JOURNAL OF APPLIED PHYSICS 102, 013906(2007) Magnetostructural transformation, microstructure, and magnetocaloric effect in Ni-Mn-Ga heusler alloys Babita Ingale Defence Metallurgical Research Laboratory, Hyderabad-500 058, India and Materials Science Centre Indian Institute of Technology, Kharagpur -721 302, India R. Gopalan, a)M. Manivel Raja, and V Chandrasekaran Defence Metallurgical Research laboratory, Hyderabad-500 058, India S Ram Materials Science Centre, Indian Institute of Technology, Kharagpur/ 302, India (Received 9 May 2007; accepted 21 May 2007: published online 5 July 2007 Magnetostructural transformation and the associated magnetic entropy change were investigated in Ni-rich ferromagnetic Heusler alloys. a direct transformation from the ferromagnetic martensite phase to the paramagnetic austenite phase was observed in selected Nis4.8 Mn2o3 Ga249 and Niss Mn,.1 two-alloy compositions. The magnetic and martensitic transformations were incurred at nearly the same temperature(351 K)in the Nis4.8Mn203Ga249 alloy. Such a typical composition involves a change of the magnetic entropy ASM as large as.0 J/kg K at 332 K in an applied magnetic field of 1.2 TO 2007 American Institute of Physics [DO:10.1063/1.2751489] . INTRODUCTION ported in detail by Albertini et al. The origin of the MC effect in the vicinity of the martensitic transition temperature Recently, interest in magnetocaloric (MC)technology in Ni-Mn-Ga alloys has been studied as a function of com- has grown significantly due to the development of new mag- position, which is expressed through the average number of etic materials such as Gd-Si-Ge, Mn-Fe-P-As, Ni-Mn-Ga, valence electron per atom (ela) and La-Fe-Si that exhibit large MC effect near room It is useful to tune the Ni-Mn-Ga alloy composition and temperature.-yIn the above series of MC materials, the fer- microstructure such that the two transitions coappear at tem- romagnetic Ni2 Ga Heusler alloys have attracted consider- perature as high as possible. Altering the stoichiometry of able interest for the application of magnetic Ni, Ga Heusler alloys toward the Ni-rich values uic entro py change ASy as Niso+ Mn 2s-xGa2s, x=0, 2, 3, and 5, the modified Ty and Tc large as-20.7 J/kg K at 333 K under 1. 8 T magnetic field transitions can shift at nearly the same temperature has been reported by Chernechukin et al. in Ni-Mn-Ga al-(2300 K). A typical Niss Mn2oGays composition met loys. It is ascribed to the coincidence of the martensite trans- such TM and Tc concurrence, with a small value ASM formation temperature TM and Curie temperature Tc in a =-0.68 J/kg K. In this work, an attempt is made to coupled magnetostructural transformation. A similar mecha- improve the properties further by tuning the Mn and Ga val nism is reported in Gds Si, Ge, alloys. -3The ASy value ues in this base alloy. The evolution of microstructural fea- (20.7 J/kg K) in Ni-Mn-Ga alloys is larger than that tures, magnetostructural transformation, and magnetic prop. (18.5 J/kg K) in Gds Si2 Gez alloys under common condi- erties is presented in this article tions near room temperature. The Ni-Mn-Ga alloys have other advantages over GdsSi, Ge, and other MC materials such as MnAs,_Sb, 13, I5 perovskite manganese oxides,,or unds. They are biocompatible, EXPERIMENTAL DETAILS easier to synthesize or fabricate in specific shape(ductile comparison to the oxides), and less expensive, especially in A Synthesis of the alloys comparison to the rare-earth or As(highly toxic) containing Three alloys, Nis4. Mn203 Ga24.9, Niss Mn18 Ga26 1, and compound Niss.2Mn18 1 Ga26., were prepared by vacuum arc melting of In a broad composition range, the Ni-Mn-Ga alloys un- ergo a martensitic transformation from a cubic(austenite)to copper crucible under high-purity argon atmosphere.The al a tetragonal/orthorhombic (martensite) phase upon cooling. loys recovered after each batch melting were reverted and The magnetostructural transformation is sensitive to the remelted(four times)to ensure homogeneous chemical dis composition.The compositional dependence of magnetic persion and alloying. The arc melted buttons were annealed and structural transformations in the martensitic at 1123 K for 2 days in vacuum followed by water quench Ni2+ Mn Gai+ (r+y+z=0) Heusler alloys has been re ing. Their chemical compositions were determined using an inductively coupled plasma-atomic emission spectroscopy Correspondingauthorelectronicmailrg_gopy(@yahoo.com (ICP-AES). Electron probe microanalysis(EPMA)was used 0021-8979/2007/102(1/013906/5/s23 102,013906-1 e 2007 American Institute of PhysicMagnetostructural transformation, microstructure, and magnetocaloric effect in Ni-Mn-Ga Heusler alloys Babita Ingale Defence Metallurgical Research Laboratory, Hyderabad – 500 058, India and Materials Science Centre, Indian Institute of Technology, Kharagpur – 721 302, India R. Gopalan,a
