- Email: [email protected]
Locally resolved photothermally modulated ferromagnetic resonance on epitaxial Fe-films deposited on laterally patterned GaAs
Journal of Magnetism and Magnetic Materials 240 (2002) 83–85
Locally resolved photothermally modulated ferromagnetic resonance on epitaxial Fe-films deposited on laterally patterned GaAs R. Meckenstock*, D. Spoddig, J. Pelzl AG Festkorperspektroskopie, Institut fur Experimentalphysik III, Ruhr-Universitat-Bochum, NB 3/70, 44780 Bochum NB, Germany . .
Abstract (0 0 1)Fe-mesa were grown on a GaAs high electron mobility transistor (HEMT)-structure. Magnetic properties were investigated by locally resolved photothermally modulated ferromagnetic resonance (PM-FMR). The surface and the in-plane anisotropy are decreased in HEMT-areas. The field independent response of PM-FMR provides information on the electrical properties of the GaAs-heterostructure and yields evidence of preparation dependent lifetime effects of the photo-carriers. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Ferromagnetic resonance; Anisotropy-local; Microwave absorption; Semiconductor
1. Introduction Recent progress has been made in growing laterally structured (0 0 1)Fe-films on semiconductor heterostructures, with desirable magnetic and electric properties for future device applications. In this paper, we present the investigations of local variations of magnetic anisotropies of (0 0 1)Fe-films and of the electric properties of the GaAs-heterostructure by a single measuring technique: the photothermally modulated ferromagnetic resonance (PM-FMR). To enlarge the accuracy of the anisotropy values, additional conventional FMR measurements were used [1]. 2. Experiments The GaAs-wafers were patterned with a high electron mobility transistor (HEMT)-structure at the University of Bochum [2]. The (0 0 1)Fe-films were grown under MBE conditions on these substrates at the University of Duisburg [3] and were afterwards photolithographically structured. Fig. 2i shows the microscopy image of the *Corresponding author. Tel.: +49-234-32-26045; fax: +489234-32-14336. E-mail address: [email protected] (R. Meckenstock).
final (0 0 1)Fe-mesa-structures on the GaAs. The HEMT structure, not visible in this image, is located below the large Fe-mesa-structures on the right side. Room temperature measurements of the magnetic properties were performed by conventional FMR and PM-FMR in the X-band. Both measurements are described by the equation of motion of the magnetization including the Gilbert damping term [4]: ! ~ ~ dM ~B ~eff Þ þ a M ~ dM ; ¼ gðM dt M dt ~¼M ~s þ m ~eiot ; M 0 iot 1 be þ qF=qMx C ~eff ¼ B B qF=qMy @ A; Bext þ qF=qMz
ð1Þ
where g is the gyromagnetic ratio, M the saturation magnetization vector, F the free energy density functional, a the intrinsic damping parameter, Beff and Bext the effective and the external magnetic field, respectively, and b the magnetic field of the microwave. Solving Eq. (1) by using the free energy density approach [1] results in the magnetic anisotropy fields of the Fe-film. To clarify the difference between conventional FMR and PM-FMR, one has to keep in
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 7 5 4 - 5
R. Meckenstock et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 83–85
mind that both signals are proportional to the imaginary part w00xx of the high frequency susceptibility tensor [5]. Conventional FMR usually is field modulated; thus, the signal is proportional to ðqw00xx =qBext Þ: In case of PMFMR, the field modulation is replaced by thermal modulation of magnetic parameters. Thus, the absorption signal is proportional to ðqw00xx =qTÞ; with w00xx ðTÞ ¼ w00xx ðMðTÞ; K1 ðTÞ; yÞ: The lateral resolution of the thermal modulation is governed by the diffusion length of the propagating thermal wave and the focus of the laser beam used for heating. In this work, a 10 mW HeNe laser with a focus of 10 mm was used for driving the thermal wave with a modulation frequency of 100 kHz. At this modulation frequency, the penetration depth of the thermal wave equals to the size of the laser spot. Details of the set-up and the technique can be found in Ref. [5]. The amplitude of the temperature modulation is of the order of 10 mK and the DC temperature rise is o1 K, thus, the resonance line positions of the PM-FMR coincide with those of conventional FMR. In addition, the heating laser of the PM-FMR induces photo carriers in the GaAs-substrate [6] even in the areas where the GaAs is covered with the Fe-film. The carriers provide a field independent background, which can be used to determine the electric behaviour of the GaAssubstrate. Thus, from one PM-FMR spectrum, one can get information on the magnetic parameters of an Fe-film and on the electrical properties of semiconducting substrate, simultaneously. 