Scanning ferromagnetic resonance microscopy and resonant heating

Scanning ferromagnetic resonance microscopy and resonant heating of magnetite nanoparticles: demonstration of thermally-detected magnetic resonance F. Sakran1,2 , A. Copty1 , M. Golosovsky1 , D. Davidov1,2 , P. Monod2 1

The Racah Institute of Physics, The Hebrew

University of Jerusalem, Jerusalem 91904, Israel 2

ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France (Dated: December 31, 2003)

Abstract We report a 9 GHz microwave scanning probe based on a slit aperture for spatially-resolved magnetic resonance detection. We use a sample consisting of dispersed magnetite F e3 O4 nanoparticles, and at small microwave power we demonstrate low-field ferromagnetic resonance images with a spatial resolution of 15 µm which is consistent with the probe size. At a medium microwave power of ∼ 300 mW we achieve localized heating of the magnetite nanoparticles via ferromagnetic resonance absorption which can be controlled by external dc magnetic field. Using our microwave probe as transmitter and an infrared detector as a receiver we demonstrate thermally-detected magnetic resonance at room temperature. PACS numbers: 68.37.Uv, 87.61.-c, 87.66.Uv Keywords: microwave microscopy, near-field microscopy, ESR imaging, thermal detection

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Scanning microwave probes have become increasingly popular [1]. They have been used for mapping of physical properties and for local spectroscopy, in particular, electron spin resonance (ESR) and ferromagnetic resonance (FMR) imaging. Several scanning ESR probes, including cavity with circular aperture [2–4], dielectric resonator with a thin slit [5], microcoils [6], and sharp tip [7], have been reported. These probes may achieve spatial resolution of 50 µm and sensitivity down to 1010 spins/Gauss which is comparable to what can be achieved in more conventional ESR microscopy with field gradient [8]. All the above mentioned scanning ESR probes employ microwave irradiation and inductive detection. Generally, higher sensitivity and/or spatial resolution in ESR detection can be achieved using alternative detection schemes such as thermomechanical [9], thermal [10], photothermal [11], and optical [12, 13] detection. The optical measuring scheme allows the detection of single molecules and few spins, although at low temperatures. In this letter we demonstrate spatially-resolved near-field FMR studies of magnetite (F e3 O4 ) nanoparticles through both (i) inductive microwave detection and (ii) thermal detection using an infrared detector. The magnetite nanoparticles are unique in several aspects. First, they demonstrate size-dependent magnetism: FMR was detected even for magnetite particles as small as few nanometers [14–16]. Second, magnetite nanoparticles strongly absorb in the visible [17] and in the microwave ranges. In the latter case the enhanced absorption is attributed to the zero-field ferromagnetic resonance [18] and this may allow FMR imaging at zero field. Third, dispersed magnetite particles in solvent allow for the fabrication of macroscopically-uniform layers and well-defined patterns. These are useful for the determination of the spatial resolution in FMR imaging. We have used here a microwave scanning spectrometer as described in our previous work [5, 20, 21]. Figure 1 shows our experimental setup. The microwave source is an HP-83623 microwave synthesizer followed by a medium-power TWT microwave amplifier (M764, Litton Electron Devices). Our microwave probe [21] is based on a dielectric resonator with a hemispherical or conical head and a thin slit aperture on its apex (Fig.1). As demonstrated previously [21], the probe exhibits relatively low impedance and is inductive. The lateral spatial resolution is determined by the slit size, by the curvature radius of the dielectric at the probe apex, and by the probe-sample distance. Due to strong divergence of the microwave beam away from the probe in the near-field regime, it is sensitive only to the surface layer of the sample with the thickness of the order of the slit width [19, 20]. The geometry of the 2

