Spin-Polarized Current in Noncollinear Antiferromagnets

Noncollinear antiferromagnets, such as ${\mathrm{Mn}}_3\mathrm{Sn}$ and ${\mathrm{Mn}}_3\mathrm{Ir}$, were recently shown to be analogous to ferromagnets in that they have a large anomalous Hall effect. Here we show that these materials are similar to ferromagnets in another aspect: the charge current in these materials is spin polarized. In addition, we show that the same mechanism that leads to the spin-polarized current also leads to a transverse spin current, which has a distinct symmetry and origin from the conventional spin Hall effect. We illustrate the existence of the spin-polarized current and the transverse spin current by performing ab initio microscopic calculations and by analyzing the symmetry. We discuss possible applications of these novel spin currents, such as an antiferromagnetic metallic or tunneling junction.

Figure 1

(a) Crystal and magnetic structure of ${\mathrm{Mn}}_3\mathrm{Sn}$ (as well as ${\mathrm{Mn}}_3\mathrm{Ga}$ and ${\mathrm{Mn}}_3\mathrm{Ge}$) and (b) ${\mathrm{Mn}}_3\mathrm{Ir}$ (as well as ${\mathrm{Mn}}_3\mathrm{Rh}$, ${\mathrm{Mn}}_3\mathrm{Pt}$).

Figure 2

$\mathrm{\Gamma }$ dependence of the odd spin currents in ${\mathrm{Mn}}_3\mathrm{Sn}$ and ${\mathrm{Mn}}_3\mathrm{Ir}$. (a),(b) The magnitude of the spin current for ${\mathrm{Mn}}_3\mathrm{Sn}$ and ${\mathrm{Mn}}_3\mathrm{Ir}$, respectively. Only the largest component is shown for clarity. The dashed line denotes a calculation without SOC. The dip in (b) corresponds to a sign change. (c),(d) The SCA for ${\mathrm{Mn}}_3\mathrm{Sn}$ and ${\mathrm{Mn}}_3\mathrm{Ir}$, respectively.

Figure 3

(a) Simplified Fermi level of a noncollinear AFM. Green line denotes the Fermi level, blue and gray arrows denote the mean values of spin and velocity, respectively. (b) The electric field causes a redistribution of electrons at the Fermi level, signified by a thicker or thinner green line. The arrows inside the circle show the corresponding spin currents. (c),(d) The main features of the Fermi level can be captured by considering only three types of electrons with velocities oriented parallel or antiparallel with their spin. (e),(f) Parallel and antiparallel states of the AFM junction. Gray dashed arrow denotes the direction of the spin current flow, blue arrows denote the spin polarization of the spin current, and red arrows denote the magnetic moments.