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Composition, structure and magnetic properties of sputter deposited Ni–Mn–Ga ferromagnetic shape memory thin films
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 630–634
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Composition, structure and magnetic properties of sputter deposited Ni–Mn–Ga ferromagnetic shape memory thin films A. Annadurai a, A.K. Nandakumar a, S. Jayakumar a, M.D. Kannan a, M. Manivel Raja b, S. Bysak b, R. Gopalan b,, V. Chandrasekaran b a b
Thin Film Center, Department of Physics, PSG College of Technology, Coimbatore 641004, India Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, Andhra Pradesh 500 058, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 10 June 2008 Received in revised form 9 September 2008 Available online 1 November 2008
Polycrystalline Ni–Mn–Ga thin films were deposited by the d.c. magnetron sputtering on well-cleaned substrates of Si(10 0) and glass at a constant sputtering power of 36 W. We report the influence of sputtering pressure on the composition, structure and magnetic properties of the sputtered thin films. These films display ferromagnetic behaviour only after annealing at an elevated temperature and a maximum saturation magnetization of 335 emu/cc was obtained for the films investigated. Evolution of martensitic microstructure was observed in the annealed thin films with the increase of sputtering pressure. The thermo-magnetic curves exhibited only magnetic transition in the temperature range of 339–374 K. The thin film deposited at high sputtering pressure of 0.025 mbar was found to be ordered L21 austenitic phase. & 2008 Elsevier B.V. All rights reserved.
Keywords: Ferromagnetic shape memory Ni–Mn–Ga thin film Sputtering and magnetization
1. Introduction Ferromagnetic shape memory alloys (FSMA) are interesting class of novel smart/multifunctional materials which can be strained magnetically. Such strain is called magnetic field induced strain (MFIS) or magnetostrain, which was first reported by Ullakko et al. in 1996 [1–3]. Among the known FSMA materials systems viz. Fe–Pt, Fe–Pd, Ni–Mn–Ga, Co–Ni–Al and Co–Ni–Ga [4,5], the Ni–Mn–Ga system has gained considerable interest among researchers due to its capability of producing large magnetostrains of about 10% [2,6–10]. The MFIS produced in this alloy system is about 50 times greater than the best performing conventional piezoelectric and magnetostrictive actuator materials [11,12]. Another advantage of this material system is its remote control operation and high-frequency response of the order of 1 kHz [2,8,13]. In addition to this, the Ni–Mn–Ga material system also possesses other technologically interesting properties viz. giant magneto caloric and large negative magneto-resistance properties [14,15], which can be exploited for magnetic refrigeration and other attractive applications in magnetic field measurement systems. In spite of unique and multifunctional properties of bulk Ni–Mn–Ga material, its high brittleness is a major drawback which limits its effective utilization in realizing potential practical applications [6,7], as the bending of the material causes fracture failure. Ni–Mn–Ga thin films exhibit superior ductility compared to the bulk materials and amenable to bending [16,17] and hence Corresponding author. Tel./fax: +91 40 2434 0884.
E-mail address: [email protected] (R. Gopalan). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.10.015
they are potential candidates in the development of micro and nano electro-mechanical systems [2,18]. Most of the research work on Ni–Mn–Ga has dealt with the study of the bulk material and only a few groups have studied the preparation and properties of Ni–Mn–Ga thin films [19–24]. Ni–Mn–Ga ferromagnetic martensitic thin films posses a great potential in the form of both free-standing structures and films attached to a substrate [25]. These films have already been successfully implemented in the prototypes of microvalves and microscanners [26] and it is also reported that the thin film magnetic-shape memory materials open up totally new possibilities for potential applications for which the extensive basic research is still needed [27,28]. The fundamental requisites for a material to produce large MFIS are (i) ferromagnetic martensitic structure and (ii) high magneto-crystalline anisotropy [9]. For a material to be used for ferromagnetic shape memory applications, two primary functional properties needed are, ferromagnetic and shape memory behaviors, which strongly depend on the structure of Ni–Mn–Ga alloys, but they are very sensitive to their composition and preparation techniques [29]. To produce thin films with these required functional properties, the deposition parameters such as sputtering pressure, sputtering power, etc., have also to be carefully optimized and hence the present work. 2. Experimental details The Ni–Mn–Ga thin films were d.c. sputter deposited on wellcleaned substrates of Si(1 0 0) and Glass in argon atmosphere at four different sputtering pressures of 0.005, 0.01, 0.015 and