probe is such that the microwave magnetic field is parallel to the long dimension of the slit, while the dc magnetic field is in the plane of the slit and is parallel to the short dimension of the slit (Fig.1). All components of the microwave probe are carefully matched in such a way that the most part of the input power comes out through the slit. This results in a very high field intensity there. The electric field in the aperture is also high, so that for input power exceeding several watts one can see sparking. In this study the microwave power is always below the sparking threshold. The sample is mounted on an XY Z positioning table and is brought to a close distance to the probe (10-100 µm) in order to operate in the near-field regime. The probe is mechanically tuned then to achieve minimal reflection. To measure magnetic resonance in the conventional, inductive way, we fix the microwave frequency, apply modulation field, sweep the dc magnetic field and detect changes in reflected power. To measure thermallydetected magnetic resonance we use a contactless infrared thermometer (Microlife Digital Infrared Thermometer, Model No. IR IDB1) with 1 sec response time and do not apply field modulation. Most of our measurements were conducted on layers of PBK11 commercial black paint on glass slide. The paint consists of magnetite nanoparticles dispersed in oil or in acrylic solvent. Scanning Electron Microscopy studies yield particle sizes between 30 nm and 230 nm with very few 0.5 µm size particles. The filling factor of the magnetite in the composite is ∼ 50%. X-ray studies indicate that the particle composition is F e3 O4 . SQUID magnetometer studies reveal hysteretic magnetization curves indicating magnetic order. Figure 2, upper panel, shows inductively measured FMR signal derivative of a uniform PBK11 layer. We observe a wide FMR signal with a resonance field of 115 mT and linewidth of 100 mT. The lower panel in Fig.2 shows integrated FMR signal. Note considerable zero-field absorption. To demonstrate the spatial resolution of our probe in the imaging mode we prepared a ”resolution target”. To this end we took three glass plates and tightly pressed one to another, leaving a narrow rectangular groove between them which we filled with the PBK11 paint (Fig.3). In such a way we obtain a 40 µm-wide stripe of magnetite nanoparticles. While both the glass and the magnetite stripe reflect microwave, the reflection from the magnetite depends on the magnetic field while the reflection from the glass does not. Hence, by modulating the magnetic field and detecting only modulated reflection with the lock-in 3

amplifier, we single out the FMR signal from the magnetite stripe. Figure 3 shows a onedimensional (1D) scan with a 15 µm-wide slit length moving perpendicular to the PBK11 stripe at a constant dc field of 80 mT. Clearly, the absorption signal is observed only when the slit is above the PBK11 stripe. The width of the peak (Fig. 3) is roughly 40 µm, and the width of the transition edge is 15 µm. The former number is consistent with the width of the stripe and the latter number is consistent with the slit width. The dashed line shows schematically the expected dependence assuming spatial resolution dominated by the slit width. As seen, the expected curve is very close to the measured one. The resulting spatial resolution of 15µm is among the best achieved so far in an inductive near-field microwave magnetic resonance microscopy operating at room temperature. To demonstrate two-dimensional imaging capabilities of our probe, we prepared a pattern consisting of a letter ”H” formed by a thin layer of PBK11 on a glass substrate. Figure 4 demonstrates inductively-detected magnetic resonance image of this pattern at a constant field of 10 mT. This low-field imaging is quite remarkable. Imaging at zero or at very low field can simplify considerably the scanning procedure in real applications. We turn now to discuss the thermal detection of FMR. Here we used slightly different geometry, namely we put the glass side upside down, so that microwave irradiation comes through the thin glass slide, while the uncovered part of the sample faces the infrared detector (Fig.1). In such a way the microwave penetrates through the 120 µm thick glass slide and heats the sample in the area of ∼ 60×500µm, while the infrared detector measures the temperature on the backside of the sample in the area of few mm2 . Figure 2 shows the temperature variation of the sample (filled symbols) as measured upon increasing the magnetic field. The measurement at each field value was taken with a 30 sec delay to allow the sample to reach its steady-state temperature. We see that the dependence of the temperature on magnetic field closely follows the FMR absorption (solid line, integrated signal) line, thus demonstrating the thermally-detected FMR. Following Kirschvink [18], we estimate the steady-state temperature rise in a magnetite single particle embedded in solvent as P , (1) 4πkr where P is the microwave power absorbed by the particle, k is the thermal conductivity of ∆T =

the medium surrounding the particle, and r is the particle radius. For the input power of 1 W and irradiated area of ∼ 60 × 500µm2 the power density at the slit plane is ∼ 107 W/m2 . 4

