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Ferromagnetic resonance in Co2MnGa films with various structural ordering
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 2271–2273 www.elsevier.com/locate/jmmm
Ferromagnetic resonance in Co2MnGa films with various structural ordering Y.V. Kudryavtseva, V.A. Oksenenkoa, V.A. Kulagina, J. Dubowikb, Y.P. Leec, a
Institute of Metal Physics, NAS of Ukraine, 36 Vernadsky Street, 03142, Kiev, Ukraine Institute of Molecular Physics, PAS, M. Smoluchowskiego 17, 60-179 Poznan´, Poland c q-psi and Department of Physics, Hanyang University, 17 Haengdang-dong, Seoul 133-791, Republic of Korea b
Available online 15 November 2006
Abstract Ferromagnetic resonance has been studied in the flash-evaporated Co2MnGa Heusler alloy (HA) films with different structural order. Depending on annealing temperature, varied from 293 to 728 K, the films are amorphous, fine crystalline with A2 (or B2) type of structure or ordered with L21 structure. The amorphous films are weakly magnetic and inhomogeneous with the saturation magnetization MS25 G at room temperature (RT) and with the magnetic regions having the effective magnetization MeffE200–300 G. The L21-ordered films are ferromagnetic with the Curie temperature of 650 K and MS (RT)E450 GE2.5 mB f.u. (lower than the bulk value of 4 mB) and Meff (RT) ¼ 500–550 G. The temperatures of ordering from the amorphous to A2 (B2) structure and from A2 (B2) to L21 structure are estimated to 430 and 550 K, respectively. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.i; 76.50.+g; 78.20.Ls; 78.66.Bz Keywords: Heusler alloy film; Structural transformation; Magnetic property
Recently, Pechan et al. [1] reported on ferromagnetic resonance (FMR) investigations of the single-crystal Co2MnGa Heusler alloy (HA) films, which are supposed to reveal nearly half-metallic behavior [2]. We present the results of FMR measurements in Co2MnGa films with different crystalline ordering. Co2MnGa alloy films of about 100 nm in thickness were prepared by flash evaporation of the crushed Co0.494Mn0.249Ga0.257 alloy simultaneously onto glass, mica and NaCl substrates in a vacuum better than 2 105 Pa. To obtain the Co2MnGa alloy films with utmost disorder, they were vapor quenched onto the substrates cooled by liquid nitrogen. The asdeposited Co2MnGa films were annealed at 293 (RT), 513, 598 and 728 K, respectively, in high vacuum conditions. The resulting different structural states of Co2MnGa HA films will be referred to as 1, 2, 3 and 4, corresponding to the above annealing conditions.
Corresponding author. Tel.: +82 2 2282 5572; fax: +82 2 2282 5573.
E-mail address: [email protected] (Y.P. Lee). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.726
The magnetic properties (the effective magnetization Meff and the saturation magnetization MS) of Co2MnGa alloy films were investigated by using FMR at 9.3 GHz at RT and at 9.08 GHz in the temperature range from 200 to 550 K as well as by measuring the magnetization hysteresis loops with a vibrating sample magnetometer. Meff was determined from the fits of the experimental out-of-plane angular dependencies of the resonance magnetic field by using the standard Kittel equations for the thin magnetic films. Possible inhomogeneous magnetic states of our films were obtained by analyzing the FMR spectra taken at the perpendicular configuration (i.e., for the magnetic field applied perpendicular to the film plane). Fig. 1 (left panel) shows transmission electron microscopy (TEM) diffraction pattern of the Co2MnGa film deposited onto a NaCl substrate cooled by liquid nitrogen. It is clearly seen that vapor quenching deposition onto cooled substrates leads to formation of an amorphous (nanocrystalline) state. After annealing at 630 K, the amorphous film becomes crystalline with the diffraction lines representing B2 HA structure (Fig. 1,
ARTICLE IN PRESS 2272
Y.V. Kudryavtsev et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2271–2273
Fig. 1. TEM diffraction patterns of the Co2MnGa alloy films deposited onto a NaCl substrate cooled by liquid nitrogen substrate (left panel) and post annealed at 630 K (right panel).