ARTICLE IN PRESS A. Annadurai et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 630–634
0.025 mbar. A constant sputtering power of 36 W was used for all the depositions. As-deposited films were post-heat-treated at 600 1C for 1 h in vacuum followed by gas quenching to room temperature. The dimensions of the target were 200 diameter and 0.5 mm thick. The targets used in the present study were wire cut from the bulk Ni–Mn–Ga alloy prepared in our laboratory using vacuum induction melting technique. The composition of the target was Ni50Mn30Ga20. The average thickness of the thin films was measured using a thickness profilometer. The film thickness ranged from 1000 to 1200 A˚. The composition of the thin films was studied by electron probe micro analysis (EPMA) and energy dispersive X-ray analysis (EDS) using transmission electron microscope (TEM). The crystal structure of the films was investigated by X-ray diffraction (XRD) technique. The bright field image and selected area diffraction pattern of the films deposited was also carried out using TEM. A detailed investigation of the magnetic properties of the films viz. M–H loops for both in and out of plane of the thin films and the temperature-dependent magnetization M(T) studies were carried out using a vibration sample magnetometer (VSM) attached with the variable temperature cryostat. The M–H loops were measured up to a maximum magnetizing field of 10 kOe at room temperature, whereas M(T) curves were carried out at a constant magnetic field of 1000 Oe for a temperature range of 150–450 K.
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amorphous nature while the post-deposition heat-treated (annealed) samples show well-defined peaks, revealing the crystalline nature of the films. Fig. 1 shows the XRD patterns obtained for post-heat-treated films deposited on glass substrates at different sputtering pressures. The observation reveals that the films posses different degrees of mixed phases of martensite and austenite. The XRD patterns of the thin films reveal the evolution of the martensitic phase in thin films as the deposition pressure is increased form 0.005 to 0.025 mbar. The films deposited at high sputtering pressure show the primary peaks obtained are of single phase L21 austenite structure. The presence of super lattice peaks (111), (2 0 0) and (3 11) suggests that the film is ordered. Formation of the austenite phase at higher pressures may be due to the shifting of the martensitic transformation towards low temperature due to the increase in the Ga content. In order to understand the crystal structure of the as-deposited films, TEM study was carried out on one of the films processed at a pressure of 0.015 mbar. The typical TEM bright field image and the selected area diffraction pattern are shown in Fig. 2. The cross-sectional TEM image displays a columnar type structure of the sputtered film. Analysis of selected area diffraction pattern revealed that the as-deposited film is composed of amorphous and crystalline phases. The crystalline phase is not of L21-type phase as expected, but it is of L12-type Ni3(Mn,Ga) phase. In order to estimate
3. Results and discussion 3.1. Structural analysis of the films The XRD patterns were recorded for both as-deposited and post-heat-treated thin film samples. As-deposited films show broad and halo diffraction peaks which are due to the quasi-
Fig. 1. X-ray diffraction pattern of annealed Ni–Mn–Ga thin films deposited on glass substrate at different sputtering gas (argon) pressures. Notations A(h k l) and M(h k l) stand for Austenite and Martensite phases, respectively.
Fig. 2. Cross-sectional transmission electron micrograph showing (a) bright field image and (b) selected area diffraction pattern of as grown Ni–Mn–Ga/Si film (0.015 mbar).
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the composition of the films we have done detailed TEM–EDS analysis. 3.2. Compositional analysis
atomic collisions due to the presence of magnetic field in magnetron sputtering system around the target by Lorenz force. Hence the atomic magnetic dipoles of Ni and Mn experience a torque as well as retardation force when they travel through such an inhomogeneous magnetic field and they might be pushed away from directly falling onto the substrate. The magnetic retardation forces do not influence the non-magnetic particles like Ga. Thus, a few of the ejected magnetic atomic particles of Ni and Mn from the target may move oblique due to magnetic retardation and they may not reach the substrate which is attributed to the reduction in the atomic percentage of such constituents in the films with the increase of sputtering pressure. At higher pressure, the increase in atomic scattering phenomenon also enhances the oblique motion of the magnetic particles.
