An electron spin resonance study

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corded on both heating and cooling using an a.c. susceptom- eter in the temperature range .... 11J. Dubowik, I. Gościańska, Y. V. Kudryavtsev, Y. P. Lee, P. Sovák, and M. Konĉ, Phys. ... Coelho, and A. M. Mansanares, Phys. Rev. B 72, 224435 ...
Coupled magnetostructural transformations in melt-spun Ni55Mn19.6Ga25.4 ribbon: An electron spin resonance study N. V. Rama Rao, R. Gopalan, J. Arout Chelvane, V. Chandrasekaran, and K. G. Suresh Citation: J. Appl. Phys. 105, 123904 (2009); doi: 10.1063/1.3148863 View online: http://dx.doi.org/10.1063/1.3148863 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v105/i12 Published by the American Institute of Physics.

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JOURNAL OF APPLIED PHYSICS 105, 123904 共2009兲

Coupled magnetostructural transformations in melt-spun Ni55Mn19.6Ga25.4 ribbon: An electron spin resonance study N. V. Rama Rao,1,2 R. Gopalan,1,a兲 J. Arout Chelvane,1 V. Chandrasekaran,1 and K. G. Suresh2,b兲 1

Defence Metallurgical Research Laboratory, Hyderabad 500 058, India Department of Physics, Indian Institute of Technology Bombay, Mumbai 400 076, India

2

共Received 27 February 2009; accepted 10 May 2009; published online 17 June 2009兲 Electron spin resonance study has been carried out on melt-spun ribbon of Ni55Mn19.6Ga25.4 exhibiting coupled magnetostructural transition. The correlation of electron spin resonance, thermal and magnetic results permitted a clear distinction of various phases and their transformations. Both structural and magnetic transitions coexist in the temperature range 300ⱕ T ⱕ 310 leading to four different magnetic phases namely paramagnetic austenite, ferromagnetic austenite, paramagnetic martensite, and ferromagnetic martensite. The sample exhibits a single paramagnetic austenite phase above 310 K while it shows a ferromagnetic martensite phase below 260 K. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3148863兴 I. INTRODUCTION

Ferromagnetic Heusler alloys Ni– Mn– X 共X = Ga, Sn, In, and Sb兲 form an important class of materials exhibiting ferromagnetic shape memory effect, giant magnetocaloric 共GMC兲 effect, giant magnetoresistance, and superelasticity.1–6 The multifunctional properties of Heusler alloys have a wide range of applications in actuators, magnetic refrigerators, magnetomechanical transducers, switching devices, etc. In particular, Ni–Mn–Ga alloy generated immense interest because of large magnetic field induced strain as well as field induced entropy change. Heusler alloys undergo a first order martensitic transition from a cubic 共austenite兲 phase to a tetragonal/orthorhombic 共martensitic兲 phase upon cooling. The martensitic transformation temperature 共Tm兲 and the Curie temperature 共TC兲 are composition dependent. Tm and TC have been found to coexist in Ni2+xMn1−xGa alloys over 0.18ⱕ x ⱕ 0.27, and this coupled magnetostructural transformation leads to large GMC effect.7 The coupled magnetostructural transformations have been characterized by magnetic, structural, and thermal measurements.8,9 Although these measurements give an idea about the transformation temperatures, a clear understanding of the magnetic nature 共ferro-/para-兲 of the phases 共austenite/ martensite兲 around the magnetostructural transformation is still elusive. Electron spin resonance 共ESR兲/ferromagnetic resonance 共FMR兲 is particularly a suitable technique for such a study, because it reveals the charge state, the site symmetry, the g value, the internal magnetic fields and their distribution, the interactions among the ions and the lattice, the magnetic ordering, and the relaxation processes, etc. However, there are only a few reports on the temperature and angular dependence of FMR in Ni–Mn–Ga alloys either in single crystal or thin film form.10–12 a兲

Present address: National Institute for Materials Science, 1-2-1 Sengen, Tsukuba- 305 0047, Japan. b兲 Author to whom correspondence should be addressed. Electronic mail: [email protected]. 0021-8979/2009/105共12兲/123904/4/$25.00

In the present investigation, we have selected melt-spun ribbon of Ni55Mn19.6Ga25.4 exhibiting coupled magnetostructural transformations and carried out magnetic, thermal, and ESR measurements as a function of temperature. The assignment of ESR/FMR lines with majority phases and the associated magnetic and structural transformations are discussed in detail. II. EXPERIMENTAL DETAILS

The precursor ingot was prepared by arc-melting the starting elements 共99.99% purity兲 under argon atmosphere. Subsequently, the ingot was induction melted in a quartz tube and melt-spun in vacuum, at a typical wheel surface speed of 17 m/s. The ribbons were annealed under high vacuum at 1075 K for 3 h and then quenched in water. Microstructure and elemental compositions were investigated by using scanning electron microscopy 共Leo 440i兲 attached to an x-ray energy dispersive spectroscopy 共EDS兲 setup. The crystal structure was identified by x-ray diffraction 共XRD兲. Temperature dependence of a.c. magnetization 共333 Hz兲 was recorded on both heating and cooling using an a.c. susceptometer in the temperature range of 77–330 K at an applied field of 0.5 Oe. The phase transformation temperatures between the martensite and austenite phases were determined by differential scanning calorimetry 共DSC兲 共TA Q100兲 at a cooling/heating rate of 20 K/min. FMR/ESR measurements were carried out on a commercial VARIAN E-15 ESR spectrometer operating at 9.3 GHz at temperatures between 140– 380 K. During this measurement the magnetic field was applied along the plane of the ribbon. III. RESULTS AND DISCUSSION