Fig. 3. Changes of FMR resonance field Hperp in the course of heating r (full and open circles) and cooling (full triangles). The numbers in circles denote corresponding films.
Fig. 2. The saturation (MS, full symbols) and effective (Meff, open symbols) magnetization of Co2MnGa films deposited onto glass (circles) and mica (triangles) as a function of annealing temperature. The numbers in circles denote corresponding films (see text).
right panel). A thorough X-ray diffraction characterization of the films annealed at T4600 K suggests even the presence of L21 ordering for film 4. Annealing of the vapor quenched Co2MnGa films results in the formation of the amorphous, A2, B2 or L21 ordering for the films 1–4, respectively. Fig. 2 shows a dependence of RT values of MS (full symbols) and Meff (open symbols) on annealing temperature corresponding to the films 1–4, respectively. The amorphous film 1 is only weakly magnetic with MSE25 G. However, the presence of several absorption peaks in FMR spectrum of the film indicates its rather inhomogeneous magnetic (and hence, nanocrystalline) structure with Meff of 240–300 G, estimated from the
position of the most intensive FMR absorption. Because Msat represents volume magnetization and Meff in this case indicates just a local one, a nearly 10 times difference between them can be explained by the formation of highly magnetized clusters in weakly magnetic matrix of amorphous film. Annealing at elevated temperatures results in a substantial increase in both MS and Meff up to 400 and 500 G, respectively. Since MS of the films 3 and 4 is practically the same, we argue that there is practically no difference in the magnetic properties of the Co2MnGa films with B2 and L21 type of ordering. However, the saturation magnetization MS (MSE 450 G–2.5 mB f.u.) of our films with the highest ordering is still lower than that of the bulk (4.05 mB) [2] and Co2MnGa epitaxial films (3.5 mB) [1]. Moreover, MS and Meff differ substantially which may be related also to a high easy plane anisotropy of Co2MnGa films due to magnetostriction [1]. Using the high-temperature FMR measurements in the perpendicular configuration, we checked a temperature range in which the amorphous Co2MnGa films are ordered. Fig. 3 shows the temperature changes of the resonance field Hperp in the course r of temperature cycling from RT to 570 K and back to RT. In a range of 420–430 K, the resonance field increases sharply, then it decreases in a way typical of a ferromagnetic film with TC of 600 K and eventually at 550 K, it grows again. In the cooling cycle, the temperature changes of the resonance field Hperp are r typical of a well ordered Co2MnGa alloy with TC of 650 K. These anomalies in the behavior of Hperp vs. r T strongly suggest the presence of two structural transformations in amorphous Co2MnGa at 430 and 550 K, respectively. The dramatic changes in resistivity upon annealing of amorphous films observed at the same temperatures also support this conclusion. Comparing
ARTICLE IN PRESS Y.V. Kudryavtsev et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2271–2273
FMR results shown in Fig. 3 with the TEM patterns, we assume that these anomalies are related to the ‘‘amorphous -A2(B2)’’and ‘‘A2(B2)-L21’’ transformations, respectively.
2273
045 25 and by KOSEF through Quantum Photonic Science Research Center.
References This work was supported in part by the project 55/05-H of the National program of Ukraine ‘‘Nanostructural systems, nanomaterials and nanotechnologies’’ and by the Polish Committee for Scientific Research—Grant 4 T08C
[1] M.J. Pechan, C. Yu, D. Carr, C.J. Palmstrøm, J. Magn. Magn. Mater. 286 (2005) 340. [2] V. Niculescu, T.J. Burch, K. Raj, J.I. Budnick, J. Magn. Magn. Mater. 5 (1977) 60.
