Structure, site-specific magnetism, and magnetotransport properties of epitaxial D022 -structure Mn2FexGa

Ferrimagnetic ${\mathrm{Mn}}_{2}{\mathrm{Fe}}_{x}\mathrm{Ga}\phantom{\rule{4pt}{0ex}}(0.26\ensuremath{\le}x\ensuremath{\le}1.12)$ thin films have been characterized by x-ray diffraction, magnetometry, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and M\"ossbauer spectroscopy with the aim of determining the structure and site-specific magnetism of this tetragonal, $\mathrm{D}{0}_{22}$-structure Heusler compound. High-quality epitaxial films with low root-mean-square surface roughness ($\ensuremath{\sim}0.6$ nm) are grown by magnetron cosputtering. The tetragonal distortion induces strong perpendicular magnetic anisotropy along the $c$ axis with a typical coercive field ${\ensuremath{\mu}}_{0}H\ensuremath{\sim}0.8\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ and an anisotropy field ranging from 6 to 8 T. On increasing the Fe content $x$, substantial uniaxial anisotropy, ${K}_{\mathrm{u}}\ensuremath{\ge}1.0\phantom{\rule{0.16em}{0ex}}{\mathrm{MJ}\phantom{\rule{0.16em}{0ex}}\mathrm{m}}^{\ensuremath{-}3}$, can be maintained over the full $x$ range, while the magnetization of the compound is reduced from 400 to $280\phantom{\rule{0.28em}{0ex}}{\mathrm{kA}\phantom{\rule{0.16em}{0ex}}\mathrm{m}}^{\ensuremath{-}1}$. The total magnetization is almost entirely given by the sum of the spin moments originating from the ferrimagnetic Mn and Fe sublattices, with the latter being coupled ferromagnetically to one of the former. The orbital magnetic moments are practically quenched and have negligible contributions to the magnetization. The films with $x=0.73$ exhibit an anomalous Hall angle of $2.5%$ and a Fermi-level spin polarization above $51%$, as measured by point contact Andreev reflection. The Fe-substituted ${\mathrm{Mn}}_{2}\mathrm{Ga}$ films are tunable with a unique combination of high anisotropy, low magnetization, appreciable spin polarization, and low surface roughness, making them strong candidates for thermally stable spin-transfer-torque switching nanomagnets with lateral dimensions down to 10 nm.