Designing Rashba–Dresselhaus effect in magnetic insulators

Abstract

One of the major strategies to control magnetism in spintronics is to utilize the coupling between electron spin and its orbital motion. The Rashba and Dresselhaus spin-orbit couplings induce magnetic textures of band electrons called spin momentum locking, which produces a spin torque by the injection of electric current. However, joule heating had been a bottleneck for device applications. Here, we propose a theory to generate further rich spin textures in insulating antiferromagnets with broken spatial inversion symmetry (SIS), which is easily controlled by a small magnetic field. In antiferromagnets, the ordered moments host two species of magnons that serve as internal degrees of freedom in analogy with electron spins. The Dzyaloshinskii-Moriya interaction introduced by the SIS breaking couples the two-magnon-degrees of freedom with the magnon momentum. We present a systematic way to design such texture and to detect it via magnonic spin current for the realization of antiferromagnetic memory.Development of tunable magnetic structure has long been a key issue for detecting and controlling magnetic domains electrically toward application to memory devices 1 . Besides the conventional domain walls that appear in real space, particular focus is given on emergent spin textures in reciprocal space, called "spin momentum locking". The spin textures are classified into Rashba-2-4 and Dresselhaus-types 5 that exhibit vortex-and antivortex geometries along the closed Fermi surfaces. Since the wave number k distinguishes the electronic state of matter, such spin texture allows for the selection of magnetic moment the state/current carries. This has brought about fundamentally important and technologically promising phenomena including spin Hall effect 6-9 , spin-orbit torque 10,11 , and Rashba-Edelstein effect [12][13][14] .In insulating magnets, an excitation is carried by the quasiparticle called magnon, which represents a quantum mechanical spin precession propagating in space. Such propagation is predominantly mediated by the standard magnetic exchange interaction JS i · S j , between spins, S i and S j . In a uniform ferromagnet, a simple exchange interaction, J(< 0), generates non-degenerate quadratic magnon bands. When the spatial inversion symmetry (SIS) is broken, an antisymmetric spin exchange called Dzyaloshinskii-Moriya (DM) interaction 15,16 , D · (S i × S j ), appears, depending on the crystal symmetry. This term bends the propagation of magnons in space in a similar manner to the cyclotron motion of electrons in the presence of magnetic flux 17 . Thus, when D is parallel to the magnetization, magnon bands in a ferromagnet become asymmetric, reflecting the "nonreciprocal" propagation 18-25 . Nevertheless, the phenomena related to magnons in non-centrosymmetric ferro or ferrimagnets lacks abundance compared to the rich counterparts of the conducting Rashba electrons. This is because the ferromagnetic magnons carry spins that are pointing in a unique direction, and cannot afford up and down spin degrees of...

Observation of Magnon Polarization

“…This "magnon squeezing" [33] and an enhancement of domain wall velocity [34] may be essential for applications of magnets in quantum information and can be measured by this technique. Our technique is also applicable for antiferromagnetic spin textures in reciprocal space [35], although there are some restrictions by the scattering geometry.…”

Bulk Rashba effect in multiferroics: A theoretical prediction for BiCoO3

“…In this case, bulk Rashba effect can emerge. 15 This possibility ultimately relies on the fact that time-reversal symmetry, which is always broken in a ferromagnet, can still form a symmetry element of the relevant magnetic space group in antiferromagnets in suitable combinations with spatial symmetry operations. Following this idea, in this paper, the electronic structure, the spin texture, and its correlation with spin canting in a noncentrosymmetric antiferromagnetic oxide will be discussed.…”

Discovering momentum-dependent magnon spin texture in insulating antiferromagnets: Role of the Kitaev interaction

“…If the bosonic BdG Hamiltonian satisfies, [H BdG (k), J µ ] = 0 (µ = x, y, z), the system has a U(1) symmetry about the µ-axis, and such a symmetry puts restriction on magnon bands and magnon spin textures in momentum space 48 .…”

“…Then, the DM interaction couples the pseudo-spins with the kinetic motion of magnons in a similar manner to the SOC that couples the spins and the momentum of electrons. Indeed, various tunable spin textures on magnon bands in momentum space are found in 1D 47 and square-lattice antiferromagnets 48 , which are the models of Ba 2 XGe 2 O 7 (X=Co, Mn) [49][50][51][52][53][54][55][56] ; it can be regarded as a magnonic Rashba-Dresselhaus effect. Also, on a honeycomb lattice, a spin Nernst effect occurs by the DM interaction [57][58][59][60][61][62] whose origin can be understood in a similar context.…”