversal is the abrupt switch of the ferrimagnetic block 18
for a negative field close to -1T (c), as a result of the Zeeman interaction. Then, the increase of the negative external field tends to progressively break the ferrimagnetic arrangement to align the DyFe2 magnetization along the field, as proved by the decrease of the Dy XMCD signal. This third reversal of the DyFe2 magnetization drives of course part of the YFe2 moments, which is again visible as a slight reduction of the XMCD Y signal. The reversal process at 250K is qualitatively identical, with an easier reversal of DyFe2 magnetization (the reversal starts and the complete reversal is achieved for larger positive field); the ferrimagnetic block also reverses for a smaller negative field. At 165K, the behavior illustrates how the reversal mechanism evolves from 100K to 200K. The magnetization in both compounds starts to reverse, as previously described at higher temperature, but this process drives the complete reversal of the YFe2 layers first, under positive field, as at 100K. The DyFe2 magnetization consequently switches back parallel to the field while interface magnetic walls annihilate, and the Dy-ferrimagnetic configuration is maintained until the negative applied field is strong enough to reverse the DyFe2 magnetization. The successive steps of the reversal mechanism are therefore the (a), (c) and (d) sketches of figure 6, as at 100K. But the process between (a) and (c) implies both compounds at 165K while it implies only YFe2 at 100K. The magnetization reversal in SLC can be characterized by the magnetic field for which the YFe2 magnetization reverses (HY) and by the minimum value of the XMCD signal at the Dy edge (Dymin) when the applied field decreases between +7T and 0T. Both these values are presented versus temperature in figure 9 (top curves). Their thermal variations are strongly related to each other and permit to define three distinct temperature ranges : (i) the “SF” region below 150K, where the Soft layers reverse First. This reversal is biased towards the positive field region (HY>0), as expected from the negative interface exchange coupling. (ii) the “HF” 19
region above 200K, where the Hard layers reverse First (Dymin