Fig. 3 shows the TEM-energy dispersive X-ray spectrum (EDS) of deposited Ni–Mn–Ga thin films (P ¼ 0.01 mbar) which confirms the presence of constituent atomic elements of the target in the thin films. In Table 1, the composition of the thin films for four different sets deposited at different sputtering pressures is given. Fig. 4 depicts the variation of different atomic constituents/ composition of the thin films with respect to sputtering pressure. The observation shows that the atomic percentage of the elements Ni and Mn decreases with the increasing sputtering pressure whereas that of the Ga increases. This may be attributed to the magnetic (Ni and Mn) and non-magnetic (Ga) nature of the elements. The magnetic dipole moment of Ni atom is 2 mB and that of Mn atom is 5 mB [17]. When these atoms are sputtered and ejected from the target by the impingement of energetic argon ions, the number of atoms ejected from the target depends on the sputtering yield of the constituents of the target. The ejected atoms from the target in the form of neutral atoms and charged particles must pass through the non-uniform/ inhomogeneous magnetic field present around the target to reach the substrate. The atomic particles having net magnetic moment experience a magnetic retardation force in addition to occurrence of normal
M–H hysteresis loops in-plane and out-of-plane of the film were systematically carried out for all the annealed thin film samples. The M–H loops obtained for thin films deposited on Si(1 0 0) and glass substrates at different sputtering pressures are shown in Figs. 5 and 6, respectively. The shape of the loops for the films deposited on Si and glass appear similar and also the magnetization curves saturate almost at same applied fields. It reveals that the sputtered films were randomly oriented polycrystalline and no significant texture developed during the deposition. The analyses of the curves showed that the annealed
Fig. 3. Typical TEM-EDS of Ni–Mn–Ga thin film (0.01 mbar) displaying characteristic peaks of constituent elements.
Fig. 4. Sputtering pressure vs. composition of the Ni–Mn–Ga thin films.
3.3. Magnetic properties of the films
Table 1 Magnetic properties of magnetron sputtered Ni–Mn–Ga thin films deposited at different sputtering pressure (sputtering power ¼ 36 W). Film ID
Film composition (at%)
Substrate
Sputtering pressure (mbar)
Thickness (A˚)
HC (Oe)
Ms (emu/cc)
Magnetic transition Tc (K)
NMG1
Ni55.5Mn26.5Ga18 Ni55Mn24.5Ga20.5
NMG3
Ni50.3Mn23Ga26.7
NMG4
Ni49.2Mn22.9Ga27.9
0.005 0.005 0.010 0.010 0.015 0.015 0.025 0.025
1250
NMG2
Si (1 0 0) Glass Si (1 0 0) Glass Si (1 0 0) Glass Si (1 0 0) Glass
86 84 91 65 90 50 97 86
240 257 289 280 208 205 320 335
360 352 347 339 – – 374 370
1170 1020 1150
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Fig. 5. M–H hysteresis loops of annealed Ni–Mn–Ga thin films deposited on silicon substrate at different sputtering pressures.
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Fig. 7. Typical M–H loops of Ni–Mn–Ga/Si thin film (0.01 mbar) measured in-plane and out-of-plane of the film.
Fig. 8. Thermo-magnetic curves of annealed Ni–Mn–Ga thin films deposited on silicon substrate at different sputtering pressures. Fig. 6. M–H loops of annealed Ni–Mn–Ga thin films deposited on glass substrate at different sputtering pressures.
films exhibit good soft magnetic properties characterized by narrow hysteresis loops, low coercivity and high-magnetic saturation values. The measured saturation magnetization (Ms) and coercivity (Hc) values of the films deposited on silicon and glass substrates are given in Table 1. The magnetization value varies from 200 to 335 emu/cc and this variation could be correlated with the change in film composition deposited at different pressures. A maximum Ms value of 335 emu/cc was observed for the film sputtered at P ¼ 0.025 mbar and this could be attributed to low Mn content compared to the other films. The
excess Mn content in Ni2MnGa will reduce the net magnetic moment. The coercivity of the films varies from 60 to 90 Oe which is of the same order of that of bulk alloys [30]. Fig. 7 shows the typical in-plane and out-of-plane M–H loops of the films deposited on Si substrate (P ¼ 0.01 mbar). The films do not display any in-plane anisotropy and the out-of-plane loops are characteristics of hard axis component, suggesting an in-plane easy axis of magnetization. The temperature-dependent magnetization measurements were also carried out for a temperature range of 150–450 K for annealed thin films deposited both on silicon as well as glass substrates to identify the structural (martensitic to austenite) and
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deposition annealing. The thermo-magnetic curves of the annealed films reflect only the magnetic transition.
