Frustrated Magnetism and Caloric Effects in Mn-antiperovskite Nitrides: $Ab~Initio$ Theory

TL;DR

This study uses ab initio methods to explore how biaxial strain affects magnetic ordering and caloric effects in Mn-antiperovskite nitrides, revealing new strain-induced phases and potential cooling applications.

Contribution

It provides the first detailed finite-temperature phase diagram of Mn$_3$GaN under strain, identifying new phases and proposing elastocaloric cooling cycles based on these transitions.

Findings

Discovery of two new strain-stabilized magnetic phases.

Identification of strain-dependent transition temperatures.

Proposal of elastocaloric cooling cycle leveraging phase transitions.

Abstract

We model changes of magnetic ordering in Mn-antiperovskite nitrides driven by biaxial lattice strain at zero and at finite temperature. We employ a non-collinear spin-polarised density functional theory to compare the response of the geometrically frustrated exchange interactions to a tetragonal symmetry breaking (the so called piezomagnetic effect) across a range of Mn$_3$AN (A = Rh, Pd, Ag, Co, Ni, Zn, Ga, In, Sn) at zero temperature. Building on the robustness of the effect we focus on Mn$_3$GaN and extend our study to finite temperature using the disordered local moment (DLM) first-principles electronic structure theory to model the interplay between the ordering of Mn magnetic moments and itinerant electron states. We discover a rich temperature-strain magnetic phase diagram with two previously unreported phases stabilised by strains larger than 0.75\% and with transition temperatures strongly dependent on strain. We propose an elastocaloric cooling cycle crossing two of the available phase transitions to achieve simultaneously a large isothermal entropy change (due to the first order transition) and a large adiabatic temperature change (due to the second order transition).