Spin Dynamics in Antiferromagnetic Systems

Abstract

This dissertation presents three research papers on spintronics-related effects. Two of the papers discuss dynamic spin phenomena in noncollinear antiferromagnets (NCAFMs). The third paper investigates the equilibrium properties of collinear antiferromagnets (CAFMs) with broken spatial inversion symmetry. The most central part of the presented work discusses the spin dynamics of NCAFMs. To this end, we develop effective action descriptions that capture the dynamics of the SO(3)-valued antiferromagnetic order parameter in response to applied currents and magnetic fields. We first theoretically investigate the ac spin pumping of NCAFMs. Starting from an effective action description of the spin system, we derive the Onsager coefficients that represents the coupling between the NCAFM and spin currents. When our theory is applied to kagome AFMs, we show that the three spin-wave bands of the material can generate ac spin currents with mutually orthogonal polarization directions when driven into resonance by an external magnetic field. Additionally, we demonstrate that the reactive and dissipative STT parameters of the kagome AFM can be extracted from the spin signal measured via the Inverse spin-Hall effect. Second, we investigate magnetic self-oscillations driven by electrically induced spin-orbit torques in kagome AFMs with broken spatial inversion symmetry. We show that the chirality of the noncollinear antiferromagnetic ground state strongly influences the dynamics. One chirality displays gapped excitations, while the opposite chirality provides gapless self-oscillations whose frequencies can be tuned from 0 Hz to the terahertz regime. Thus, the NCAFMs offer unique chiral magnetic properties that makes them especially appealing for bridging the gap between technologies operating in the microwave and infrared regions. Motivated by recent observations of magnetic surface twist states induced by Dzyaloshinskii-Moriya interaction (DMI) in noncentrosymmetric ferromagnets, we study the effects of DMI at the boundaries of noncentrosymmetric CAFMs. We demonstrate that the DMI leads to a boundary-induced twist state in the antiferromagnetic order parameter and a large surface magnetization. The magnetization couples directly to the variation in the order parameter field. As a result, the surface magnetization demonstrates ultrafast THz dynamics and offers effective means to investigate and control the spin dynamics of AFMs and detect the antiferromagnetic DMI.