Acknowledgements The financial support for the present study was extended by the Defence Research and Development Organization (DRDO), Government of India. The keen interest shown by the Director, DMRL in this work is gratefully acknowledged. The contribution of the PSG college of Technology in this work is also gratefully acknowledged. References
Fig. 9. Thermo-magnetic curves of annealed Ni–Mn–Ga thin films deposited on glass substrate at different sputtering pressures.
magnetic transition (Curie) temperatures. The thermo-magnetic curves for all the films deposited on silicon substrate are shown in Fig. 8 and that of the films deposited on glass are shown in Fig. 9. These curves provide the signatures of structural transition (TM) by showing a sudden rise in magnetization and magnetic transition (TC) by showing a drop in magnetization upon heating. But the observation of thermo-magnetic curves reveals that they reflect only magnetic transition and no signature of structural transition was noticed. The Curie temperature of the films varies from 339 to 374 K. However, the film deposited at 0.015 mbar does not show any transition even up to 450 K. Even though, the ferromagnetic to paramagnetic Curie transition was clearly observed in the films, the absence of martensitic transition in the thermo-magnetic curves may be due to the small change in the magnetization value during the occurrence of martensitic transition in the films and needs further studies to understand.
4. Conclusions In conclusion, the Ni–Mn–Ga thin films deposited at lowsputtering pressures (0.005–0.015 mbar), after annealing at 600 1C exhibited mixed phases of both austenite and martensite. On the other hand, highly ordered L21 austenite phase was formed in the films while deposited at higher sputtering pressure (0.025 mbar). It is clear that the sputtering pressure influences the composition and the evolution of martensitic structure in thin films. The annealed thin films show good soft magnetic properties and ferromagnetic behavior only after they are subjected to post-
[1] K. Ullakko, J.K. Huang, C. Kantner, R.C. O’Handley, V.V. Kokorin, Appl. Phys. Lett. 69 (1996) 1966. [2] O. So¨derberg, Y. Ge, A. Sozinov, S.P. Hannula, V.K. Lindroos, Smart. Mater. Struct. 14 (2005) 223. [3] O. Heczko, J. Magn. Magn. Mater. 290 (2005) 787. [4] A. Vassiliev, J. Magn. Magn. Mater. 242 (2002) 66. [5] P.J. Brown, A.P. Gandy, K. Ishida, R. Kainuma, T. Kanomata, M. Matsumoto, H. Morito, K.U. Neumann, K. Oikawa, B. Ouladdiaf, K.R.A. Ziebeck, J. Magn. Magn. Mater. 310 (2007) 2755. [6] S. Besseghini, E. Villa, F. Passaretti, M. Pini, F. Bonfanti, Mater. Sci. Eng. A 378 (2004) 415. [7] N.V. Rama Rao, R. Gopalan, M. Manivel Raja, J. Arout Chelvane, B. Majumdar, V. Chandrasekaran, Scr. Mater. 56 (2007) 405. [8] M.A. Marioni, R.C. O’Handley, S.M. Allen, S.R. Hall, D.I. Paul, M.L. Richard, J. Feuchtwanger, B.W. Peterson, J.M. Chambers, R. Techapiesancharoenkij, J. Magn. Magn. Mater. 290 (2005) 35. [9] H. Rumpf, C.M. Craciunescu, H. Modrow, Kh. Olimov, E. Quandt, M. Wuttig, J. Magn. Magn. Mater. 302 (2006) 421. [10] S. Banik, R. Ranjan, A. Chakrabarti, S. Bhardwaj, N.P. Lalla, A.M. Awasthi, V. Sathe, D.M. Phase, P.K. Mukhopadhyay, D. Pandey, S.R. Barman, Phys. Rev. B 75 (2007) 104107. [11] F. Xiong, Y. Liu, E. Pagounis, J. Magn. Magn. Mater. 