Now a days Ni-Mn-X (X= Ga, Sb, In, Sn,) Heusler based alloy systems gained attention due to their vast applications in magnetic refrigeration, magnetic actuated devices and spintronic devices [1-2], By selectively tuning the composition of this alloy system shows wide physical properties such as magnetic field induced transition, inverse magnetocaloric effect (IMCE), giant magnetoresistance, giant Hall effect, giant magneto thermal conductivity, magnetic superelasticity effects, exchange bias and shape memory effect [3, 4, 5, 6, 7-8] The Ni-Mn-Sn alloy system is particularly interesting class of materials due to the large reported IMCE [9], The structural and magnetic properties of these alloys are very sensitive to the composition. Generally, the magnetic moment decreases in the martensite phase for the composition with excess Mn which couples antiferromagnetically with Mn atoms. The effect of variation of Mn/Sn concentration on structural and magnetic properties in Ni-Mn-Sn alloys has been studied extensively [9,10,11], however; there are very few reports on the effect of variation of Ni/Mn concentration on the magneto-structural properties in Ni-Mn-Sn alloys 12, Hence, we have taken up this study to investigate in detail, the influence of Ni/Mn content on the structural and magnetic properties in Ni₄₈Mn₃₉Sn₁₃ alloy. In addition to this, we have also studied the IMCE and the effect of magnetic field on the structural transition in these alloys.

The ingot of Ni₄₈Mn₃₉Sn₁₃ alloy was prepared by melting the high purity starting elements (99.9% pure) using an arc melting furnace under argon atmosphere. The alloy was remelted four times to ensure homogeneity and it was annealed under high vacuum at 1175 K for 6 h, then quenched with Ar gas. The structural analysis was carried out using Philips 3121 X-ray diffractometer with Cu-Ka radiation (not discussed). The magnetization measurements were performed by means of a physical property measurement system (PPMS-9T) using vibrating sample magnetometer (VSM) module (Quantum Design, USA).

The ZFC and FC magnetization curve for Ni₄₈Mn₃₉Sn₁₃ alloy at 5 mT is shown in figure 1. The sample shows complex magnetic behaviour with distinct transitions which correspond to first and second order transformations. At high temperature end, the ZFC and FC curve shows a second order transition at 303 K that corresponds to the Curie temperature of austenite phase T£, where the sample transform from paramagnetic to ferromagnetic austenite. With decreasing temperature, the magnetization increases and reaches a peak value at 299 K in the FC curve. In the temperature range 151–201 K, the FC and ZFC curve display hysteresis indicating the first order structural transition from austenite to martensite phase. The characteristic transformation temperatures, martensite start and finish (M_s, M_{), and austenite start and finish (A_s, A_f), where the magnetization changes its path, have been indicated on the ZFC and FC curve. As the temperature lowers below M_s the magnetization decreases up to M_f in the FC curve as the fraction of the austenite phase progressively decreases. Lowering the temperatures, i.e. below M_f, there is a splitting between the ZFC and FC curves and also the ZFC curve shows a step like behaviour in magnetization. This transition is denoted as T_EBwhich corresponds to the exchange bias blocking temperature of the sample. The splitting between ZFC and FC curves indicates that the sample is magnetically inhomogeneous. This behaviour is also observed in Ni-Mn-X (X= Sn, Sb, In) alloys [13, 14, 15-16]. The observation of magnetic inhomogeneity and exchange bias blocking temperature can be attributed to the presence of antiferromagnetic (AF) interactions arising out of the AF coupling between the Mn atoms in the Mn sites and the Mn atoms in the Ni/Sn sites. The presence of AF interaction in this material system was confirmed through the Neutron diffraction and ferromagnetic resonance studies else were [17-18]. In order to study the effect of magnetic field on the structural transformations, we have measured the magnetization as a function of temperature during heating and cooling cycles in a field of 5 T. The alloy shows a shift in M_s towards lower temperatures as the magnetic field increases. A maximum shift of 12 K has been observed for this alloy. The lowering of the martensitic transition in Ni-Mn-Sn system implies that the external magnetic field favours the formation of the austenitic phase. It is due to the fact that the saturation magnetization of the martensitic phase is smaller than that of the austenite phase. A similar behaviour has also been observed in Ni-Mn-Sb and Ni-Mn-ln alloys. This behaviour is in contrary to that observed in Ni-Mn-Ga alloys, wherein the martensitic transition shifts to higher temperatures upon increasing the magnetic field [19].

The magnetic behaviour of the Ni₄₈Mn₃₉Sn₁₃alloy was studied by measuring the isothermal magnetization curve in the vicinity of martensitic and magnetic transitions (not shown). In the region of martensitic transition, the isothermal magnetization curves are complex with the signature of a field induced transition. At temperatures 184 and 187 K, the metamagnetism can be seen clearly, where the magnetization suddenly starts to rise around a field of 3.8 T. It is due to the field induced reverse transition from low magnetic martensite to high magnetic austenite phase.

Magnetic entropy change (“S_M) is calculated from the isothermal magnetization curves using the Maxwell relation:

The “S_M value calculated at different temperatures for Ni₄₇Mn₄₀Sn₁₃ is shown in figure 2b. The sign of “S_M is determined by the sign of and the positive sign of “S_M represents the inverse magnetocaloric effect. This sample displays the highest “S_M value of 12.12 Jkg⁻¹K⁻¹ around 225 K for a field change of 5 T. Previously, Krenke et al. have reported a highest “S_M value of 18 Jkg⁻¹K⁻¹ near room temperature for a field of 5 T in Ni₀₅₀Mn_{0 50x}Sn_x(x = 0.13) alloy [9]. For composition Ni_{50 x}Mn_39+xSn₁₁ (x = 7), a maximum “S_M value of 10.4 Jkg⁻¹ K⁻¹ has been reported in a low field of 1 T near 200 K [20].

We have studied the structural and magnetic transformations in Ni₄₈Mn₃₉Sn₁₃ alloy. We also studied the magnetic entropy change for Ni₄₈Mn₃₉Sn₁₃ alloy. This alloy shows a highest “S_M value of 12.12 Jkg ¹K¹ for a field change of 5T. The observation of giant magnetocaloric effect and economical concerns make the Ni₄₈Mn₃₉Sn₁₃ alloy as potential candidates for magnetic refrigeration applications.

The authors acknowledge the DST and UGC for their financial support. The authors also acknowledge Defence Metallurgical Research Laboratory (DMRL), Flyderabad.

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