3. Results Fig. 1 shows a typical PM-FMR spectrum of an epitaxially grown Fe-film on a (0 0 1) GaAs-substrate. Two FMR-modes are well resolved and have a good signal-to-noise ratio. These modes are expected for an external magnetic field along the Fe-[1 1 0] direction and microwave frequencies in the X-band [5]. As mentioned above the PM-FMR signals are superimposed with a field independent background (dashed line in Fig. 1), which is caused by photo carriers induced in the GaAs and is two orders of magnitude larger than the FMR signal. For a patterned Fe-film, the conventional FMRspectra can show multiple resonances due to the integral character of the signal and the non-uniform magnetic parameter of the film. The upper spectrum in Fig. 2iii shows such a conventional FMR absorption derivative of the structured Fe-film described above. The FMRsignals are deformed and clearly a superposition of several modes. To interpret the spectrum, one needs to investigate the local variations in the magnetic and electric properties with the PM-FMR. The two lower spectra in Fig. 2iii display the PMFMR absorption derivative versus the external magnetic
PM-FMR amplitude (mV)
84
1.0092
reflected microwave power 1.0086
1.0080
1.0074
1.0068 20
40
60
80
100
120
external magnetic field (mT) Fig. 1. PM-FMR signal amplitude versus the external magnetic field along the [1 1 0] orientation of a Fe-film on GaAs.
field of two different positions of the patterned Fe-film. In order to reduce the measuring time scanning the entire sample, the signal-to-noise ratio was not optimized to achieve the exact line shape. Nevertheless, one can see that the line position is clearly pronounced and the linewidth is smaller compared to the conventional signal. The middle spectrum corresponds to the sample area marked by a in Fig. 2i. In this area, the GaAssubstrate has no HEMT-structure and the Fe-film was etched to a mesa with a diameter of 60 mm. The lower spectrum was measured in position b on a rectangular 500 500 mm2 Fe-mesa with HEMT-structure underneath. Comparing the three spectra in Fig. 2iii it is obvious that the shape of the conventional FMR is a superposition of the signal of the Fe-film on pure GaAs and on GaAs with HEMT-structure. Based on this behaviour one can calculate the anisotropy parameters by angle dependent FMR-measurements. As a result the surface anisotropy of the Femesa deposited on the HEMT-structure (b) is decreased by a factor of 2 compared to the Fe-mesa directly on GaAs (a). The uniaxial in-plane anisotropy was slightly decreased by about 10% between positions b and a: Fig. 2ii shows a greyscale image of the locally resolved field independent microwave reflection signal as indicated in Fig. 1. The Fig. 2ii covers the same area as optical microscope image in Fig. 2i. Comparing the microwave reflection and the microscope image it is found that the sample was etched non-uniformly in the Fe-patterning step. The intensity of the microwave reflection signal increases with the degree of etching of the GaAs-surface, being 3 times higher at position A than at position B: The lowest background signal was found inside the HEMT-structure, seen as the dark vertical stripe on the right side of Fig. 2ii. The low intensity is explained by the lower number of photo carriers induced by the laser light due to the nonconducting layer of the HEMT-structure [2]. We gain more insight into the electronic contributions of the photo modulated microwave absorption and are
R. Meckenstock et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 83–85
85
reflected microwave power (W)
Fig. 2. (i) Optical microscope image of structured Fe surface. (ii) Greyscale image of the field independent PM-FMR-amplitude. (iii) Microwave response versus external magnetic field in the [1 1 0] direction. Spectra top to bottom: integral conventional FMR-spectrum; PM-FMR-spectrum of Fe-mesa on pure GaAs (position a in i and ii); PM-FMR-spectrum of Fe-mesa with HEMT below (position b in i and ii). photomodulated microwave absorption 10-2
10-3
10-4
10 6
Fe covered GaAs, C GaAs, B GaAs strongly etched, A modulation frequency (Hz)
HEMT, the signal decrease is shifted to a higher value (2.7 MHz). This is in agreement with the shorter lifetime of the photo carriers in the GaAs. At position A; the GaAs was strongly etched, removing the As-termination for the Fe-film preparation [3]. In this area, the signal minimum is shifted to even higher values (4.2 MHz), indicating a shorter lifetime of the photo carriers. Acknowledgements
5 10 6
Fig. 3. Field independent PM-FMR-amplitude versus modulation frequency. A, B, and C correspond to marked positions in Fig. 2ii.