The room temperature XRD and microstructure 共figure not shown兲 of the heat treated ribbon indicate that the sample is formed in single phase with orthorhombic structure. EDS analysis revealed a highly homogeneous chemical composition without any microchemical segregation and the uniform composition was found to be Ni55Mn19.6Ga25.4. Figure 1共a兲

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© 2009 American Institute of Physics

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FIG. 2. ESR/FMR spectra at different temperatures for Ni55Mn19.6Ga25.4 melt-spun ribbon. FIG. 1. 共Color online兲 共a兲 DSC curves and 共b兲 a.c. magnetic susceptibility during cooling and heating cycles for Ni55Mn19.6Ga25.4 melt-spun ribbon.

shows the DSC curve for Ni55Mn19.6Ga25.4 ribbon during heating and cooling cycles. The phase transformation temperatures, namely, martensite start 共M s兲, martensite finish 共M f 兲, austenite start 共As兲, and austenite finish 共A f 兲 temperatures are found to be 306, 297, 300, and 312 K respectively. The phase transformation and magnetic transition temperatures determined from the a.c. magnetic susceptibility 共␹-T兲

measurements are shown in Fig. 1共b兲. It can be seen from ␹-T curve that the heating curve does not coincide with the cooling curve but exhibits a narrow thermal hysteresis which is characteristic of a first order structural transformation. It is also evident from Fig. 1共b兲 that ferromagnetic to paramagnetic transition nearly coincides with the martensitic transition. The fact that the values As = 300 K, A f = 312 K 共obtained from DSC兲, and TC = 311 K indicates that the magnetic transition takes place just before the completion of

FIG. 3. 共Color online兲 Typical ESR/ FMR spectra with corresponding fit 共solid line兲 at different temperature regimes for Ni55Mn19.6Ga25.4 melt-spun ribbon.

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structural transition 共A f 兲. The coexistence of the austenite and martensite phases at magnetic transition leads to complex magnetic behavior in the ribbon. In order to distinctly identify the magnetic nature of these phases and the related transformations an ESR study was carried out. Figure 2 shows a sequence of ESR spectra in the temperature range of 140–380 K. The temperature dependence of line shape can be separated in four regions viz., 共i兲 at high temperatures 共T ⬎ 310 K兲—one resonance with Dysonian line shape, 共ii兲 in the intermediate temperature region 共300 ⱕ T ⱕ 310兲—four distinct resonances with Lorentzian line shape, 共iii兲 in the region of intermediate temperature 共260 ⱕ T ⬍ 300兲—two distinct resonances with Lorentzian line shape, and 共iv兲 at low temperatures 共T ⬍ 260兲—one resonance with Lorentzian line shape. Figure 3 shows a typical spectrum in these temperature regions. The variation of resonance fields 共Hres兲 as function of temperature is displayed in Fig. 4共a兲. In the high temperature regime 共T ⬎ 310 K兲 a single resonance line with Dysonian line shape is observed. The weak temperature dependence of Hres and the Dysonian line shape indicate a typical paramag-

neticlike behavior.13 Corroborating the above results with thermal and magnetization studies, the observed Hres in the high temperature regime corresponds to paramagnetic austenite 共PA兲 phase. In the intermediate temperature range 共300ⱕ T ⱕ 310兲, four distinct resonances with Lorentzian line shape is observed. In this temperature regime, both structural and magnetic transitions coexist, leading to different magnetic phases such as PA, ferromagnetic austenite 共FM兲, paramagnetic martensite 共PM兲, and ferromagnetic martensite 共FM兲. Around the Curie temperature the ferromagnetic correlations starts with a mixture of coexisting ferro-/paramagnetic phases. As a result, the coexisting austenite and martensite phases would contain ferromagnetic and paramagnetic components, leading to four different resonance lines. The Hres around 680 kA/m correspond to austenite ferromagnetic phase while the value around 635 kA/m corresponds to austenite paramagnetic phase. Similarly the Hres around 342 and 208 kA/m correspond to martensite ferromagnetic and martensite paramagnetic phases, respectively. It is reported that the magnetization for martensite phase is higher than that of austenite phase,14 and hence the lower Hres value is assigned to the martensite phase. The above prediction is based on the resonance condition considering the demagnetizing field effects, for example, in the form of a plate or ribbon with magnetic field applied in its plane.15 In the temperature regime of 260ⱕ T ⬍ 300 K, the mixed FM and austenite phases give rise to two distinct resonance lines. The increase in resonance field with decreasing temperature is the characteristic of a ferromagnetic behavior. Although the DSC and ␹-T measurements show austenite start temperature at As = 300 K, the FMR signal reveals the presence of austenite phase up to 260 K. This shows that the microscopic variation in the spin environment determined by local probelike FMR is more sensitive than bulk techniques such as a.c. magnetization and DSC. In the low temperature regime 共T ⬍ 260 K兲, the presence of single resonance is attributed to the FM phase. The variation of the resonance field with temperature indicates a typical ferromagnetic metal. From the above analysis, it is clear that the Ni55Mn19.6Ga25.4 ribbon exhibits a transition from FM to PA phase through a complex coexisting phase region of FM and PM. Figure 4共b兲 shows the temperature dependence of resonance linewidths 共⌬H兲. The ⌬H of the single phase martensitic region decreases gradually with increasing temperature up to 260 K, following the saturation magnetization behavior 关inset of Fig. 4共b兲兴. Similarly in the paramagnetic region the ESR linewidths exhibit a weak temperature dependence corroborating the magnetization results. However, in the intermediate temperature region a complex behavior is observed owing to the presence of both structural and magnetic inhomogeneities. IV. CONCLUSIONS

