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Ferromagnetic resonance study of magnetic-shape-memory Ni2MnGa films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 2031–2032
Ferromagnetic resonance study of magnetic-shape-memory Ni2MnGa films M.D. Huanga, N.N. Leea, Y.H. Hyuna, J. Dubowikb, Y.P. Leea,* a
q-Psi and Department of Physics, Hanyang University, 17 Haengdang-Dong, Seongdong-Gu, Seoul 133-791, South Korea b Institute of Molecular Physics, Poznan, Poland
Abstract The magnetic properties of Ni2MnGa films, deposited onto glass and mica, were investigated by ferromagnetic resonance. The effects of temperature on resonance field and linewidth of the films were measured for both in-plane and out-of-plane configurations in the temperature range from 75 to 400 K. The resonance field gradually decreases as the applied field rotates from normal to parallel directions with respect to the sample surface. The results have been discussed and analyzed. r 2003 Elsevier B.V. All rights reserved. PACS: 61.18.Fs; 76.50.+g; 81.30.Kf Keywords: Ni2MnGa; Magnetic shape memory; Ferromagnetic resonance; Magnetic anisotropy; Hysteresis loop
Ni2MnGa is the only known Heusler alloy with the magnetic-shape-memory effect and thus has been investigated intensively. The ferromagnetic resonance (FMR) method, which can be used to measure the values of magnetization and all the parameters of magnetic anisotropy and also to study the dynamic process of magnetism and details of the magnetic structure, was applied to Ni2MnGa films in this work. In a vacuum better than 5 105 Pa, the films, with a thickness of about 160 nm, were prepared by flash evaporation onto glass and mica substrates, heated up to 720 K in order to get ordered structure. The resonance field and line width of the as-deposited films as a function of temperature from 75 to 400 K, covering from the temperature below the martensitic transformation to that above the Curie point, were studied by using an electron paramagnetic resonance (EPR) spectrometer of X-band at a microwave frequency of 9.08 GHz, and an external field ranging from 0 to 1.5 T. Angle-dependent resonance field was also measured at 78 and 293 K with an applied field rotating from normal to parallel orientations. *Corresponding author. Tel.: +82-2-2281-0916; fax: +82-22281-5573. E-mail address: [email protected] (Y.P. Lee).
The normalized hysteresis loops of Ni2MnGa films (not shown) at room temperature reveals that the magnetically easy axis lies in the film plane. Fig. 1 displays the temperature-dependent resonance field Hres of the films deposited onto glass and mica substrates for both in-plane and out-of-plane configurations. As can be seen, Hres increases with decreasing temperature in case of the out-of-plane configuration, except for a slightly anomalous decrease in 300–330 K. The fact that the Hres increases with decreasing temperature can be explained since the saturated magnetization increases as the temperature decreases [1]. On the other hand, inplane Hres decreases monotonously with decreasing temperature, which may be ascribed to the internal stresses in the thin films [2]. The temperature dependence of Hres manifests that the precession frequency of atomic magnetic moment changes with temperature and the applied field orientation, and the different values of Hres for the out-of-plane and in-plane configurations indicate the presence of magnetic anisotropy in the thin films [3]. The substrates have also some effects on Hres ; as shown in Fig. 1, which can be attributed to the different temperature dependence of the anisotropy field [1]. The effects of temperature and orientation on the resonance linewidth DH of Ni2MnGa films are displayed
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.809
ARTICLE IN PRESS M.D. Huang et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2031–2032
2032
8 6
6 4
out-of-plane glass out-of-plane mica in-plane mica in-plane glass
4
Resonance field (kOe)
Resonance field (kOe)
(a)
78 K 293 K
8
2
100
200
300
2
0
(b)
78 K 293 K
8
6
400
Temperature (K)
4
Fig. 1. Effect of temperature on resonance field in the out-ofplane and in-plane configurations for Ni2MnGa films deposited on glass and mica.
2
0 0
mica mica glass glass
Resonance linewidth (kOe)
3
1
0 100
150
200
250
300
350
400
20
30
40
50
60
70
80
90
Fig. 3. Angle-dependent Hres of Ni2MnGa films deposited onto (a) glass and (b) mica.