able to demonstrate the high sensitivity of the microwave reflection by studying the signal as a function of modulation frequency. The frequency spectra of the three representative positions A; B; and C mentioned above are shown in Fig. 3. All spectra show decreases in the signal at characteristic frequencies. These decreases are governed not only by the lifetime of the photo carriers, but also by experimental parameters such as the microwave cavity quality factor and the sample size. Thus, a quantitative measure of the photo carrier lifetime is only possible with a calibrated set-up, which has not been possible yet. For the spectra corresponding to position C in Fig. 3 (HEMT-structure), one can see the signal decrease at about 1.5 MHz. In the HEMT-structure, we expect the longest lifetime of the excited photo carriers. In position B; representing the slightly etched GaAs-surface with no
The authors would like to acknowledge the financial support of the Deutsche Forschungsgemeinschaft (SFB 491). References [1] R. Meckenstock, et al., Magnetic properties of Fe/ZnSe and Fe/GaAs heterostructures investigated by ferromagnetic resonance and SQUID measurements, Proceedings of the MML 2001, Manuscript No. 527, J. Magn. Magn. Mater., to be published. [2] A.D. Wieck, D. Reuter, Institute of Physics Conference Series No. 166, 2000, p. 51. [3] M. Doi, et al., Magnetic and structural properties of epitaxial Fe thin films on selectively doped AlxGa1x/GaAs heterostructure, Proceedings of the MML 2001, J. Magn. Magn. Mater., to be published. [4] G.V. Skrotskii, L.V. Kurbatov, in: S.V. Vonsovskii (Ed.), Ferromagnetic Resonance, Pergamon, New York, 1966. [5] R. Meckenstock, et al., J. Appl. Phys. 77 (1995) 6439. [6] W. Kiepert, et al., Progress in Natural Science (Supplement to Vol. 6), Taylor & Francis, London and Washington, 1996, p. 515.
Locally resolved photothermally modulated ferromagnetic resonance on epitaxial Fe-films deposited on laterally patterned GaAs R. Meckenstock*, D. Spoddig, J. Pelzl AG Festkorperspektroskopie, Institut fur Experimentalphysik III, Ruhr-Universitat-Bochum, NB 3/70, 44780 Bochum NB, Germany . .
Abstract (0 0 1)Fe-mesa were grown on a GaAs high electron mobility transistor (HEMT)-structure. Magnetic properties were investigated by locally resolved photothermally modulated ferromagnetic resonance (PM-FMR). The surface and the in-plane anisotropy are decreased in HEMT-areas. The field independent response of PM-FMR provides information on the electrical properties of the GaAs-heterostructure and yields evidence of preparation dependent lifetime effects of the photo-carriers. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Ferromagnetic resonance; Anisotropy-local; Microwave absorption; Semiconductor
1. Introduction Recent progress has been made in growing laterally structured (0 0 1)Fe-films on semiconductor heterostructures, with desirable magnetic and electric properties for future device applications. In this paper, we present the investigations of local variations of magnetic anisotropies of (0 0 1)Fe-films and of the electric properties of the GaAs-heterostructure by a single measuring technique: the photothermally modulated ferromagnetic resonance (PM-FMR). To enlarge the accuracy of the anisotropy values, additional conventional FMR measurements were used [1]. 2. Experiments The GaAs-wafers were patterned with a high electron mobility transistor (HEMT)-structure at the University of Bochum [2]. The (0 0 1)Fe-films were grown under MBE conditions on these substrates at the University of Duisburg [3] and were afterwards photolithographically structured. Fig. 2i shows the microscopy image of the *Corresponding author. Tel.: +49-234-32-26045; fax: +489234-32-14336. E-mail address: [email protected] (R. Meckenstock).