FIG. 4. 共Color online兲 共a兲 Variation of resonance fields 共Hres兲 and 共b兲 linewidths 共⌬H兲 as function of temperature for Ni55Mn19.6Ga25.4 melt-spun ribbon. Saturation magnetization as function of temperature is displayed in inset 共b兲.

In conclusion, coupled magnetostructural transformation in melt-spun ribbon of Ni55Mn19.6Ga25.4 was studied through ESR/FMR and corroborated with the thermal and magnetic data. The ESR/FMR results show a clear distinction of mag-

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netic and structural inhomogeneities around the transformation. The variation in Hres and ⌬H with temperature gives evidence for these inhomogeneities. The present study, therefore, confirms that ESR/FMR as local probe is more sensitive than the bulk magnetic and thermal measurements. ACKNOWLEDGMENTS

This work was supported by Defense Research and Development Organization 共DRDO兲, India. The keen interest shown by the Director, DMRL in this work is gratefully acknowledged. The authors would like to thank Dr. B. Majumdar for his help in carrying out melt spinning experiments. 1

K. Ullakko, J. K. Huang, C. Kanter, V. V. Kokorin, and R. C. O’Handley, Appl. Phys. Lett. 69, 1966 共1996兲. F. X. Hu, B. G. Shen, and J. R. Sun, Appl. Phys. Lett. 76, 3460 共2000兲. 3 T. Krenke, E. Duman, M. Acet, E. F. Wassermann, X. Moya, L. Mañosa, and A. Planes, Nature Mater. 4, 450 共2005兲. 4 K. Oikawa, W. Ito, Y. Imano, Y. Sutou, R. Kainuma, K. Ishida, S. Okamoto, O. Kitakami, and T. Kanomata, Appl. Phys. Lett. 88, 122507 共2006兲. 2

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Rama Rao et al.

M. Khan, N. Ali, and S. Stadler, J. Appl. Phys. 101, 053919 共2007兲. C. Biswas, R. Rawat, and S. R. Barman, Appl. Phys. Lett. 86, 202508 共2005兲. 7 A. N. Vasil’ev, A. D. Bozhko, I. E. Dikshtein, V. G. Shavrov, V. D. Buchel’nikov, M. Matsumoto, S. Suzuki, T. Takagi, and J. Tani, Phys. Rev. B 59, 1113 共1999兲. 8 V. V. Khovailo, T. Takagi, A. D. Bozhko, M. Matsumoto, J. Tani, and V. G. Shavrov, J. Phys.: Condens. Matter 13, 9655 共2001兲. 9 V. V. Khovaylo, V. D. Buchel’nikov, R. Kainuma, V. V. Koledov, M. Ohtsuka, V. G. Shavrov, T. Takagi, S. V. Taskaev, and A. N. Vasil’ev, Phys. Rev. B 72, 224408 共2005兲. 10 V. G. Gavriljuk, A. Dobrinsky, B. D. Shanina, and S. P. Kolesnik, J. Phys.: Condens. Matter 18, 7613 共2006兲. 11 J. Dubowik, I. Gościańska, Y. V. Kudryavtsev, Y. P. Lee, P. Sovák, and M. Konĉ, Phys. Status Solidi C 3, 143 共2006兲. 12 S. I. Patil, D. Tan, S. E. Lofland, S. M. Bhagat, I. Takeuchi, O. Famodu, J. C. Read, K.-S. Chang, C. Craciunescu, and M. Wuttig, Appl. Phys. Lett. 81, 1279 共2002兲. 13 M. J. M. Pires, A. Magnus, G. Carvalho, S. Gama, E. C. Da Silva, A. Coelho, and A. M. Mansanares, Phys. Rev. B 72, 224435 共2005兲. 14 V. V. Khovailo, V. Novosad, T. Takagi, D. A. Filippov, R. Z. Levitin, and A. N. Vasil’ev, Phys. Rev. B 70, 174413 共2004兲. 15 C. Kittel, Introduction to Solid State Physics, 5th ed. 共Wiley, New York, 1979兲. 5 6

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