285 (2005) 410. [12] G. Pirge, C.V. Hyatt, S. Altintas, J. Mater. Process. Technol. 155 (2004) 1266. [13] C. Jiang, J. Liu, J. Wang, L. Xu, H. Xu, Acta Mater. 53 (2005) 1111. [14] D. Borisenko, V.V. Koledov, V.V. Khovalio, V.G. Shavrov, J. Magn. Magn. Mater. 300 (2006) 486. [15] C. Biswas, S.R. Barman, Appl. Surf. Sci. 252 (2006) 3380. [16] M. Kohl, D. Brugger, M. Ohtsuka, T. Takagi, Sensors Actuators A 114 (2004) 445. [17] M. Ohtsuka, M. Matsumoto, K. Itagaki, Mater. Sci. Eng. A 438 (2006) 935. [18] M. Ohtsuka, M. Sanada, M. Matsumoto, K. Itagaki, Mater. Sci. Eng. A 378 (2004) 377. [19] J.P. Ahn, N. Cheng, T. Lograsso, K.M. Krishnan, IEEE Trans. Magn. 37 (2001) 2141. [20] J.W. Dong, L.C. Chen, C.J. Palmstrom, R.D. James, S. McKernan, Appl. Phys. Lett. 75 (1999) 1443. [21] J.W. Dong, J.O. Xie, J. Lu, C. Adelmann, C.J. Palmstrom, J. Cui, Q. Pan, T.W. Shield, R.D. James, S. McKernan, J. Appl. Phys. 95 (2004) 2593. [22] M. Wuttig, C. Craciunescu, J. Li, Mater. Trans. JIM 41 (2000) 933. [23] S.K. Wu, K.H. Tseng, J.Y. Wang, Thin Solid Films 408 (2002) 316. [24] F.J. Castano, B.N. Cheeseman, R.C. O’Handley, C.A. Ross, C. Redondo, F. Castano, J. Appl. Phys. 93 (2003) 8492. [25] M.V.A. Chernenko, M. Khol, M. Ohtsuka, T. Takagi, V.A. L’vov, V.M. Kniazkyi, Mater. Sci. Eng. A 438 (2006) 944. [26] M. Kohl, A. Agarwal, V.A. Chernenko, M. Ohtsuka, K. Seemann, Mater. Sci. Eng. 438 (2006) 940. [27] O. Soderberg, I. Aaltio, Y. Ge, O. Heczko, S.-P. Hannula, Mater. Sci. Eng. A (2007). [28] V.A. Chernenko, S. Besseghini, M. Hagler, P. Mullner, M. Ohtsuka, F. Stortiero, Mater. Sci. Eng. A (2007). [29] A. Vovk, L. Malkinski, V. Golub, C. O’Connor, J. Appl. Phys. 97 (10C) (2005) 503. [30] I. Babita, R. Gopalan, M. Manivel Raja, V. Chandrasekaran, S. Ram, J. Appl. Phys. 102 (2007) 013906.
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Composition, structure and magnetic properties of sputter deposited Ni–Mn–Ga ferromagnetic shape memory thin films A. Annadurai a, A.K. Nandakumar a, S. Jayakumar a, M.D. Kannan a, M. Manivel Raja b, S. Bysak b, R. Gopalan b,, V. Chandrasekaran b a b
Thin Film Center, Department of Physics, PSG College of Technology, Coimbatore 641004, India Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, Andhra Pradesh 500 058, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 10 June 2008 Received in revised form 9 September 2008 Available online 1 November 2008
Polycrystalline Ni–Mn–Ga thin films were deposited by the d.c. magnetron sputtering on well-cleaned substrates of Si(10 0) and glass at a constant sputtering power of 36 W. We report the influence of sputtering pressure on the composition, structure and magnetic properties of the sputtered thin films. These films display ferromagnetic behaviour only after annealing at an elevated temperature and a maximum saturation magnetization of 335 emu/cc was obtained for the films investigated. Evolution of martensitic microstructure was observed in the annealed thin films with the increase of sputtering pressure. The thermo-magnetic curves exhibited only magnetic transition in the temperature range of 339–374 K. The thin film deposited at high sputtering pressure of 0.025 mbar was found to be ordered L21 austenitic phase. & 2008 Elsevier B.V. All rights reserved.
