Ab initio study on the magneto-structural properties of MnAs

TL;DR

This study uses ab initio methods to analyze the magnetic and structural phase transitions of MnAs, revealing how lattice constants influence magnetic states and phase stability, and providing insights into the Curie temperature dependence.

Contribution

It offers a detailed ab initio analysis of MnAs's phase stability, magnetic ordering, and structural transitions, including an analytic approximation for orthorhombic distortion effects.

Findings

Ferromagnetic state favored at large lattice constants.

Orthorhombic distortion occurs in antiferromagnetic states.

Paramagnetic phase transitions at about 400 K.

Abstract

The magnetic and structural properties of MnAs are studied with ab initio methods, and by mapping total energies onto a Heisenberg model. The stability of the different phases is found to depend mainly on the volume and on the amount of magnetic order, confirming previous experimental findings and phenomenological models. It is generally found that for large lattice constants the ferromagnetic state is favored, whereas for small lattice constants different antiferromagnetic states can be stabilized. In the ferromagnetic state the structure with minimal energy is always hexagonal, whereas it becomes orthorhombically distorted if there is an antiferromagnetic component in the hexagonal plane. For the paramagnetic state the stable cell is found to be orthorhombic up to a critical lattice constant of about 3.7 Angstrom, above which it remains hexagonal. This leads to the second order structural phase transition between paramagnetic states at about 400 K, where the lattice parameter increases above this critical value with rising temperature due to the thermal expansion. For the paramagnetic state an analytic approximation for the magnitude of the orthorhombic distortion as a function of the lattice constant is given. Within the mean field approximation the dependence of the Curie temperature on the volume and on the orthorhombic distortion is calculated. For orthorhombically distorted cells the Curie temperature is much smaller than for hexagonal cells. This is mainly due to the fact that some of the exchange coupling constants in the hexagonal plane become negative for distorted cells. With these results a description of the susceptibility as function of temperature is given.