Mike Alexander Lund

• Lund, Mike Alexander; Rodrigues, Davi R.; Everschor-Sitte, Karin & Hals, Kjetil Magne Dørheim (2023). Voltage-Controlled High-Bandwidth Terahertz Oscillators Based on Antiferromagnets. Physical Review Letters. ISSN 0031-9007. 131(15). doi: 10.1103/PhysRevLett.131.156704. Full text in Research Archive Show summary

  Producing compact voltage-controlled frequency generators and sensors operating in the terahertz (THz) regime represents a major technological challenge. Here, we show that noncollinear antiferromagnets (NCAFM) with kagome structure host gapless self-oscillations whose frequencies are tunable from 0 Hz to the THz regime via electrically induced spin-orbit torques (SOTs). The auto-oscillations' initiation, bandwidth, and amplitude are investigated by deriving an effective theory, which captures the reactive and dissipative SOTs. We find that the dynamics strongly depends on the ground state's chirality, with one chirality having gapped excitations, whereas the opposite chirality provides gapless self-oscillations. Our results reveal that NCAFMs offer unique THz functional components, which could play a significant role in filling the THz technology gap.

• Lund, Mike Alexander; Salimath, Akshaykumar & Hals, Kjetil Magne Dørheim (2021). Spin pumping in noncollinear antiferromagnets. Physical review B (PRB). ISSN 2469-9950. 104(17). doi: 10.1103/PhysRevB.104.174424. Show summary

  The ac spin pumping of noncollinear antiferromagnets is theoretically investigated. Starting from an effective action description of the spin system, we derive the Onsager coefficients connecting the spin pumping and spin-transfer torque associated with the dynamics of the SO(3)-valued antiferromagnetic order parameter. Our theory is applied to a kagome antiferromagnet resonantly driven by a uniform external magnetic field. We demonstrate that the reactive (dissipative) spin-transfer torque parameter can be extracted from the pumped ac spin current in-phase (in quadrature) with the driving field. Furthermore, we find that the three spin-wave bands of the kagome AF generate spin currents with mutually orthogonal polarization directions. This offers a unique way of controlling the spin orientation of the pumped spin current by exciting different spin-wave modes.

• Troncoso, Roberto; Lund, Mike Alexander; Brataas, Arne & Kamra, Akashdeep (2021). Cross-sublattice spin pumping and magnon level attraction in van der Waals antiferromagnets. Physical review B (PRB). ISSN 2469-9950. 103(14). doi: 10.1103/PhysRevB.103.144422.
• Lund, Mike Alexander; Everschor-Sitte, Karin & Hals, Kjetil Magne Dørheim (2020). Large surface magnetization in noncentrosymmetric antiferromagnets. Physical review B (PRB). ISSN 2469-9950. 102(18). doi: 10.1103/PhysRevB.102.180412. Show summary

  Thin-film antiferromagnets (AFs) with Rashba spin-orbit coupling are theoretically investigated. We demonstrate that the relativistic Dzyaloshinskii-Moriya interaction (DMI) produces a large surface magnetization and a boundary-driven twist state in the antiferromagnetic Néel vector. We predict a magnetization on the order of 2.3×10^4 A/m, which is comparable to the magnetization of ferromagnetic semiconductors. Importantly, the magnetization is characterized by ultrafast terahertz dynamics and provides different approaches for efficiently probing and controlling the spin dynamics of AFs as well as detecting the antiferromagnetic DMI. Notably, the magnetization does not lead to any stray magnetic fields except at the corners where weak magnetic monopole fields appear

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• Lund, Mike Alexander; Rodrigues, Davi R.; Everschor-Sitte, Karin & Hals, Kjetil Magne Dørheim (2024). Voltage-Controlled High-Bandwidth Terahertz Oscillators Based On Antiferromagnets. Show summary

  The terahertz (THz) technology gap refers to a frequency range of electromagnetic radiation in the THz regime where current technologies are inefficient for generating and detecting radiation. Here, we show that noncollinear antiferromagnets (NCAFM) with kagome structure host gapless self-oscillations whose frequencies are tunable from 0 Hz to the THz regime via electrically induced spin-orbit torques (SOTs) [1]. The auto-oscillations' initiation, bandwidth, and amplitude are investigated by deriving an effective theory, which captures the reactive and dissipative SOTs. We find that the dynamics strongly depends on the ground state's chirality, with one chirality having gapped excitations, whereas the opposite chirality provides gapless self-oscillations. Our results demonstrate that NCAFMs offer unique THz functional components, which could play a significant role in bridging the gap between technologies operating in the microwave and infrared regions. [1] M. A. Lund, D. R. Rodrigues, K. Everschor-Sitte, and K. M. D. Hals, Phys. Rev. Lett. 131, 156704 (2023).