Keywords: Ferromagnetic shape memory Ni–Mn–Ga thin film Sputtering and magnetization
1. Introduction Ferromagnetic shape memory alloys (FSMA) are interesting class of novel smart/multifunctional materials which can be strained magnetically. Such strain is called magnetic field induced strain (MFIS) or magnetostrain, which was first reported by Ullakko et al. in 1996 [1–3]. Among the known FSMA materials systems viz. Fe–Pt, Fe–Pd, Ni–Mn–Ga, Co–Ni–Al and Co–Ni–Ga [4,5], the Ni–Mn–Ga system has gained considerable interest among researchers due to its capability of producing large magnetostrains of about 10% [2,6–10]. The MFIS produced in this alloy system is about 50 times greater than the best performing conventional piezoelectric and magnetostrictive actuator materials [11,12]. Another advantage of this material system is its remote control operation and high-frequency response of the order of 1 kHz [2,8,13]. In addition to this, the Ni–Mn–Ga material system also possesses other technologically interesting properties viz. giant magneto caloric and large negative magneto-resistance properties [14,15], which can be exploited for magnetic refrigeration and other attractive applications in magnetic field measurement systems. In spite of unique and multifunctional properties of bulk Ni–Mn–Ga material, its high brittleness is a major drawback which limits its effective utilization in realizing potential practical applications [6,7], as the bending of the material causes fracture failure. Ni–Mn–Ga thin films exhibit superior ductility compared to the bulk materials and amenable to bending [16,17] and hence Corresponding author. Tel./fax: +91 40 2434 0884.
E-mail address: [email protected] (R. Gopalan). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.10.015
they are potential candidates in the development of micro and nano electro-mechanical systems [2,18]. Most of the research work on Ni–Mn–Ga has dealt with the study of the bulk material and only a few groups have studied the preparation and properties of Ni–Mn–Ga thin films [19–24]. Ni–Mn–Ga ferromagnetic martensitic thin films posses a great potential in the form of both free-standing structures and films attached to a substrate [25]. These films have already been successfully implemented in the prototypes of microvalves and microscanners [26] and it is also reported that the thin film magnetic-shape memory materials open up totally new possibilities for potential applications for which the extensive basic research is still needed [27,28]. The fundamental requisites for a material to produce large MFIS are (i) ferromagnetic martensitic structure and (ii) high magneto-crystalline anisotropy [9]. For a material to be used for ferromagnetic shape memory applications, two primary functional properties needed are, ferromagnetic and shape memory behaviors, which strongly depend on the structure of Ni–Mn–Ga alloys, but they are very sensitive to their composition and preparation techniques [29]. To produce thin films with these required functional properties, the deposition parameters such as sputtering pressure, sputtering power, etc., have also to be carefully optimized and hence the present work. 2. Experimental details The Ni–Mn–Ga thin films were d.c. sputter deposited on wellcleaned substrates of Si(1 0 0) and Glass in argon atmosphere at four different sputtering pressures of 0.005, 0.01, 0.015 and
ARTICLE IN PRESS A. Annadurai et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 630–634
0.025 mbar. A constant sputtering power of 36 W was used for all the depositions. As-deposited films were post-heat-treated at 600 1C for 1 h in vacuum followed by gas quenching to room temperature. The dimensions of the target were 200 diameter and 0.5 mm thick. The targets used in the present study were wire cut from the bulk Ni–Mn–Ga alloy prepared in our laboratory using vacuum induction melting technique. The composition of the target was Ni50Mn30Ga20. The average thickness of the thin films was measured using a thickness profilometer. The film thickness ranged from 1000 to 1200 A˚. The composition of the thin films was studied by electron probe micro analysis (EPMA) and energy dispersive X-ray analysis (EDS) using transmission electron microscope (TEM). The crystal structure of the films was investigated by X-ray diffraction (XRD) technique. The bright field image and selected area diffraction pattern of the films deposited was also carried out using TEM. A detailed investigation of the magnetic properties of the films viz. M–H loops for both in and out of plane of the thin films and the temperature-dependent magnetization M(T) studies were carried out using a vibration sample magnetometer (VSM) attached with the variable temperature cryostat. The M–H loops were measured up to a maximum magnetizing field of 10 kOe at room temperature, whereas M(T) curves were carried out at a constant magnetic field of 1000 Oe for a temperature range of 150–450 K.
