Stabilization of the skyrmion crystal phase and transport in thin-film antiferromagnets

We investigate the stability and dynamics of the skyrmion lattice in antiferromagnetic thin films subjected to fieldlike torques such as, e.g., those induced by an electric current in CuMnAs and ${\mathrm{Mn}}_2\mathrm{Au}$ via the inverse spin-galvanic effect. The skyrmion crystal phase represents the ground state of the antiferromagnet in a substantial area of the phase diagram, the latter being parametrized by the effective staggered field and uniaxial anisotropy constant. Experimental signatures of the skyrmion crystal phase and readout schemes based on topological transport (e.g., the spin Hall effect) are discussed. We also estimate qualitatively the effect of thermal and current fluctuations, including shot noise, on the stability of the skyrmion lattice.

Figure 1

AFM thin film deposited on a heavy-metal substrate. (a) The MCA axis lies along the normal to the film. Blue dots depict skyrmions located at the sites of the static hexagonal lattice. Inset: Spatial dependence of the staggered order parameter for an isolated Néel skyrmion. (b) Crystallographic structure of the CuMnAs sample. The film is grown along the [110] direction. Red/blue spheres illustrate Mn atoms belonging to different magnetic sublattices, whereas red/blue arrows denote the direction of the resultant staggered field.

Figure 2

Zero-temperature phase diagram in the anisotropy-staggered field phase space $(\mathcal{K},{\mathcal{B}}_{\mathrm{sg}})$, obtained from a circular-cell minimization of the energy (1). The AFM film has an effective easy-axis (easy-plane) anisotropy for $\mathcal{K}<0\left(\mathcal{K}>0\right)$. The insets show the spatial configuration of the Néel order (yellow arrow) within the AFM phase. The averaged skyrmion density ${\rho }_{\mathrm{sky}}$ is measured in arbitrary units.