VP14-05 Néel-order spin-orbit torques in CuMnAs from first principles ballistic transport
Creators: Maria Stamenova and Andrea Droghetti
Publisher: name: IEEE
Year: 2025

The tetragonal metallic antiferromagnet CuMnAs has been proposed for devices relying on electrical Néel vector switching [1] or electrical/optical heat-assisted toggling into a nano-fragmented domain state with a GMR-like level of resistance change [2]. CuMnAs in its tetragonal phase, belongs to a special class of non-centrosymmetric lattice AFMs, in which the two magnetic sublattices form inversion partners (see Figure 1). Inversion symmetry breaking in magnetic crystals, combined with relativistic effects, gives rise to key spintronic effects like the inverse spin-galvanic effect, in which the electron flow induces local spin-polarisations on the two sublattices. These give rise to atomically-staggered local fields, generating opposite torques, so called Néel-order spin-orbit torques (SOTs), on the two sublattices [3]. As the AFMs are immune to external magnetic field, this mechanism is essential for the manipulation of their order parameter in novel AFM-spintronic devices. The Néel-order SOTs’ mechanism has been demonstrated theoretically and first principle results based on Kubo linear response formalism provide quantitative insights – staggered fields of about 3x10-3 T per 107 A/cm2 locally at each sublattice [1]. We have set out to instead investigate this effect from the other theoretical extreme – ballistic transport from first principles (DFT+NEGF) [4,5] close to the linear response regime in wide slabs of CuMnAs. Our calculated atomically-resolved Néel-order SOTs (Fig. 1) reproduce the desired staggered (switching-friendly) behaviour of the SOTs shown in Ref. [1]. Quantitatively, however, for ballistic conductivity of the slab of 3.4x1014 A/(V.m2), we find average SOT fields of about 70 T/V, suggesting a substantially larger ballistic SOT effect. We investigate further the role of disorder in suppressing the ballistic SOT effect.References: [1] P. Wadley, et al., Science 351, 587 (2016). [2] Z. Kaspar, et al., Nature Electronics 4, 30 (2022). [3] J. Zelezný, et al., Phys. Rev. Lett. 113, 157201 (2014). [4] A. R. Rocha, et al., Phys. Rev. B 73, 085414 (2006) ; J.M. Soler, et al, J. Phys. Condens. Matter 14, 2745 (2002). [5] I. Rungger, et al, (2020). Non-equilibrium Green’s Function Methods for Spin Transport and Dynamics. In: Andreoni, W., Yip, S. (eds) Handbook of Materials Modeling. Springer, Cham.
The tetragonal metallic antiferromagnet CuMnAs has been proposed for devices relying on electrical Néel vector switching [1] or electrical/optical heat-assisted toggling into a nano-fragmented domain state with a GMR-like level of resistance change [2]. CuMnAs in its tetragonal phase, belongs to a special class of non-centrosymmetric lattice AFMs, in which the two magnetic sublattices form inversion partners (see Figure 1). Inversion symmetry breaking in magnetic crystals, combined with relativistic effects, gives rise to key spintronic effects like the inverse spin-galvanic effect, in which the electron flow induces local spin-polarisations on the two sublattices. These give rise to atomically-staggered local fields, generating opposite torques, so called Néel-order spin-orbit torques (SOTs), on the two sublattices [3]. As the AFMs are immune to external magnetic field, this mechanism is essential for the manipulation of their order parameter in novel AFM-spintronic devices. The Néel-order SOTs’ mechanism has been demonstrated theoretically and first principle results based on Kubo linear response formalism provide quantitative insights – staggered fields of about 3x10-3 T per 107 A/cm2 locally at each sublattice [1]. We have set out to instead investigate this effect from the other theoretical extreme – ballistic transport from first principles (DFT+NEGF) [4,5] close to the linear response regime in wide slabs of CuMnAs. Our calculated atomically-resolved Néel-order SOTs (Fig. 1) reproduce the desired staggered (switching-friendly) behaviour of the SOTs shown in Ref. [1]. Quantitatively, however, for ballistic conductivity of the slab of 3.4x1014 A/(V.m2), we find average SOT fields of about 70 T/V, suggesting a substantially larger ballistic SOT effect. We investigate further the role of disorder in suppressing the ballistic SOT effect.References: [1] P. Wadley, et al., Science 351, 587 (2016). [2] Z. Kaspar, et al., Nature Electronics 4, 30 (2022). [3] J. Zelezný, et al., Phys. Rev. Lett. 113, 157201 (2014). [4] A. R. Rocha, et al., Phys. Rev. B 73, 085414 (2006) ; J.M. Soler, et al, J. Phys. Condens. Matter 14, 2745 (2002). [5] I. Rungger, et al, (2020). Non-equilibrium Green’s Function Methods for Spin Transport and Dynamics. In: Andreoni, W., Yip, S. (eds) Handbook of Materials Modeling. Springer, Cham.