Effect of chemical and external hydrostatic pressure on magnetic and magnetocaloric properties of Pt doped Ni₂MnGa shape memory Heusler alloys

Highlights

• The magnetic and magnetocaloric property is investigated for Pt doped Ni_2−xPt_xMnGa (x = 0.2, 0.3 and 1.0) alloys at ambient pressure.

• The pressure also induces the AFM/FM interaction at low temperature for x = 0.2 alloy, where previously not observed in Ni-Mn based alloys under pressure.

• The metamagnetic transition and crossover of magnetization are suppressed for x = 1.0 alloy.

• The application of chemical and hydrostatic pressure increases the T_M and decreases ΔS_M.

Abstract

The magnetocaloric effect (MCE) on Ni_2−xPt_xMnGa (x = 0.2, 0.3 and 1.0) shape memory Heusler alloys around martensite phase transition temperature (T_M) is investigated by varying chemical pressure (Pt concentration). The magnetic entropy change (ΔS_M) decreases with increasing chemical pressure for various external applied magnetic fields up to 3 T, and the width of thermal hysteresis increases with the increases of Pt concentration. The effect of hydrostatic pressure on both T_M and ΔS_M for Ni_1.8Pt_0.2MnGa is also investigated. We observed that the application of hydrostatic pressure increases T_M (3.5 K/GPa) and stabilizes the martensite phase. The maximum magnetic entropy change (ΔS_max) of 9.31 J Kg⁻¹ K^-1is observed for a field change of 9 T at ambient pressure for Ni_1.8Pt_0.2MnGa. Further, the application of external pressure leads to the decrease of ΔS_max to 5.52 J Kg⁻¹ K⁻¹ at 0.91 GPa.

Introduction

Shape memory Heusler alloys (SMHAs) are of current interest due to their potential applications in magnetic refrigeration [1], magnetic actuation [2], and spintronics devices [3], etc. These alloys exhibit both structural and magnetic transitions [4], [5], [6]. During the structural phase transition, the high temperature austenite phase, which has cubic crystal structure, transforms to the low temperature martensite phase which has lower symmetry structure [7]. A good control of magnetic phase transition and magnetic property of Ni-Mn-X (X = In, Ga, Sn) SMHA’s show multifunctional properties [8], [9], [10]. Among SMHAs, the novel properties of Ni-Mn-Ga alloys received huge attention due to its large magnetic field induced strain (MFIS) owing to their potential applications in sensors and actuators [9], [11]. The MFIS of 10% has been observed in Ni₂MnGa ferromagnetic shape memory alloy (FSMA) which is believed to be related with its modulated orthorhombic structure of the martensite phase [12], [13]. The martensite phase transformation exhibits in these alloys not only to provide large MFIS but responsible for other multifunctional properties such as the shape memory effect (SME) [2], magneto-resistance (MR) [14], [15], magneto caloric effect (MCE) [16], [17] and exchange bias phenomenon [18], [19]etc. From the above mentioned properties, MCE shows the potential application in solid-state cooling technology [4], [5]. Recently Pt doped Ni₂MnGa have created a lot of attention due to their higher transition temperatures, and both theoretical and experimental studies indicate that these alloys may exhibit better magnetic and mechanical properties [6], [18], [20], [21], [22]. In general, in Ni-Mn-X (X = In, Ga, Sn) SMHAs, the structure, and their magnetic properties are very sensitive to their chemical composition [23]. Apart from the chemical composition, external parameters such as the magnetic field and pressure are expected to strongly influence their structure and magnetic properties [24], [25] The hydrostatic pressures are known to play a significant role in the structure and magnetic properties of these systems [24], [26], [27], [28]. The relative stability of the high temperature cubic austenite phase and the low temperature martensite phase could be influenced by pressure [15], [29]. For the composition of Ni-Mn-Ga FSMAs, the exchange interaction of Mn-Mn is strongly dependent on the Mn-Mn distance which can be easily altered by either hydrostatic pressure or chemical substitutions [9], [10].

In the present manuscript, the magnetic and magnetocaloric properties of Pt substituted Ni_2−xPt_xMnGa (x = 0.2, 0.3, and 1.0) at the Ni site is reported. By increasing of Pt (x) concentration in Ni_2−xPt_xMnGa (x = 0.2, 0.3 and 1.0) the martensite transition (T_M) increases, whereas magnetic entropy change (ΔS_M) decreases. Hence, the high magnetic entropy value of about 9.31 J Kg⁻¹ K⁻¹ for x = 0.2 at 288 K is observed at 9 T of the magnetic field. Therefore, it is interesting to note that an effect of chemical and hydrostatic pressure on magnetic and magnetocaloric properties of Ni_1.8Pt_0.2MnGa quaternary Heusler alloy is explored in this article to have a better understanding and application on these materials.