final (0 0 1)Fe-mesa-structures on the GaAs. The HEMT structure, not visible in this image, is located below the large Fe-mesa-structures on the right side. Room temperature measurements of the magnetic properties were performed by conventional FMR and PM-FMR in the X-band. Both measurements are described by the equation of motion of the magnetization including the Gilbert damping term [4]: ! ~ ~ dM ~B ~eff Þ þ a M ~ dM ; ¼ gðM dt M dt ~¼M ~s þ m ~eiot ; M 0 iot 1 be þ qF=qMx C ~eff ¼ B B qF=qMy @ A; Bext þ qF=qMz
ð1Þ
where g is the gyromagnetic ratio, M the saturation magnetization vector, F the free energy density functional, a the intrinsic damping parameter, Beff and Bext the effective and the external magnetic field, respectively, and b the magnetic field of the microwave. Solving Eq. (1) by using the free energy density approach [1] results in the magnetic anisotropy fields of the Fe-film. To clarify the difference between conventional FMR and PM-FMR, one has to keep in
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 7 5 4 - 5
R. Meckenstock et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 83–85
mind that both signals are proportional to the imaginary part w00xx of the high frequency susceptibility tensor [5]. Conventional FMR usually is field modulated; thus, the signal is proportional to ðqw00xx =qBext Þ: In case of PMFMR, the field modulation is replaced by thermal modulation of magnetic parameters. Thus, the absorption signal is proportional to ðqw00xx =qTÞ; with w00xx ðTÞ ¼ w00xx ðMðTÞ; K1 ðTÞ; yÞ: The lateral resolution of the thermal modulation is governed by the diffusion length of the propagating thermal wave and the focus of the laser beam used for heating. In this work, a 10 mW HeNe laser with a focus of 10 mm was used for driving the thermal wave with a modulation frequency of 100 kHz. At this modulation frequency, the penetration depth of the thermal wave equals to the size of the laser spot. Details of the set-up and the technique can be found in Ref. [5]. The amplitude of the temperature modulation is of the order of 10 mK and the DC temperature rise is o1 K, thus, the resonance line positions of the PM-FMR coincide with those of conventional FMR. In addition, the heating laser of the PM-FMR induces photo carriers in the GaAs-substrate [6] even in the areas where the GaAs is covered with the Fe-film. The carriers provide a field independent background, which can be used to determine the electric behaviour of the GaAssubstrate. Thus, from one PM-FMR spectrum, one can get information on the magnetic parameters of an Fe-film and on the electrical properties of semiconducting substrate, simultaneously. 3. Results Fig. 1 shows a typical PM-FMR spectrum of an epitaxially grown Fe-film on a (0 0 1) GaAs-substrate. Two FMR-modes are well resolved and have a good signal-to-noise ratio. These modes are expected for an external magnetic field along the Fe-[1 1 0] direction and microwave frequencies in the X-band [5]. As mentioned above the PM-FMR signals are superimposed with a field independent background (dashed line in Fig. 1), which is caused by photo carriers induced in the GaAs and is two orders of magnitude larger than the FMR signal. For a patterned Fe-film, the conventional FMRspectra can show multiple resonances due to the integral character of the signal and the non-uniform magnetic parameter of the film. The upper spectrum in Fig. 2iii shows such a conventional FMR absorption derivative of the structured Fe-film described above. The FMRsignals are deformed and clearly a superposition of several modes. To interpret the spectrum, one needs to investigate the local variations in the magnetic and electric properties with the PM-FMR. The two lower spectra in Fig. 2iii display the PMFMR absorption derivative versus the external magnetic
PM-FMR amplitude (mV)
84
1.0092
reflected microwave power 1.0086
1.0080
1.0074
1.0068 20
40
60
80
100
120
external magnetic field (mT) Fig. 1. PM-FMR signal amplitude versus the external magnetic field along the [1 1 0] orientation of a Fe-film on GaAs.
