Magnetic order and magnetotransport in half-metallic ferrimagnetic MnyRuxGa thin films

Abstract

The ruthenium content of half-metallic ${\mathrm{Mn}}_{2}{\mathrm{Ru}}_{x}\mathrm{Ga}$ thin films, with a biaxially strained inverse Heusler structure, controls the ferrimagnetism that determines their magnetic and electronic properties. An extensive study of ${\mathrm{Mn}}_{y}{\mathrm{Ru}}_{x}\mathrm{Ga}$ films on MgO (100) substrates with $1.8\ensuremath{\le}y\ensuremath{\le}2.6$ and $x=0.5, 0.7 \mathrm{or} 0.9$, including crystallographic, magnetic order, magneto-transport, and spin polarization, is undertaken to map specific composition-dependent properties in this versatile ternary system. A comparison of experimental densities obtained from x-ray reflectivity with calculated densities indicates full-site occupancy for all compositions, which implies chemical disorder. All moments lie on a Slater-Pauling plot with slope 1 and all except $x=0.5$, $y=2.2$ exhibit magnetic compensation at ${T}_{\text{comp}}$ below 500 K. The coercivity near ${T}_{\text{comp}}$ exceeds 10 T. Increasing the Mn or Ru content raises ${T}_{\text{comp}}$, but increasing Ru also decreases the spin polarization determined by point contact Andreev reflection. Molecular-field theory is used to model the temperature dependence of the net ferrimagnetic moment and three principal exchange coefficients are deduced. Marked differences in the shape of anomalous Hall and net magnetization hysteresis loops are explained by substantial canting of the small net moment by up to ${40}^{\ensuremath{\circ}}$ relative to the $c$ axis in zero field, which is a result of slight noncollinearity of the ${\mathrm{Mn}}^{4c}$ sublattice moments due to competing intrasublattice exchange interactions arising from antisite disorder and excess Mn in the unit cell. Consequences are reduced-spin polarization and an enhanced intrinsic contribution to the anomalous Hall effect. The systematic investigation of the physical properties as a function of $x$ and $y$ will guide the selection of compositions to meet the requirements for magnonic and spintronic MRG-based devices.