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amorphous nature while the post-deposition heat-treated (annealed) samples show well-defined peaks, revealing the crystalline nature of the films. Fig. 1 shows the XRD patterns obtained for post-heat-treated films deposited on glass substrates at different sputtering pressures. The observation reveals that the films posses different degrees of mixed phases of martensite and austenite. The XRD patterns of the thin films reveal the evolution of the martensitic phase in thin films as the deposition pressure is increased form 0.005 to 0.025 mbar. The films deposited at high sputtering pressure show the primary peaks obtained are of single phase L21 austenite structure. The presence of super lattice peaks (111), (2 0 0) and (3 11) suggests that the film is ordered. Formation of the austenite phase at higher pressures may be due to the shifting of the martensitic transformation towards low temperature due to the increase in the Ga content. In order to understand the crystal structure of the as-deposited films, TEM study was carried out on one of the films processed at a pressure of 0.015 mbar. The typical TEM bright field image and the selected area diffraction pattern are shown in Fig. 2. The cross-sectional TEM image displays a columnar type structure of the sputtered film. Analysis of selected area diffraction pattern revealed that the as-deposited film is composed of amorphous and crystalline phases. The crystalline phase is not of L21-type phase as expected, but it is of L12-type Ni3(Mn,Ga) phase. In order to estimate
3. Results and discussion 3.1. Structural analysis of the films The XRD patterns were recorded for both as-deposited and post-heat-treated thin film samples. As-deposited films show broad and halo diffraction peaks which are due to the quasi-
Fig. 1. X-ray diffraction pattern of annealed Ni–Mn–Ga thin films deposited on glass substrate at different sputtering gas (argon) pressures. Notations A(h k l) and M(h k l) stand for Austenite and Martensite phases, respectively.
Fig. 2. Cross-sectional transmission electron micrograph showing (a) bright field image and (b) selected area diffraction pattern of as grown Ni–Mn–Ga/Si film (0.015 mbar).
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the composition of the films we have done detailed TEM–EDS analysis. 3.2. Compositional analysis
atomic collisions due to the presence of magnetic field in magnetron sputtering system around the target by Lorenz force. Hence the atomic magnetic dipoles of Ni and Mn experience a torque as well as retardation force when they travel through such an inhomogeneous magnetic field and they might be pushed away from directly falling onto the substrate. The magnetic retardation forces do not influence the non-magnetic particles like Ga. Thus, a few of the ejected magnetic atomic particles of Ni and Mn from the target may move oblique due to magnetic retardation and they may not reach the substrate which is attributed to the reduction in the atomic percentage of such constituents in the films with the increase of sputtering pressure. At higher pressure, the increase in atomic scattering phenomenon also enhances the oblique motion of the magnetic particles.
Fig. 3 shows the TEM-energy dispersive X-ray spectrum (EDS) of deposited Ni–Mn–Ga thin films (P ¼ 0.01 mbar) which confirms the presence of constituent atomic elements of the target in the thin films. In Table 1, the composition of the thin films for four different sets deposited at different sputtering pressures is given. Fig. 4 depicts the variation of different atomic constituents/ composition of the thin films with respect to sputtering pressure. The observation shows that the atomic percentage of the elements Ni and Mn decreases with the increasing sputtering pressure whereas that of the Ga increases. This may be attributed to the magnetic (Ni and Mn) and non-magnetic (Ga) nature of the elements. The magnetic dipole moment of Ni atom is 2 mB and that of Mn atom is 5 mB [17]. When these atoms are sputtered and ejected from the target by the impingement of energetic argon ions, the number of atoms ejected from the target depends on the sputtering yield of the constituents of the target. The ejected atoms from the target in the form of neutral atoms and charged particles must pass through the non-uniform/ inhomogeneous magnetic field present around the target to reach the substrate. The atomic particles having net magnetic moment experience a magnetic retardation force in addition to occurrence of normal
M–H hysteresis loops in-plane and out-of-plane of the film were systematically carried out for all the annealed thin film samples. The M–H loops obtained for thin films deposited on Si(1 0 0) and glass substrates at different sputtering pressures are shown in Figs. 5 and 6, respectively. The shape of the loops for the films deposited on Si and glass appear similar and also the magnetization curves saturate almost at same applied fields. It reveals that the sputtered films were randomly oriented polycrystalline and no significant texture developed during the deposition. The analyses of the curves showed that the annealed
Fig. 3. Typical TEM-EDS of Ni–Mn–Ga thin film (0.01 mbar) displaying characteristic peaks of constituent elements.
Fig. 4. Sputtering pressure vs. composition of the Ni–Mn–Ga thin films.
