Spin Dynamics in Antiferromagnetic Systems
Original version
Lund, M. A. (2025). Spin Dynamics in Antiferromagnetic Systems [Doctoral dissertation]. University of Agder.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.
Has parts
Paper I: Lund, M. A., Everschor-Sitte, K. & Hals, K. M. D. (2020). Large surface magnetization in noncentrosymmetric antiferromagnets. Physical review B (PRB), 102 (18). https://doi.org/10.1103/PhysRevB.102.180412 Published version. Full-text is available in AURA as a separate file: https://hdl.handle.net/11250/3153361Paper II: Lund, M. A., Salimath, A. & Hals, K. M. D. (2021). Spin pumping in noncollinear antiferromagnets. Physical review B (PRB), 104 (17). https://doi.org/10.1103/PhysRevB.104.174424 Published version. Full-text is available in AURA as a separate file: https://hdl.handle.net/11250/3159816
Paper III: Lund, M. A., Rodrigues, D. R., Everschor-Sitte, K. & Hals, K. M. D. (2023). Voltage-Controlled High-Bandwidth Terahertz Oscillators Based on Antiferromagnets. Physical Review Letters, 131(15),1-6. https://doi.org/10.1103/PhysRevLett.131.156704 Published version. Full-text is available in AURA as a separate file: https://hdl.handle.net/11250/3116388