• Hals, Kjetil Magne Dørheim; Lund, Mike Alexander; Everschor-Sitte, Karin & Rodrigues, Davi R. (2023). Voltage-Controlled High-Bandwidth Terahertz Oscillators Based On Antiferromagnets. Show summary

  The terahertz (THz) technology gap refers to a frequency range of electromagnetic radiation in the THz regime where current technologies are inefficient for generating and detecting radiation. Here, we show that noncollinear antiferromagnets (NCAFM) with kagome structure host gapless self-oscillations whose frequencies are tunable from 0 Hz to the THz regime via electrically induced spin-orbit torques (SOTs). The auto-oscillations' initiation, bandwidth, and amplitude are investigated by deriving an effective theory, which captures the reactive and dissipative SOTs. We find that the dynamics strongly depends on the ground state's chirality, with one chirality having gapped excitations, whereas the opposite chirality provides gapless self-oscillations. Our results demonstrate that NCAFMs offer unique THz functional components, which could play a significant role in bridging the gap between technologies operating in the microwave and infrared regions. *We acknowledge funding from the Research Council of Norway Project No. 286889, the German Research Foundation (DFG) Project No. 320163632 and the TRR 173 – 268565370 Spin + X (project B12).

• Lund, Mike Alexander; Salimath, Akshaykumar & Hals, Kjetil Magne Dørheim (2023). Spin pumping in noncollinear antiferromagnets. Show summary

  The spin pumping and spin-transfer torque (STT) mechanisms in antiferromagnets have been theoretically and experimentally investigated in recent years. However, most of these works have concentrated on collinear antiferromagnets, leaving the spin dynamics of the more complex noncollinear antiferromagnets largely unexplored. In this talk, I will present our latest work [1] on ac spin pumping in non-collinear antiferromagnets. Starting from an effective theory of the spin system, we derive the Onsager coefficients connecting the spin pumping and STT associated with the dynamics of the SO(3)-valued antiferromagnetic order parameter. Our theory is applied to a kagome AFM resonantly driven by a uniform external magnetic field. We demonstrate that the reactive (dissipative) STT parameter can be extracted from the pumped ac spin-current in phase (in quadrature) with the driving field. Furthermore, we find that the three spin-wave bands of the kagome AFM generate spin currents with mutually orthogonal polarization directions. This offers a unique way of controlling the spin orientation of the pumped spin current by exciting different spin-wave modes. [1] M. A. Lund, A. Salimath, K. M. D. Hals, Phys. Rev. B 104, 174424 (2021). *We acknowledge funding from the Research Council of Norway Project No. 286889

• Lund, Mike Alexander; Salimath, Akshaykumar & Hals, Kjetil Magne Dørheim (2022). Spin pumping in noncollinear antiferromagnets.
• Lund, Mike Alexander; Everschor-Sitte, Karin & Hals, Kjetil Magne Dørheim (2021). Large Surface Magnetization in Noncentrosymmetric Antiferromagnets. Show summary

  Thin-film antiferromagnets (AFs) with Rashba spin-orbit coupling are theoretically investigated. We demonstrate that the relativistic Dzyaloshinskii-Moriya interaction (DMI) produces a large surface magnetization and a boundary-driven twist state in the antiferromagnetic N´eel vector. We predict a magnetization on the order of 2.3 · 10^4 A/m, which is comparable to the magnetization of ferromagnetic semiconductors. Importantly, the magnetization is characterized by ultra-fast terahertz dynamics and provides new approaches for efficiently probing and controlling the spin dynamics of AFs as well as detecting the antiferromagnetic DMI. Notably, the magnetization does not lead to any stray magnetic fields except at corners where weak magnetic monopole fields appear

View all works in Cristin