Abstract

It is demonstrated that in an antiferromagnetic metal a steady-state transport current generates a current-induced out-of-plane spin density, resulting in torques on the magnetization. This spin density is parameterized by a velocity that is proportional to the current. The generalization of the non-linear sigma model equation of motion for antiferromagnetic
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magnetization dynamics in an antiferromagnetic metal in the presence of a transport current is presented. From this equation of motion a current-induced shift of the spin-wave dispersion is found, as well as current-induced torques that lead to current-driven antiferromagnetic domain wall motion. A key finding is that the form of the current-induced spin density, expressed in terms of the N\'{e}\`{e}l vector is similar to the current-induced spin density in ferromagnets with the N\'{e}\`{e}l vector replaced by the magnetization direction. The velocity that characterizes the efficiency of the coupling between current and magnetization is calculated, using linear-response theory in the Boltzmann transport regime. In the absence of dissipation, current-driven antiferromagnetic domain walls are found to move with this characteristic velocity. When magnetization damping is included, the domain wall moves a finite amount and then stops, similar to the intrinsic pinning of current-driven ferromagnetic domain walls. Like in the latter case, including dissipative coupling or non-adiabatic effects between current and magnetization removes this intrinsic pinning. In magnetic insulators there is no transport of electronic charge. Still, there can be transport of spin in the form of spin waves, or, in their quantized form, magnons. Spin transport, carried by quasi-equilibrium magnons, in a magnetic insulator within a Boltzmann transport framework is considered. The spin resistivity of quasi-equilibrium magnons is found to be strongly reduced in comparison with equilibrium magnons, a property that may be useful in designing magnon spintronics applications. The contribution of magnon interactions to their resistivity has been studied for the reason that these dominate in the materials that are experimentally relevant, such as yttrium-iron-garnet (YIG). Firstly, the spin resistivity of a magnon gas using the simplest model, the Heisenberg model, is used. The investigation of material-specific models, in particular for YIG, is then presented. Despite the simplified model, the main qualitative result, the reduction of spin resistivity as a result of enhancing the magnon number by pumping, remains valid. This is because it only depends on the bosonic nature of the magnon excitations that leads to enhanced scattering into states that are already occupied
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