High spin polarization in epitaxial films of ferrimagnetic Mn${}_3$Ga

Ferrimagnetic Mn${}_3$Ga exhibits a unique combination of low saturation magnetization ($M_s=0.11$ MA m${}^{-1}$) and high perpendicular anisotropy with a uniaxial anisotropy constant of $K_{\mathrm{u}}$ $=$ 0.89 MJ m${}^{-3}$. Epitaxial $c$-axis films exhibit spin polarization as high as $58\%$, measured using point-contact Andreev reflection. These epitaxial films will be able to support thermally stable sub-10-nm bits for spin-transfer torque memories.

Figure 1

(a) The cubic unit cell (dashed line) of the $L$$2_1$ Heusler alloys. The tetragonal unit cell of the $D$$0_{22}$ structure is indicated by the red (solid) line. (b) In Mn${}_3$Ga, it is stretched along the $c$ axis. The $X$ and $Y$ positions are occupied by Mn, and the $Z$ position is occupied by Ga. The magnetic couplings in $X$ and $Y$ sublattices are antiferromagnetic and ferromagnetic, respectively, but the stronger antiferromagnetic intersublattice coupling between $X$ and $Y$ sites frustrates the $X$ site moments, leading to a ferrimagnetic structure.

Figure 2

(a) X-ray diffraction patterns of Mn${}_3$Ga layers grown on Pt, Cr, and MgO seed layers. (b) Substrate temperature ($T_{\mathrm{s}}$) dependence of Mn${}_3$Ga layers grown on Pt (001); the inset shows the order parameter $S$ vs $T_{\mathrm{s}}$; at $T_{\mathrm{s}}$ $=$ 375 °C the sample is not single phase. (c) ϕ scans showing the fourfold symmetry of the films and the MgO (001) substrate. (d) Atomic force microscope images of films grown on MgO, (e) Pt, (f) Cr, all grown at $T_{\mathrm{s}}$ $=$ 350 °C.

Figure 3

(a) Magnetization curves for a 90-nm-thick film of tetragonal Mn${}_3$Ga measured at room temperature with the field applied perpendicular and parallel to the plane of the film. The anisotropy field at 300 K was determined by extrapolating in-plane data through the origin, and is 16 T. (b) Magnetization of the Mn${}_3$Ga/Cr/Mn${}_3$Ga bilayer structure. (c) Point-contact Andreev reflection spectrum of a Mn${}_3$Ga film grown on Pt.