We assume that the magnetite particle absorbs 100% of the power that it intercepts. For the particle radius of r = 100 nm, and k = 0.5 W/m K we find ∆T ≈ 10 C. In a layer of these particles with 50 % filling factor they heat one another through thermal conduction, therefore Eq.1 underestimates the total temperature rise. Clearly, the power density in the near-field of the slit is high enough to produce considerable heating even with input microwave power of less than 1 W. The irradiated spot is small, of the order of the slit width. However, the microwave absorption leads to the temperature increase not only in the irradiated spot but also in the surrounding area (due to thermal diffusion). The IR detector exhibits larger opening compared to the size of irradiated spot and thus the measured temperature in Fig.2 (open circles) represents only an average temperature around the irradiated spot, while the actual temperature in the center of irradiated spot is even higher. The temperature increases also upon increasing the input microwave power, although at the incident power of 2-3 W the FMR signal already shows saturation. Figure 5 shows temperature variation in the irradiated region of a thin PBK11 layer upon applying different combinations of microwave irradiation and dc magnetic field. Clearly, applying input microwave power of 300 mW in the absence of magnetic field results in temperature increase up to 320 C due to absorption in both acrylic matrix and magnetite nanoparticles. Steady-state temperature is achieved after several minutes. If we apply now magnetic field of 90 mT, the temperature rises immediately by ∼ 20 C; in a field of 115 mT there is a temperature rise of ∼ 30 C due to ferromagnetic resonant absorption at this specific value of magnetic field. The temperature variation upon increasing and decreasing of the magnetic field is reversible, with a time constant of about a minute. At a constant microwave power, the maximum temperature occurs at resonance field, consistent with the data in Fig. 2. We note that the application of a higher field of 300 mT, which is above the resonance field (but at the same mw irradiation level) leads to cooling! In summary, we demonstrate miniature microwave probe, which can localize efficiently microwave radiation within few tens of microns. We proved that our probe can perform FMR imaging with the spatial resolution determined by the aperture size which presently is 15 µm but may be made smaller. The sensitivity of our IR thermometer is rather low and its time response is slow. There are, however, available in the market much better (in terms of sensitivity and time constant) and smaller IR detector. Such IR detection may exhibit higher spin sensitivity compared to microwave detection. 5

Our technique may be useful in biology or medicine. For example, using the technique of decoration with magnetite nanoparticles, our probe can be used for local magnetic resonance and tissue imaging, even at zero field. Another application may be heat treatment by selective irradiation of tissues and cells decorated with magnetite nanoparticles, in close similarity to laser hyperthermia [17]. Local magnetic resonance of magnetic nanoparticles opens also the possibility for controlled localized and selective heating using an external magnetic field. F.S. and A.C. have been supported by the Israeli Ministry of Science and Technology. D.D. is grateful to PARIS-SCIENCES chair for support and hospitality while in France; and to H. Mohwald for valuable discussions.

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References

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Figure captions

Fig.1 Measurement setup. Microwave irradiation is from the one side of the sample and IR detector (optional) is mounted on another side. The inset shows probe geometry. Fig.2 FMR signal of a PBK11 black paint. Open symbols show FMR derivative signal from a uniform layer of the paint. The solid line shows integrated microwave absorption. Filled symbols show corresponding temperature rise as measured by contactless IR detector with 30 sec sampling time. Note that the temperature closely follows resonant absorption, thus demonstrating ”thermally-detected FMR”. Slit width here is w = 60µm, the input microwave power is 300 mW. Fig.3 A linear scan over a 40 µm wide and 1 mm thick magnetite stripe on a glass substrate. The slit width is 15 µm, the magnetic field is 120 mT, modulation field is 1.2 mT, microwave power is 200 mW, microwave frequency is 9.2 GHz. The sample is schematically shown in the upper part of the Figure. Fig.4 Scanning ferromagnetic resonance image of a thin layer of the PBK11 black paint on a 1 mm thick glass substrate. The paint forms letter ”H”. The slit width is 20 µm, the magnetic field is 10 mT, modulation field is 1.2 mT, microwave power is 200 mW. The inset shows optical image of the sample. Fig.5 Temperature variation upon combined action of microwave irradiation and magnetic field. Upon application of microwave power and in the absence of magnetic field the sample heats due to pronounced zero-field microwave absorption in magnetite (see Fig.2). Magnetic field of 90 mT reversibly heats the sample. The maximum heating is achieved by application of the field 115 mT corresponding to the peak of resonance absorption (Fig.2). Further increase or decrease of the magnetic field leads to the temperature drop since this corresponds to the deviation from the ferromagnetic resonance conditions. Microwave power is 300 mW, the slit width is 60 µm.

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