Cubic ${\mathrm{Mn}}_2\mathrm{Ga}$ Thin Films: Crossing the Spin Gap with Ruthenium

Cubic ${\mathrm{Mn}}_2\mathrm{Ga}$ films with the half-Heusler $C{1}_b$ structure are grown on V (001) epitaxial films. The phase is a soft ferrimagnet, with Curie temperature $T_C=225\text{K}$ and magnetization $M_s=280\text{kA}{\text{m}}^{-1}$, equivalent to $1.65{\mu }_B$ per formula. Adding ruthenium leads to an increase of $T_C$ up to 550 K in cubic ${\text{Mn}}_2{\text{Ru}}_x\text{Ga}$ films with $x=0.33$ and a collapse of the net magnetization. The anomalous Hall effect changes sign at $x=0.5$, where the sign of the magnetization changes and the magnetic easy direction flips from in plane to perpendicular to the film. The ${\text{Mn}}_2{\text{Ru}}_{0.5}\text{Ga}$ compound with a valence electron count of 21 is identified as a zero-moment ferrimagnet with high spin polarization, which shows evidence of half-metallicity.

Figure 1

(a) Crystal structure of an $L{2}_1$ full Heusler $X_{2}YZ$ with $X$ (green), $Y$ (gray), and $Z$ (red) occupying $8c$, $4b$, and $4a$ positions. (b) A $C{1}_b$ half-Heusler $XYZ$ with $X$, $Y$, and $Z$ occupying $4c$, $4b$, and $4a$ positions. (c) X-ray diffraction patterns of $C{1}_b$-${\mathrm{Mn}}_2\mathrm{Ga}$ and $L{2}_1$-${\mathrm{Mn}}_2\mathrm{RuGa}$ thin films grown on MgO (001) substrates. The $C{1}_b$ phase is grown on a (001) oriented vanadium seed layer. (d) Extended x-ray absorption fine structure at the $K$ edges of a ${\mathrm{Mn}}_2{\mathrm{Ru}}_{0.52}\mathrm{Ga}$ sample. Fits are discussed in the text.

Figure 2

(a) Thermal scans in 200 mT of the magnetization of $C{1}_b$-${\mathrm{Mn}}_2{\mathrm{Ru}}_x\mathrm{Ga}$ films with different Ru content. (b) In-plane and perpendicular magnetization of a cubic $C{1}_b$-${\mathrm{Mn}}_2\mathrm{Ga}$ film at 100 K and a ${\mathrm{Mn}}_2{\mathrm{Ru}}_{0.52}\mathrm{Ga}$ film at 300 K. (c) Variation of Curie temperature and moment with Ru concentration $x$ and number of valence electrons per formula unit. The magnetic anisotropy changes from in-plane to perpendicular when $x>0.5$, and the moment is reversed in sign. Its variation close to $x=0.5$ is indicated by a red line with slope 2. The half-metallic region is lightly shaded. The lower panel shows the variation of the lattice parameter $a_{\perp }$, normal to the substrate, as a function of $x$. (d) Schematic density of states showing the effect of Ru addition in ${\mathrm{Mn}}_2{\mathrm{Ru}}_x\mathrm{Ga}$, which illustrates the Fermi level crossing the spin gap.

Figure 3

Transport on half-metallic ${\mathrm{Mn}}_2{\mathrm{Ru}}_{0.33}\mathrm{Ga}$ (right) and metallic ${\mathrm{Mn}}_2{\mathrm{Ru}}_{0.83}\mathrm{Ga}$ (left) showing evidence of the change of magnetic and electronic structure on crossing the spin gap. Panels show resistivity (top), Hall effect (middle), and magnetoresistance (bottom).

Figure 4

(a) Point contact Andreev reflection spectroscopy of cubic ${\mathrm{Mn}}_2{\mathrm{Ru}}_{0.48}\mathrm{Ga}$ film with in-plane anisotropy and (b) cubic ${\mathrm{Mn}}_2\mathrm{RuGa}$ with perpendicular anisotropy.