M. Manivel Raja, and V. Chandrasekaran Defence Metallurgical Research Laboratory, Hyderabad – 500 058, India S. Ram Materials Science Centre, Indian Institute of Technology, Kharagpur – 721 302, India
Received 9 May 2007; accepted 21 May 2007; published online 5 July 2007 Magnetostructural transformation and the associated magnetic entropy change were investigated in Ni-rich ferromagnetic Heusler alloys. A direct transformation from the ferromagnetic martensite phase to the paramagnetic austenite phase was observed in selected Ni54.8Mn20.3Ga24.9 and Ni55Mn18.9Ga26.1 two-alloy compositions. The magnetic and martensitic transformations were incurred at nearly the same temperature
351 K in the Ni54.8Mn20.3Ga24.9 alloy. Such a typical composition involves a change of the magnetic entropy
SM as large as −7.0 J/kg K at 332 K in an applied magnetic field of 1.2 T. © 2007 American Institute of Physics. DOI: 10.1063/1.2751489 I. INTRODUCTION Recently, interest in magnetocaloric
MC technology has grown significantly due to the development of new mag￾netic materials such as Gd-Si-Ge, Mn-Fe-P-As, Ni-Mn-Ga, and La-Fe-Si that exhibit large MC effect near room temperature.1–9 In the above series of MC materials, the fer￾romagnetic Ni2MnGa Heusler alloys have attracted consider￾able interest for the application of magnetic refrigeration.6,7,10–14 A magnetic entropy change
SM as large as −20.7 J/kg K at 333 K under 1.8 T magnetic field has been reported by Chernechukin et al.9 in Ni-Mn-Ga al￾loys. It is ascribed to the coincidence of the martensite trans￾formation temperature TM and Curie temperature TC in a coupled magnetostructural transformation.9 A similar mecha￾nism is reported in Gd5Si2Ge2 alloys.1–3 The
SM value
−20.7 J/kg K in Ni-Mn-Ga alloys is larger than that
−18.5 J/kg K in Gd5Si2Ge2 alloys under common condi￾tions near room temperature.1,2 The Ni-Mn-Ga alloys have other advantages over Gd5Si2Ge2 and other MC materials such as MnAs1−xSbx, 13,15 perovskite manganese oxides,9,16 or rare-earth-based compounds.10 They are biocompatible, easier to synthesize or fabricate in specific shape
ductile in comparison to the oxides, and less expensive, especially in comparison to the rare-earth or As
highly toxic containing compounds. In a broad composition range, the Ni-Mn-Ga alloys un￾dergo a martensitic transformation from a cubic
austenite to a tetragonal/orthorhombic
martensite phase upon cooling. The magnetostructural transformation is sensitive to the composition.9–13 The compositional dependence of magnetic and structural transformations in the martensitic Ni2+xMn1+yGa1+z
x+y+z=0 Heusler alloys has been re￾ported in detail by Albertini et al.17 The origin of the MC effect in the vicinity of the martensitic transition temperature in Ni-Mn-Ga alloys has been studied as a function of com￾position, which is expressed through the average number of valence electron per atom
e/a. 12 It is useful to tune the Ni-Mn-Ga alloy composition and microstructure such that the two transitions coappear at tem￾perature as high as possible. Altering the stoichiometry of Ni2MnGa Heusler alloys toward the Ni-rich values Ni50+xMn25−XGa25, x=0, 2, 3, and 5, the modified TM and TC transitions can shift at nearly the same temperature
300 K. 17,18 A typical Ni55Mn20Ga25 composition met such TM and TC concurrence, with a small value
SM =−0.68 J/kg K.18 In this work, an attempt is made to improve the properties further by tuning the Mn and Ga val￾ues in this base alloy. The evolution of microstructural fea￾tures, magnetostructural transformation, and magnetic prop￾erties is presented in this article. II. EXPERIMENTAL DETAILS A. Synthesis of the alloys Three alloys, Ni54.8Mn20.3Ga24.9, Ni55Mn18.9Ga26.1, and Ni55.2Mn18.1Ga26.7, were prepared by vacuum arc melting of high-purity
99.99% starting elements in a water-cooled copper crucible under high-purity argon atmosphere. The al￾loys recovered after each batch melting were reverted and remelted
four times to ensure homogeneous chemical dis￾persion and alloying. The arc melted buttons were annealed at 1123 K for 2 days in vacuum followed by water quench￾ing. Their chemical compositions were determined using an inductively coupled plasma-atomic emission spectroscopy
ICP-AES. Electron probe microanalysis
EPMA was used a Corresponding author; electronic mail: rg_gopy@yahoo.com JOURNAL OF APPLIED PHYSICS 102, 013906
2007 0021-8979/2007/1021
/013906/5/$23.00 © 2007 American Institute of Physics 102, 013906-1