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Magnetic, phase transformation and magnetocaloric studies in ferromagnetic Ni55Mn20Ga25 Heusler alloy

The phase transformation and magnetic entropy change (ΔSM) in the Ni55Mn20Ga25 Heusler alloy has been studied. The temperature dependence of magnetization study shows the direct transition from ferromagnetic martensitic phase to paramagnetic austenitic phase occurred at 353 K. By compositional tuning the first order martensitic transformation (TM ∼200 K for parent compound Ni2MnGa) and second order magnetic phase transition temperatures (TC ∼375 K for parent compound Ni2MnGa) can be merged. This occurs for Ni55Mn20Ga25 alloy at 353 K. The magnetic entropy change, ΔSM-value of -7.0 Jkg-1K-1 has been obtained in a field change of 1.2 T. The origin of enhancement in ΔSM -value is attributed to the essential coincidence of TM and TC.


Introduction
Recently interest in magnetocaloric technology has grown significantly due to the development of new magnetic materials such as Gd-Si-Ge, Mn-Fe-P-As, Ni-Mn-Ga, La-(Fe-Si) etc. that exhibit large magnetocaloric (MC) effect near room temperature [1][2][3][4][5][6][7][8][9][10]. In the above series of MC materials the ferromagnetic Ni-Mn-Ga Heusler alloys have attracted considerable attention for the application of magnetic refrigeration. In a broad composition range the Ni-Mn-Ga alloys undergoes martensitic transition from a cubic (austenite) to tetragonal/orthorhombic/monoclinic (martensite) phase [11,12]. In both these phases, the magnetic coupling is ferromagnetic however with different ferromagnetic exchange. The difference between the two crystalline structures at low and high temperatures bring about the fundamental difference in their magnetic behavior, causing an abrupt change of magnetization in presence of magnetic field which results in MC effect (MCE). A large ΔS M is reported in Gd 5 Si 2 Ge 2 alloy, in which the structural and magnetic transitions occur simultaneously. In the case of Ni-Mn-Ga alloy, the magnetostructural transformation is very sensitive to the composition and needs fine tuning to occur them simultaneously near to room temperature. By altering the stoichiometry of Ni 50 Mn 25 Ga 25 Heusler alloy towards Ni rich composition (such that in a series of Ni 50+x Mn 25-x Ga 25 alloys), T M and T C can be tuned to occur simultaneously at ≥ 300K. ΔS M as large as -20.7 Jkg -1 K -1 at 333 K for the field change of 1.8 T has been reported by Chernechukin et al. [10] in Ni-Mn-Ga alloys. The Ni-Mn-Ga alloys have other advantages over Gd 5 Si 2 Ge 2 and other MC materials such as MnAs 1-x Sb [11,13], perovskite manganese oxides [10], or rare-earth based compounds [1,2]. 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 this work, an attempt is made to improve the ΔS M -value by tuning the T M and T C . The evolution of microstructural features, magneto-structural transformation, and magnetic properties are presented in this paper.

Experimental Details
The Ni 55 Mn 20 Ga 25 alloy was prepared by vacuum arc-melting technique of high purity Ni, Mn and Ga as starting elements (99.99% purity). The alloy recovered after each melting batch reverted and remelted (four times) to ensure chemical homogeneity of the alloy. The chemical composition of the alloy was determined using atomic absorption spectroscopy. In addition, electron probe microanalysis was performed for confirming the alloy chemistry. The crystalline structure of the alloys was analyzed with X-ray diffractograms (XRD) using a Philips model-P.W. 1710. The microstructure was studied using a scanning electron microscope (SEM) of JEOL model 35C. Magnetic proprieties were measure with a DMS-1600 model vibrating sample magnetometer with a maximum field up to 12 kOe. Thermomagnetic measurement was carried out at a biased field of 500 Oe in order to determine the T M and T C values. The data was collected during heating cycle at a rate of 10 K/min and the temperature was controlled within an accuracy of ±1 K. Isothermal magnetization curves has been carried out in the magneto-structural transformation region. A TA instrument Q100 differential scanning calorimeter (DSC) was used to monitor heat-flow during the two kinds of the transitions. The data were collected during the heating as well as the cooling cycles at a given 20 K/min rate. A hysteresis of the thermogram occurs in the two cycles is characteristic of thermochemistry of the sample for MC effects of interest in this work.

Phase transformation and crystal structure
Analysis of XRD patterns reveals that the alloy has a nearly single phase with no detectable secondary phases ( Figure 1). The rietveld refinement of the diffracted intensities suggests a tetragonal I4/mmm symmetric structure with a = 3.881 Å and c = 6.502 Å. Ni atoms occupy 4d (0, 0.5, 0.25) positions, while Mn and Ga atoms occupy 2b (0, 0, 0.5) and 2a (0, 0, 0) positions, respectively. Wedel et al [14] reported the same structure in their work from XRD. Microstructure of the alloy was studied by both optical and scanning electron microscopy techniques. Typical SEM backscattered electron image of the alloy was shown in Figure 2. The alloy exhibits plate like martensite structure at equal spacing. Martensite twins (stripes) are found randomly oriented. Thus, a common implication of the SEM and XRD two independent studies show the presence of martensite phase at room temperature for Ni 55 Mn 20 Ga 25 alloy.   Figure 4. It can be seen that the magnetization decreases as we increase the temperature in the same magnetic field. Near the T M a large change in magnetization was observed. It is interesting to note that the initial magnetization curves (below 340 K) are hard to saturate as it exhibits martensitic phase, having a much larger anisotropy field (H a =0.8 T) than that of austenite phase (H a = 0.15 T) [11].
The ΔS M can be estimated from magnetization isotherms data by the Maxwell relation, namely

Conclusions
The tetragonal martensite phase has been observed at room temperature for Ni 55 Mn 20 Ga 25 alloy. It displays an enhanced MCE with ΔS M = -7.0 J/kg K in a field of 1.2 T at 332 K. The origin of the ΔS M is related to the essential coincidence of structural and magnetic transition temperature i.e. magnetostructural transformation temperature. This co-incidence is achieved through compositional tuning.