Journal of Magnetism and Magnetic Materials 310 (2007) 2271–2273 www.elsevier.com/locate/jmmm
Ferromagnetic resonance in Co2MnGa films with various structural ordering Y.V. Kudryavtseva, V.A. Oksenenkoa, V.A. Kulagina, J. Dubowikb, Y.P. Leec, a
Institute of Metal Physics, NAS of Ukraine, 36 Vernadsky Street, 03142, Kiev, Ukraine Institute of Molecular Physics, PAS, M. Smoluchowskiego 17, 60-179 Poznan´, Poland c q-psi and Department of Physics, Hanyang University, 17 Haengdang-dong, Seoul 133-791, Republic of Korea b
Available online 15 November 2006
Abstract Ferromagnetic resonance has been studied in the flash-evaporated Co2MnGa Heusler alloy (HA) films with different structural order. Depending on annealing temperature, varied from 293 to 728 K, the films are amorphous, fine crystalline with A2 (or B2) type of structure or ordered with L21 structure. The amorphous films are weakly magnetic and inhomogeneous with the saturation magnetization MS25 G at room temperature (RT) and with the magnetic regions having the effective magnetization MeffE200–300 G. The L21-ordered films are ferromagnetic with the Curie temperature of 650 K and MS (RT)E450 GE2.5 mB f.u. (lower than the bulk value of 4 mB) and Meff (RT) ¼ 500–550 G. The temperatures of ordering from the amorphous to A2 (B2) structure and from A2 (B2) to L21 structure are estimated to 430 and 550 K, respectively. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.i; 76.50.+g; 78.20.Ls; 78.66.Bz Keywords: Heusler alloy film; Structural transformation; Magnetic property
Recently, Pechan et al. [1] reported on ferromagnetic resonance (FMR) investigations of the single-crystal Co2MnGa Heusler alloy (HA) films, which are supposed to reveal nearly half-metallic behavior [2]. We present the results of FMR measurements in Co2MnGa films with different crystalline ordering. Co2MnGa alloy films of about 100 nm in thickness were prepared by flash evaporation of the crushed Co0.494Mn0.249Ga0.257 alloy simultaneously onto glass, mica and NaCl substrates in a vacuum better than 2 105 Pa. To obtain the Co2MnGa alloy films with utmost disorder, they were vapor quenched onto the substrates cooled by liquid nitrogen. The asdeposited Co2MnGa films were annealed at 293 (RT), 513, 598 and 728 K, respectively, in high vacuum conditions. The resulting different structural states of Co2MnGa HA films will be referred to as 1, 2, 3 and 4, corresponding to the above annealing conditions.
Corresponding author. Tel.: +82 2 2282 5572; fax: +82 2 2282 5573.
E-mail address: [email protected] (Y.P. Lee). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.726
The magnetic properties (the effective magnetization Meff and the saturation magnetization MS) of Co2MnGa alloy films were investigated by using FMR at 9.3 GHz at RT and at 9.08 GHz in the temperature range from 200 to 550 K as well as by measuring the magnetization hysteresis loops with a vibrating sample magnetometer. Meff was determined from the fits of the experimental out-of-plane angular dependencies of the resonance magnetic field by using the standard Kittel equations for the thin magnetic films. Possible inhomogeneous magnetic states of our films were obtained by analyzing the FMR spectra taken at the perpendicular configuration (i.e., for the magnetic field applied perpendicular to the film plane). Fig. 1 (left panel) shows transmission electron microscopy (TEM) diffraction pattern of the Co2MnGa film deposited onto a NaCl substrate cooled by liquid nitrogen. It is clearly seen that vapor quenching deposition onto cooled substrates leads to formation of an amorphous (nanocrystalline) state. After annealing at 630 K, the amorphous film becomes crystalline with the diffraction lines representing B2 HA structure (Fig. 1,
ARTICLE IN PRESS 2272
Y.V. Kudryavtsev et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2271–2273
Fig. 1. TEM diffraction patterns of the Co2MnGa alloy films deposited onto a NaCl substrate cooled by liquid nitrogen substrate (left panel) and post annealed at 630 K (right panel).
