Site-specific order and magnetism in tetragonal Mn${}_3$Ga thin films

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 $2a$ sites and Mn on $2b$ and $4d$ sites. The magnetic structure is basically ferrimagnetic, with the larger Mn moment of about 3 ${\mu }_B$ on the $2b$ site, which is coordinated by 8 Mn $4d$ and 4 Ga, and the smaller one on the $4d$ site, which is coordinated by 4 Mn $2b$, 4 Ga, and 4 Mn $4d$. The Mn $d$-band occupancy is close to 5 on both sites, and the orbital moments are small, $<$$0.2$ ${\mu }_B$. The material nevertheless exhibits substantial uniaxial anisotropy, $K_u=1.0{\mathrm{MJ}\mathrm{m}}^{-3}$, which originates from the $4d$ site. The $2b$ 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${}^{-3}$) than Mn${}_3$Ga.