Ab Initio and Monte Carlo Approaches For the Magnetocaloric Effect in Co- and In-Doped Ni-Mn-Ga Heusler Alloys

Vladimir Sokolovskiy,Anna Grünebohm,Peter Entel,Vasiliy Buchelnikov

doi:10.3390/e16094992

Abstract

The complex magnetic and structural properties of Co-doped Ni-Mn-Ga Heusler alloys have been investigated by using a combination of first-principles calculations and classical Monte Carlo simulations. We have restricted the investigations to systems with 0, 5 and 9 at% Co. Ab initio calculations show the presence of the ferrimagnetic order of austenite and martensite depending on the composition, where the excess Mn atoms on Ga sites show reversed spin configurations. Stable ferrimagnetic martensite is found for systems with 0 (5) at% Co and a c=a ratio of 1.31 (1.28), respectively, leading to a strong competition of ferro- and antiferro-magnetic exchange interactions between nearest neighbor Mn atoms. The Monte Carlo simulations with ab initio exchange coupling constants as input parameters allow one to discuss the behavior at finite temperatures and to determine magnetic transition temperatures. The Curie temperature of austenite is found to increase with Co, while the Curie temperature of martensite decreases with increasing Co content. This behavior can be attributed to the stronger Co-Mn, Mn-Mn and Mn-Ni exchange coupling constants in austenite compared to the corresponding ones in martensite. The crossover from a direct to inverse magnetocaloric effect in Ni-Mn-Ga due to the substitution of Ni by Co leads to the appearance of a “paramagnetic gap” in the martensitic phase. Doping with In increases the magnetic jump at the martensitic transition temperature. The simulated magnetic and magnetocaloric properties of Co- and In-doped Ni-Mn-Ga alloys are in good qualitative agreement with the available experimental data.

Highlights

Among ferromagnetic (FM) materials having acceptable magnetocaloric properties, Ni-Mn-GaHeusler alloys have been widely investigated by experimental and theoretical methods in view of their potential applications as intelligent functional materials [1,2,3,4,5]
There is an extreme sensitivity of the crystal structure of the martensitic phase with respect to composition in off-stoichiometric Ni2+x Mn1−x Ga or Ni2 Mn1+x Ga1−x alloys [21,24,25,26,27,28], showing that in dependence of composition and thermo-mechanical treatments, the crystal structure of low-temperature martensite can be tetragonal or monoclinic, with five-layered modulation (5M) [29], orthorhombic or monoclinic, with seven-layered modulation (7M) [30,31], or non-modulated tetragonal (NM) [32,33]
In this paper, we present the results of ab initio calculations and Monte Carlo simulations of substitutional processes, we have gathered in Table 1 the investigations from the literature [19,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50]

Summary

Introduction

Among ferromagnetic (FM) materials having acceptable magnetocaloric properties, Ni-Mn-Ga. The effect of Co content on the magnetic property and phase stability of Ni2−y Coy MnGa (0 ≤ y ≤ 0.75) alloys has been investigated by Chang-Long et al [52] They show that both the phase stability of austenite and the Curie temperature increase with increasing Co content. We extend the zero-temperature first-principles calculations of Co-doped Ni-Mn-Ga within the DFT scheme to finite-temperature Monte Carlo (MC) simulations, where exchange coupling constants, magnetic moments, and anisotropy energy are taken from ab initio calculations This allows us to simulate the temperature dependence of magnetic and thermodynamic properties, as well as the magnetocaloric effect of Ni-Co-Mn-Ga across a magnetostructural phase transformation

Construction of the Model Hamiltonian

Computational Details

Results of Ab Initio Calculations

Results of Monte Carlo Simulations

Conclusions