2
50
10
Angle from normal (Degree)
out-of-plane in-plane in-plane out-of-plane
450
Temperature (K)
Fig. 2. The same as Fig. 1 except for the resonance linewidth.
in Fig. 2. As can be seen, DH increases for both directions as temperature decreases. This is due to the motional-narrowing effect [4]. The linewidth for an actual lattice is DH ¼ ðDHÞ20 t [5], where ðDHÞ0 is the line width in the rigid lattice and t is the average time for the atom to remain in a site. It is well known that t is inversely proportional to temperature, therefore, t; and thus DH; increases with decreasing temperature. The effect of internal stress in the thin films also plays a role in the line broadening, showing a monotonic increase of DH when temperature increases [2]. An abnormal decrease, observed below 270 K in the case of out-ofplane configuration for the films on mica, suggests an inhomogeneous magnetic structure [6]. When the applied field rotates from normal to parallel with respect to the film plane, Hres gradually decreases, as shown in Fig. 3. These results confirm that the Hres is affected by temperature and substrates. The dependence
of Hres on angle also reflects the demagnetization factors [6], and the maximum at 0 indicates the easy axis existing in the plane [3], which is consistent with the magnetization loop results mentioned above. Based on the FMR study of Ni2MnGa films deposited onto glass and mica, we can draw the following conclusions. The easy axis of Ni2MnGa is in the plane of film. The values of Hres are not only affected by the temperature and substrates but also affected by the orientations of applied field. The resonance line width is verified to be dependent on temperature and substrate. This work was supported by the KOSEF through q-Psi, by a KRF Grant (2001-015-DS0015), and by MOST, Korea.
References [1] S.V. Lebedev, C.E. Patton, M.A. Wittenauer, J. Appl. Phys. 91 (2002) 4426. [2] C. Chappert, K.L. Dang, P. Beauvillain, Phys. Rev. B 34 (1986) 3192. [3] M. Farle, B. Mirwald-Schulz, A.N. Anisimov, W. Platow, K. Baberschke, Phys. Rev. B 55 (1997) 3708. [4] N. Bloembergen, E.M. Purcell, R.V. Pound, Phys. Rev. 73 (1948) 679. [5] C. Kittel, Introduction to Solid State Physics, 7th Edition, Wiley, New York, 1996, p. 495. [6] B.D. Shanina, et al., J. Magn. Magn. Mater. 237 (2001) 309.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 2031–2032
Ferromagnetic resonance study of magnetic-shape-memory Ni2MnGa films M.D. Huanga, N.N. Leea, Y.H. Hyuna, J. Dubowikb, Y.P. Leea,* a
q-Psi and Department of Physics, Hanyang University, 17 Haengdang-Dong, Seongdong-Gu, Seoul 133-791, South Korea b Institute of Molecular Physics, Poznan, Poland
Abstract The magnetic properties of Ni2MnGa films, deposited onto glass and mica, were investigated by ferromagnetic resonance. The effects of temperature on resonance field and linewidth of the films were measured for both in-plane and out-of-plane configurations in the temperature range from 75 to 400 K. The resonance field gradually decreases as the applied field rotates from normal to parallel directions with respect to the sample surface. The results have been discussed and analyzed. r 2003 Elsevier B.V. All rights reserved. PACS: 61.18.Fs; 76.50.+g; 81.30.Kf Keywords: Ni2MnGa; Magnetic shape memory; Ferromagnetic resonance; Magnetic anisotropy; Hysteresis loop
Ni2MnGa is the only known Heusler alloy with the magnetic-shape-memory effect and thus has been investigated intensively. The ferromagnetic resonance (FMR) method, which can be used to measure the values of magnetization and all the parameters of magnetic anisotropy and also to study the dynamic process of magnetism and details of the magnetic structure, was applied to Ni2MnGa films in this work. In a vacuum better than 5 105 Pa, the films, with a thickness of about 160 nm, were prepared by flash evaporation onto glass and mica substrates, heated up to 720 K in order to get ordered structure. The resonance field and line width of the as-deposited films as a function of temperature from 75 to 400 K, covering from the temperature below the martensitic transformation to that above the Curie point, were studied by using an electron paramagnetic resonance (EPR) spectrometer of X-band at a microwave frequency of 9.08 GHz, and an external field ranging from 0 to 1.5 T. Angle-dependent resonance field was also measured at 78 and 293 K with an applied field rotating from normal to parallel orientations. *Corresponding author. Tel.: +82-2-2281-0916; fax: +82-22281-5573. E-mail address: [email protected] (Y.P. Lee).
