Site-specific order and magnetism in tetragonal Mn3Ga thin films

Abstract

Mn 3 Ga bulk material and thin films deposited on several different substrates have been investigated using x-ray and neutron diffraction, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and electronic structure calculations using density-functional theory with the aim of determining the atomic site occupancy, magnetic moments, and magnetic structure of this tetragonal D0 22 -structure compound. The Mn 3 Ga has close to ideal site occupancy, with Ga on 2 a sites and Mn on 2 b and 4 d sites. The magnetic structure is basically ferrimagnetic, with the larger Mn moment of about 3 ? B on the 2 b site, which is coordinated by 8 Mn 4 d and 4 Ga, and the smaller one on the 4 d site, which is coordinated by 4 Mn 2 b , 4 Ga, and 4 Mn 4 d .TheMn d -band occupancy is close to 5 on both sites, and the orbital moments are small, < 0 . 2 ? B . The material nevertheless exhibits substantial uniaxial anisotropy, K u = 1 . 0MJm ? 3 , which originates from the 4 d site. The 2 b site has hard axis anisotropy, which together with an oscillatory exchange coupling from the first and second nearest neighbors, leads to a soft component of the magnetization in the c plane, coexisting with c -axis hysteresis loops exhibiting coercivity of up to 1.2 T, and magnetization in the range 110?220 kA m ? 1 at room temperature, depending on preparation conditions. Tetragonal Mn 2 Ga films behave similarly. Manganese is lost from both sites, but the films have substantially larger magnetization (480 kA m ? 1 ) and anisotropy constant (2.35 MJ m ? 3 )thanMn 3 Ga.