ELECTRONIC PROPERTIES AND MAGNETIC ORDER OF COMPOSITIONALLY AND STRUCTURALLY RE-ARRANGED HEUSLER COMPOUND Mn2CoAl

Abstract

Heusler compound Mn2CoAl has been reported to possess the spin gapless property. It has been proposed as a candidate for application in fabrication of spin-logic and energy efficient spintronic devices. While some theoretical studies have reported a ferromagnetic order in the ordered alloy, some have reported a ferrimagnetic order. Some reports have found the regular L21 crystal structure more stable, while others have reported the inverse Heusler structure as being more stable. Recently, its ferrimagnetic order was observed experimentally using the synchrotron-based x-ray magnetic circular dichroism technique. These results have been explained using theoretical investigations of the interaction between the atom resolved states and magnetic moments using the Density Functional Theory (DFT), as implemented in the Quantum ESPRESSO package, for the compositionally ordered and disordered systems as well as distorted and un distorted crystal structures. The exchange-correlation potential is treated with the Generalized Gradient Approximation (GGA), employing the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional (PBE-GGA). The inverse Heusler structure is found to be more stable than the L21 structure. There is a near zero gap in the majority states, while a clear gap at the Fermi level is evident in the minority states. Tetragonalizing the structure causes an increased intersection of the Mn d states at the Fermi level, causing the electronic structure to tend towards a half metal rather than a spin gapless semiconductor. Higher structural distortions destroy the half metallic gap, resulting in a metallic electronic structure. Intermixing of the states results in a half metallic electronic structure. The Mn magnetic moments couple antiferromagnetically in the optimized, slightly distorted and compositionally rearranged structures. A ferromagnetic coupling is found in the more distorted structures.