Fig. 3. Changes of FMR resonance field Hperp in the course of heating r (full and open circles) and cooling (full triangles). The numbers in circles denote corresponding films.
Fig. 2. The saturation (MS, full symbols) and effective (Meff, open symbols) magnetization of Co2MnGa films deposited onto glass (circles) and mica (triangles) as a function of annealing temperature. The numbers in circles denote corresponding films (see text).
right panel). A thorough X-ray diffraction characterization of the films annealed at T4600 K suggests even the presence of L21 ordering for film 4. Annealing of the vapor quenched Co2MnGa films results in the formation of the amorphous, A2, B2 or L21 ordering for the films 1–4, respectively. Fig. 2 shows a dependence of RT values of MS (full symbols) and Meff (open symbols) on annealing temperature corresponding to the films 1–4, respectively. The amorphous film 1 is only weakly magnetic with MSE25 G. However, the presence of several absorption peaks in FMR spectrum of the film indicates its rather inhomogeneous magnetic (and hence, nanocrystalline) structure with Meff of 240–300 G, estimated from the
position of the most intensive FMR absorption. Because Msat represents volume magnetization and Meff in this case indicates just a local one, a nearly 10 times difference between them can be explained by the formation of highly magnetized clusters in weakly magnetic matrix of amorphous film. Annealing at elevated temperatures results in a substantial increase in both MS and Meff up to 400 and 500 G, respectively. Since MS of the films 3 and 4 is practically the same, we argue that there is practically no difference in the magnetic properties of the Co2MnGa films with B2 and L21 type of ordering. However, the saturation magnetization MS (MSE 450 G–2.5 mB f.u.) of our films with the highest ordering is still lower than that of the bulk (4.05 mB) [2] and Co2MnGa epitaxial films (3.5 mB) [1]. Moreover, MS and Meff differ substantially which may be related also to a high easy plane anisotropy of Co2MnGa films due to magnetostriction [1]. Using the high-temperature FMR measurements in the perpendicular configuration, we checked a temperature range in which the amorphous Co2MnGa films are ordered. Fig. 3 shows the temperature changes of the resonance field Hperp in the course r of temperature cycling from RT to 570 K and back to RT. In a range of 420–430 K, the resonance field increases sharply, then it decreases in a way typical of a ferromagnetic film with TC of 600 K and eventually at 550 K, it grows again. In the cooling cycle, the temperature changes of the resonance field Hperp are r typical of a well ordered Co2MnGa alloy with TC of 650 K. These anomalies in the behavior of Hperp vs. r T strongly suggest the presence of two structural transformations in amorphous Co2MnGa at 430 and 550 K, respectively. The dramatic changes in resistivity upon annealing of amorphous films observed at the same temperatures also support this conclusion. Comparing
ARTICLE IN PRESS Y.V. Kudryavtsev et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2271–2273
FMR results shown in Fig. 3 with the TEM patterns, we assume that these anomalies are related to the ‘‘amorphous -A2(B2)’’and ‘‘A2(B2)-L21’’ transformations, respectively.
2273
045 25 and by KOSEF through Quantum Photonic Science Research Center.
References This work was supported in part by the project 55/05-H of the National program of Ukraine ‘‘Nanostructural systems, nanomaterials and nanotechnologies’’ and by the Polish Committee for Scientific Research—Grant 4 T08C
[1] M.J. Pechan, C. Yu, D. Carr, C.J. Palmstrøm, J. Magn. Magn. Mater. 286 (2005) 340. [2] V. Niculescu, T.J. Burch, K. Raj, J.I. Budnick, J. Magn. Magn. Mater. 5 (1977) 60.