2020 Abstracts YRLGW

Vivek Amin

Ferromagnets generate spin currents under an applied electric field. For example, charge currents in ferromagnets are spin-polarized because majority and minority carriers have different conductivities. However, in the presence of spin-orbit coupling, electrons can carry a substantial spin current flowing perpendicularly to the electric field with spin directions both longitudinal and transverse to the magnetization.
In this talk, we discuss several mechanisms to electrically generate spin currents in ferromagnets. These mechanisms are closely related to the anomalous and planar Hall effects but yield spin currents with spin directions transverse to the magnetization. Such spin currents can be detected through the torques they exert at layer boundaries [1]. We present first principles transport calculations giving the strength and magnetization dependence of the electrically generated spin currents allowed by symmetry via both intrinsic [2] and extrinsic [3] mechanisms. We find that in transition metal ferromagnets, the spin currents with spin direction transverse to the magnetization can have an associated conductivity comparable to the spin Hall conductivity in Pt.

[1] W. Wang, T. Wang, V. P. Amin, Y. Wang, A. Radhakrishnan, A. Davidson, S. R. Allen, T. J. Silva, H. Ohldag, D. Balzar, B. L. Zink, P. M. Haney, J. Q. Xiao, D. G. Cahill, V. O. Lorenz, and X. Fan, Anomalous Spin-Orbit Torques in Magnetic Single-Layer Films, Nature Nanotechnology, 14, 819-824, 2019
[2] V. P. Amin, J. Li, M. D. Stiles, and P. M. Haney, Intrinsic Spin Currents in Ferromagnets, Phys. Rev. B 99, 220405(R), 2019
[3] V. P. Amin, J. Zemen, and M. D. Stiles, Interface generated spin currents, Phys. Rev. Lett. 121, 136805 (2018)

Libor Šmejkal

Relativistic bandstructure of solids generates functionalities of modern quantum, topological and spintronics materials1. Common collinear antiferromagnets exhibit Kramers spin degenerate bands2 and for many decades were believed to be excluded from spin splitting physics. Our recent prediction of crystal time-reversal symmetry breaking by anisotropic magnetization densities due to the collinear antiferromagnetism combined with nonmagnetic atoms3 changes this perspective. Unlike the conventional spin-orbit interaction induced spin splitting, our antiferromagnetic spin splitting is of exchange origin, can reach giant eV values, and can preserve spin quantum number.
In this talk, we will discuss the basic properties of this new type of antiferromagnetic spin splitting, its local magnetic symmetry origin and symmetry criteria for its emergence and we will catalogue broad class of material candidates. Furthermore, we will show that this antiferromagnetic spin splitting generates a crystal Hall effect controllable via rearrangement of nonmagnetic atoms3. Finally, we will present an experimental discovery of crystal Hall effect in ruthenium dioxide antiferromagnet4.

[1] Šmejkal, L., Mokrousov, Y., Yan, B. & MacDonald, A. H. Topological antiferromagnetic spintronics. Nat. Phys. 14, 242 (2018).
[2] Šmejkal, L., Železný, J., Sinova, J. & Jungwirth, T. Electric Control of Dirac Quasiparticles by Spin-Orbit Torque in an Antiferromagnet. Phys. Rev. Lett. 118, 106402 (2017), arXiv (2016)
[3] Šmejkal, L., González-Hernández, R., Jungwirth, T. & Sinova, J. Crystal time-reversal symmetry breaking and spontaneous Hall effect in collinear antiferromagnets. Sci. Adv. 6, eaaz8809 (2020), arXiv (2019)
[4] Feng, Z., Zhou, X., Šmejkal, L. et al. Observation of the Crystal Hall Effect in a Collinear Antiferromagnet. arXiv (2020)