field of two different positions of the patterned Fe-film. In order to reduce the measuring time scanning the entire sample, the signal-to-noise ratio was not optimized to achieve the exact line shape. Nevertheless, one can see that the line position is clearly pronounced and the linewidth is smaller compared to the conventional signal. The middle spectrum corresponds to the sample area marked by a in Fig. 2i. In this area, the GaAssubstrate has no HEMT-structure and the Fe-film was etched to a mesa with a diameter of 60 mm. The lower spectrum was measured in position b on a rectangular 500 500 mm2 Fe-mesa with HEMT-structure underneath. Comparing the three spectra in Fig. 2iii it is obvious that the shape of the conventional FMR is a superposition of the signal of the Fe-film on pure GaAs and on GaAs with HEMT-structure. Based on this behaviour one can calculate the anisotropy parameters by angle dependent FMR-measurements. As a result the surface anisotropy of the Femesa deposited on the HEMT-structure (b) is decreased by a factor of 2 compared to the Fe-mesa directly on GaAs (a). The uniaxial in-plane anisotropy was slightly decreased by about 10% between positions b and a: Fig. 2ii shows a greyscale image of the locally resolved field independent microwave reflection signal as indicated in Fig. 1. The Fig. 2ii covers the same area as optical microscope image in Fig. 2i. Comparing the microwave reflection and the microscope image it is found that the sample was etched non-uniformly in the Fe-patterning step. The intensity of the microwave reflection signal increases with the degree of etching of the GaAs-surface, being 3 times higher at position A than at position B: The lowest background signal was found inside the HEMT-structure, seen as the dark vertical stripe on the right side of Fig. 2ii. The low intensity is explained by the lower number of photo carriers induced by the laser light due to the nonconducting layer of the HEMT-structure [2]. We gain more insight into the electronic contributions of the photo modulated microwave absorption and are
R. Meckenstock et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 83–85
85
reflected microwave power (W)
Fig. 2. (i) Optical microscope image of structured Fe surface. (ii) Greyscale image of the field independent PM-FMR-amplitude. (iii) Microwave response versus external magnetic field in the [1 1 0] direction. Spectra top to bottom: integral conventional FMR-spectrum; PM-FMR-spectrum of Fe-mesa on pure GaAs (position a in i and ii); PM-FMR-spectrum of Fe-mesa with HEMT below (position b in i and ii). photomodulated microwave absorption 10-2
10-3
10-4
10 6
Fe covered GaAs, C GaAs, B GaAs strongly etched, A modulation frequency (Hz)
HEMT, the signal decrease is shifted to a higher value (2.7 MHz). This is in agreement with the shorter lifetime of the photo carriers in the GaAs. At position A; the GaAs was strongly etched, removing the As-termination for the Fe-film preparation [3]. In this area, the signal minimum is shifted to even higher values (4.2 MHz), indicating a shorter lifetime of the photo carriers. Acknowledgements
5 10 6
Fig. 3. Field independent PM-FMR-amplitude versus modulation frequency. A, B, and C correspond to marked positions in Fig. 2ii.
able to demonstrate the high sensitivity of the microwave reflection by studying the signal as a function of modulation frequency. The frequency spectra of the three representative positions A; B; and C mentioned above are shown in Fig. 3. All spectra show decreases in the signal at characteristic frequencies. These decreases are governed not only by the lifetime of the photo carriers, but also by experimental parameters such as the microwave cavity quality factor and the sample size. Thus, a quantitative measure of the photo carrier lifetime is only possible with a calibrated set-up, which has not been possible yet. For the spectra corresponding to position C in Fig. 3 (HEMT-structure), one can see the signal decrease at about 1.5 MHz. In the HEMT-structure, we expect the longest lifetime of the excited photo carriers. In position B; representing the slightly etched GaAs-surface with no
The authors would like to acknowledge the financial support of the Deutsche Forschungsgemeinschaft (SFB 491). References [1] R. Meckenstock, et al., Magnetic properties of Fe/ZnSe and Fe/GaAs heterostructures investigated by ferromagnetic resonance and SQUID measurements, Proceedings of the MML 2001, Manuscript No. 527, J. Magn. Magn. Mater., to be published. [2] A.D. Wieck, D. Reuter, Institute of Physics Conference Series No. 166, 2000, p. 51. [3] M. Doi, et al., Magnetic and structural properties of epitaxial Fe thin films on selectively doped AlxGa1x/GaAs heterostructure, Proceedings of the MML 2001, J. Magn. Magn. Mater., to be published. [4] G.V. Skrotskii, L.V. Kurbatov, in: S.V. Vonsovskii (Ed.), Ferromagnetic Resonance, Pergamon, New York, 1966. [5] R. Meckenstock, et al., J. Appl. Phys. 77 (1995) 6439. [6] W. Kiepert, et al., Progress in Natural Science (Supplement to Vol. 6), Taylor & Francis, London and Washington, 1996, p. 515.