Manipulating Spins in Antiferromagnets with External Forces

This thesis presents five papers on theoretical spintronics. Four of the papers describe antiferromagnets and the interactions between the magnetic order parameter and external forces induced by charge currents, spin waves, magnetic fields, and stochastic temperature fluctuations. The fifth paper studies the ultrafast magnetic response of a ferromagnet after rapid heating by a laser pulse. A significant part of the presented work is devoted to describing dynamics of the antiferromagnetic order parameter. To this end, we develop the conceptually simple collective coordinate equations of motion and demonstrate that the dynamics of antiferromagnetic textures are equivalent to the inertial motion of classical particles subject to external forces and dissipation-induced friction. We apply the collective coordinate method to describe the dynamics of antiferromagnetic domain walls. First, we study a domain wall under the influence of a charge current. Second, we describe the interactions between a domain wall and antiferromagnetic spin waves and show that the resulting domain wall dynamics depend crucially on the polarization of the spin waves. Third, we demonstrate the possibility of accurately controlling the position of an antiferromagnetic domain wall using external magnetic fields. The interactions between inhomogeneous magnetic fields and antiferromagnetic domain walls result from the intrinsic magnetization of textures in the order parameter. By applying the fluctuation-dissipation theorem, we include finite temperature effects to the dynamic equations of motion for a homogeneous antiferromagnet. We calculate that the heat current between an antiferromagnetic insulator and an adjacent normal metal is substantial and carried by a staggered spin current. We complete the transition to the high-temperature regime by developing a theory of out-of-equilibrium ultrafast spin dynamics in a ferromagnetic metal based on electron-magnon scattering. The theory includes the effects from a nonthermalized magnon distribution function and the out-of-equilibrium spin accumulation among the itinerant electrons