3.3. Magnetic properties of the films
Table 1 Magnetic properties of magnetron sputtered Ni–Mn–Ga thin films deposited at different sputtering pressure (sputtering power ¼ 36 W). Film ID
Film composition (at%)
Substrate
Sputtering pressure (mbar)
Thickness (A˚)
HC (Oe)
Ms (emu/cc)
Magnetic transition Tc (K)
NMG1
Ni55.5Mn26.5Ga18 Ni55Mn24.5Ga20.5
NMG3
Ni50.3Mn23Ga26.7
NMG4
Ni49.2Mn22.9Ga27.9
0.005 0.005 0.010 0.010 0.015 0.015 0.025 0.025
1250
NMG2
Si (1 0 0) Glass Si (1 0 0) Glass Si (1 0 0) Glass Si (1 0 0) Glass
86 84 91 65 90 50 97 86
240 257 289 280 208 205 320 335
360 352 347 339 – – 374 370
1170 1020 1150
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Fig. 5. M–H hysteresis loops of annealed Ni–Mn–Ga thin films deposited on silicon substrate at different sputtering pressures.
633
Fig. 7. Typical M–H loops of Ni–Mn–Ga/Si thin film (0.01 mbar) measured in-plane and out-of-plane of the film.
Fig. 8. Thermo-magnetic curves of annealed Ni–Mn–Ga thin films deposited on silicon substrate at different sputtering pressures. Fig. 6. M–H loops of annealed Ni–Mn–Ga thin films deposited on glass substrate at different sputtering pressures.
films exhibit good soft magnetic properties characterized by narrow hysteresis loops, low coercivity and high-magnetic saturation values. The measured saturation magnetization (Ms) and coercivity (Hc) values of the films deposited on silicon and glass substrates are given in Table 1. The magnetization value varies from 200 to 335 emu/cc and this variation could be correlated with the change in film composition deposited at different pressures. A maximum Ms value of 335 emu/cc was observed for the film sputtered at P ¼ 0.025 mbar and this could be attributed to low Mn content compared to the other films. The
excess Mn content in Ni2MnGa will reduce the net magnetic moment. The coercivity of the films varies from 60 to 90 Oe which is of the same order of that of bulk alloys [30]. Fig. 7 shows the typical in-plane and out-of-plane M–H loops of the films deposited on Si substrate (P ¼ 0.01 mbar). The films do not display any in-plane anisotropy and the out-of-plane loops are characteristics of hard axis component, suggesting an in-plane easy axis of magnetization. The temperature-dependent magnetization measurements were also carried out for a temperature range of 150–450 K for annealed thin films deposited both on silicon as well as glass substrates to identify the structural (martensitic to austenite) and
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deposition annealing. The thermo-magnetic curves of the annealed films reflect only the magnetic transition.
Acknowledgements The financial support for the present study was extended by the Defence Research and Development Organization (DRDO), Government of India. The keen interest shown by the Director, DMRL in this work is gratefully acknowledged. The contribution of the PSG college of Technology in this work is also gratefully acknowledged. References
Fig. 9. Thermo-magnetic curves of annealed Ni–Mn–Ga thin films deposited on glass substrate at different sputtering pressures.
magnetic transition (Curie) temperatures. The thermo-magnetic curves for all the films deposited on silicon substrate are shown in Fig. 8 and that of the films deposited on glass are shown in Fig. 9. These curves provide the signatures of structural transition (TM) by showing a sudden rise in magnetization and magnetic transition (TC) by showing a drop in magnetization upon heating. But the observation of thermo-magnetic curves reveals that they reflect only magnetic transition and no signature of structural transition was noticed. The Curie temperature of the films varies from 339 to 374 K. However, the film deposited at 0.015 mbar does not show any transition even up to 450 K. Even though, the ferromagnetic to paramagnetic Curie transition was clearly observed in the films, the absence of martensitic transition in the thermo-magnetic curves may be due to the small change in the magnetization value during the occurrence of martensitic transition in the films and needs further studies to understand.
4. Conclusions In conclusion, the Ni–Mn–Ga thin films deposited at lowsputtering pressures (0.005–0.015 mbar), after annealing at 600 1C exhibited mixed phases of both austenite and martensite. On the other hand, highly ordered L21 austenite phase was formed in the films while deposited at higher sputtering pressure (0.025 mbar). It is clear that the sputtering pressure influences the composition and the evolution of martensitic structure in thin films. The annealed thin films show good soft magnetic properties and ferromagnetic behavior only after they are subjected to post-
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