Phase transitions and magnetocaloric effect in Mn₃Cu_0.89N_0.96

Abstract

We report a large magnetic entropy change observed in the antiperovskite Mn₃Cu_0.89N_0.96. Based on the heat flow peak measured by differential scanning calorimetry, the total entropy change according to the structural transition (tetragonal to cubic) was calculated to be ∼60 J kg^–1 K^–1 while the magnetic entropy change accounts for ∼22.5% of the total entropy under a 50 kOe magnetic field at 145 K. To clarify the origin of the magnetic entropy change, we managed to control the structure transition using a magnetic field. It was found that the magnetic entropy change originates from the transformation from antiferromagnetic (AFM) to ferromagnetic (FM) as well as from the phase transition from cubic to tetragonal under the magnetic field. In the tetragonal phase, a magnetic field can drive the AFM component to transform gradually to the FM component. The magnetic field can also change the phase fraction in the tetragonal and cubic two-phase coexistence region, a similar behavior to that induced by temperature. In the current system, only 5.4% of the cubic phase transforms to the tetragonal phase at 5 Tesla (T), indicating there is still much latent entropy in Mn₃Cu_0.89N_0.96.

Introduction

Materials with a large magnetocaloric effect (MCE) have attracted much attention in recent years. Most of these materials with large MCE contain expensive and sometimes toxic rare-earth elements [1]. Development of low-cost and innocuous materials with large MCE is therefore critical for practical applications.

Some antiperovskite compounds with large MCE have potential for magnetocooling technology. Mn₃GaC has been found to undergo an abrupt first-order transition from antiferromagnetic (AFM) to ferromagnetic (FM) at ∼165 K, and this transition can be induced by a magnetic field [2], [3], [4], [5]. A large magnetic entropy change of 15 J kg^–1 K^–1 below 20 kOe was estimated from the Maxwell equation for the isothermal magnetization curves of Mn₃GaC [2], [3]. Carbon content was found to be capable of altering the magnetic state of Mn₃GaC and influencing the entropy variation of the phase transition [6], [7], [8]. It was reported that by introducing 22% vacancies at carbon positions, the magnetic entropy change can be reduced to 3.7 J kg^–1 K^–1 under a magnetic field of 50 kOe in Mn₃GaC_0.78 [7]. In addition, substitution of Co for Mn in Mn₃GaC can lower the first-order magnetic transition temperature from 160 to 100 K without significant loss of MCE. As a result, Co doping broadens the region of the large MCE in Mn_3−xCo_xGaC [9]. In order to generate large magnetic entropy, a sharp change of magnetization is necessary. Mn₃CuN undergoes a first-order structural transition from cubic to tetragonal accompanied by a paramagnetic (PM) to ferrimagnetic (FIM) magnetic transition [6], [10]. Mn₃SnC has a similar magnetic structure to Mn₃CuN [6]. Wang et al. reported that Mn₃SnC has magnetic entropy −ΔS_m = 80.69 mJ cm^–3 K^–1 (corresponding to 10.37 J kg^–1 K^–1) and −ΔS_m = 133 mJ cm^–3 K^–1 (corresponding to 17 J kg^–1 K^–1) at 20 kOe and 48 kOe, respectively [11]. The structure variation and the resulting giant magnetostriction property of Mn₃SbN are analogous to those of Mn₃CuN [12]. In Mn₃SbN, the total entropy of the phase transition process is 10.2 J mol^–1 K^–1 (corresponding to 34.93 J kg^–1 K^–1) and the magnetic entropy induced by a magnetic field of 50 kOe is 2.1 J mol^–1 K^–1 (corresponding to 7.19 J kg^–1 K^–1), hence most of the entropy originates from lattice entropy [13]. To the best of our knowledge, there is no report so far on the total and magnetic entropy of Mn₃CuN. According to Ref. [6], the Mn₂ atom in Fig. 1b has an AFM moment of 2.85 μ_B and a FM moment of 0.2 (0.15) μ_B, the Mn₁ atom in Fig. 1b has a FM moment of 0.65 (0.15) μ_B at 4.2 K in Mn₃CuN, from which it can be seen that Mn₃CuN displays weak FM component for neutron detection. Hence we have attempted to prepare Mn₃Cu_xN_y so as to obtain a large MCE. It would be of great interest to investigate the entropy in Mn₃Cu_xN_y due to its intriguing coupled structural and magnetic phase transitions and the possible resultant MCE.

A variety of fascinating physical properties related to its magnetic, electronic and lattice properties have been found in Mn₃Cu_xA_1−xN_y (where A is a doping element). Chi et al. reported a near-zero temperature coefficient of resistivity (NZ-TCR) in Mn₃CuN and found that the TCR was ∼46 ppm K^–1 at about room temperature [14] and could be improved by partial substitution of Cu by Ni or Ag [15], [16]. Negative thermal expansion (NTE) behaviors were found in some Mn-based antiperovskites, such as Mn₃AN (A = Cu, Zn, Ga), and partial substitution of A by Ge can induce and adjust the temperature range of the NTE property [17], [18], [19], [20], [21]. The magnetic–lattice coupling can be adjusted by inducing vacancies in some antiperovskite structures [22], [23]. Furthermore, anisotropic magnetostriction behavior was found in Mn₃CuN_x [24], [25]. All these properties were related to electronic entropy, lattice entropy and magnetic entropy.

In this paper, we use differential scanning calorimetry (DSC) and a superconducting quantum interference device (SQUID) magnetometer to study the entropy changes of the phase transitions in Mn₃Cu_0.89N_0.96. We found that the magnetic entropy in a magnetic field of 50 kOe is much less than the total transitional entropy. To understand such a large entropy and reveal its mechanism, we used neutron powder diffraction (NPD) to study the crystal and magnetic structures, and the structural transitions in Mn₃Cu_0.89N_0.96 under magnetic fields, and to clarify the correlations between the structure and the magnetic and entropy changes.