The normalized hysteresis loops of Ni2MnGa films (not shown) at room temperature reveals that the magnetically easy axis lies in the film plane. Fig. 1 displays the temperature-dependent resonance field Hres of the films deposited onto glass and mica substrates for both in-plane and out-of-plane configurations. As can be seen, Hres increases with decreasing temperature in case of the out-of-plane configuration, except for a slightly anomalous decrease in 300–330 K. The fact that the Hres increases with decreasing temperature can be explained since the saturated magnetization increases as the temperature decreases [1]. On the other hand, inplane Hres decreases monotonously with decreasing temperature, which may be ascribed to the internal stresses in the thin films [2]. The temperature dependence of Hres manifests that the precession frequency of atomic magnetic moment changes with temperature and the applied field orientation, and the different values of Hres for the out-of-plane and in-plane configurations indicate the presence of magnetic anisotropy in the thin films [3]. The substrates have also some effects on Hres ; as shown in Fig. 1, which can be attributed to the different temperature dependence of the anisotropy field [1]. The effects of temperature and orientation on the resonance linewidth DH of Ni2MnGa films are displayed
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.809
ARTICLE IN PRESS M.D. Huang et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2031–2032
2032
8 6
6 4
out-of-plane glass out-of-plane mica in-plane mica in-plane glass
4
Resonance field (kOe)
Resonance field (kOe)
(a)
78 K 293 K
8
2
100
200
300
2
0
(b)
78 K 293 K
8
6
400
Temperature (K)
4
Fig. 1. Effect of temperature on resonance field in the out-ofplane and in-plane configurations for Ni2MnGa films deposited on glass and mica.
2
0 0
mica mica glass glass
Resonance linewidth (kOe)
3
1
0 100
150
200
250
300
350
400
20
30
40
50
60
70
80
90
Fig. 3. Angle-dependent Hres of Ni2MnGa films deposited onto (a) glass and (b) mica.
2
50
10
Angle from normal (Degree)
out-of-plane in-plane in-plane out-of-plane
450
Temperature (K)
Fig. 2. The same as Fig. 1 except for the resonance linewidth.
in Fig. 2. As can be seen, DH increases for both directions as temperature decreases. This is due to the motional-narrowing effect [4]. The linewidth for an actual lattice is DH ¼ ðDHÞ20 t [5], where ðDHÞ0 is the line width in the rigid lattice and t is the average time for the atom to remain in a site. It is well known that t is inversely proportional to temperature, therefore, t; and thus DH; increases with decreasing temperature. The effect of internal stress in the thin films also plays a role in the line broadening, showing a monotonic increase of DH when temperature increases [2]. An abnormal decrease, observed below 270 K in the case of out-ofplane configuration for the films on mica, suggests an inhomogeneous magnetic structure [6]. When the applied field rotates from normal to parallel with respect to the film plane, Hres gradually decreases, as shown in Fig. 3. These results confirm that the Hres is affected by temperature and substrates. The dependence
of Hres on angle also reflects the demagnetization factors [6], and the maximum at 0 indicates the easy axis existing in the plane [3], which is consistent with the magnetization loop results mentioned above. Based on the FMR study of Ni2MnGa films deposited onto glass and mica, we can draw the following conclusions. The easy axis of Ni2MnGa is in the plane of film. The values of Hres are not only affected by the temperature and substrates but also affected by the orientations of applied field. The resonance line width is verified to be dependent on temperature and substrate. This work was supported by the KOSEF through q-Psi, by a KRF Grant (2001-015-DS0015), and by MOST, Korea.
References [1] S.V. Lebedev, C.E. Patton, M.A. Wittenauer, J. Appl. Phys. 91 (2002) 4426. [2] C. Chappert, K.L. Dang, P. Beauvillain, Phys. Rev. B 34 (1986) 3192. [3] M. Farle, B. Mirwald-Schulz, A.N. Anisimov, W. Platow, K. Baberschke, Phys. Rev. B 55 (1997) 3708. [4] N. Bloembergen, E.M. Purcell, R.V. Pound, Phys. Rev. 73 (1948) 679. [5] C. Kittel, Introduction to Solid State Physics, 7th Edition, Wiley, New York, 1996, p. 495. [6] B.D. Shanina, et al., J. Magn. Magn. Mater. 237 (2001) 309.