Kjetil Magne Dørheim Hals

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Nothhelfer, Jonas; Díaz, Sebastián A.; Kessler, Stephan; Meng, Tobias; Rizzi, Matteo & Hals, Kjetil Magne Dørheim [Vis alle 7 forfattere av denne artikkelen] (2022). Steering Majorana braiding via skyrmion-vortex pairs: A scalable platform. Physical review B (PRB). ISSN 2469-9950. 105(22). doi: 10.1103/PhysRevB.105.224509. Vis sammendrag

  Majorana zero modes are quasiparticles that hold promise as building blocks for topological quantum computing. However, the litmus test for their detection, the observation of exotic non- abelian statistics revealed by braiding, has so far eluded experimental efforts. Here we take advantage of the fact that skyrmion-vortex pairs in superconductor-ferromagnet heterostructures harboring Majorana zero modes can be easily manipulated in two spatial dimensions. We adiabatically braid the hybrid topological structures and explicitly confirm the non-abelian statistics of the Majorana zero modes numerically using a self-consistent calculation of the superconducting order parameter. Our proposal of controlling skyrmion-vortex pairs provides the necessary leeway toward a scalable topological quantum computing platform.

• Rodrigues, Davi R.; Salimath, Akshaykumar; Everschor-Sitte, Karin & Hals, Kjetil Magne Dørheim (2022). Dzyaloshinskii-Moriya induced spin-transfer torques in kagome antiferromagnets. Physical review B (PRB). ISSN 2469-9950. 105(17). doi: 10.1103/PhysRevB.105.174401. Fulltekst i vitenarkiv Vis sammendrag

  In recent years antiferromagnets (AFMs) have become very promising for nanoscale spintronic applications due to their unique properties, such as THz dynamics and the absence of stray fields. Manipulating antiferromagnetic textures is currently, however, limited to very few exceptional material symmetry classes allowing for staggered torques on the magnetic sublattices. In this work, we predict for kagome AFMs with broken mirror symmetry a new coupling mechanism between antiferromagnetic domain walls (DWs) and spin currents, produced by the relativistic Dzyaloshinskii-Moriya interaction (DMI). We microscopically derive the DMI's free-energy contribution for the kagome AFMs. Unlike ferromagnets and collinear AFMs, the DMI does not lead to terms linear in the spatial derivatives, but instead renormalizes the spin-wave stiffness and anisotropy energies. Importantly, we show that the DMI induces a highly nontrivial, twisted DW profile that is controllable via two linearly independent components of the spin accumulation. This texture manipulation mechanism goes beyond the concept of staggered torques and implies a higher degree of tunability for the current-driven DW motion compared to conventional ferromagnets and collinear AFMs.

• Rodrigues, Davi; Salimath, Akshaykumar; Everschor-Sitte, Karin & Hals, Kjetil Magne Dørheim (2021). Spin-Wave Driven Bidirectional Domain Wall Motion in Kagome Antiferromagnets. Physical Review Letters. ISSN 0031-9007. 127(15). doi: 10.1103/PhysRevLett.127.157203. Vis sammendrag

  We predict a mechanism to controllably manipulate domain walls in kagome antiferromagnets via a single linearly polarized spin-wave source. We show by means of atomistic spin dynamics simulations of antiferromagnets with kagome structure that the speed and direction of the domain wall motion can be regulated by only tuning the frequency of the applied spin-wave. Starting from microscopics, we establish an effective action and derive the corresponding equations of motion for the spin-wave-driven domain wall. Our analytical calculations reveal that the coupling of two spin-wave modes inside the domain wall explains the frequency-dependent velocity of the spin texture. Such a highly tunable spin-wave-induced domain wall motion provides a key component toward next-generation fast, energy-efficient, and Joule-heating-free antiferromagnetic insulator devices.

• 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. Vis sammendrag

  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.

• 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. Vis sammendrag

  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

• Hals, Kjetil Magne Dørheim & Everschor-Sitte, Karin (2019). Twists in ferromagnetic monolayers with trigonal prismatic symmetry. Physical review B (PRB). ISSN 2469-9950. 99(10). doi: 10.1103/PhysRevB.99.104422. Fulltekst i vitenarkiv
• Mulkers, Jeroen; Hals, Kjetil Magne Dørheim; Leliaert, Jonathan; Milosevic, Milorad V; Van Waeyenberge, Bartel & Everschor-Sitte, Karin (2018). Effect of boundary-induced chirality on magnetic textures in thin films. Physical review B (PRB). ISSN 2469-9950. 98(6). doi: 10.1103/PhysRevB.98.064429.
• Hals, Kjetil Magne Dørheim & Everschor-Sitte, Karin (2017). New Boundary-Driven Twist States in Systems with Broken Spatial Inversion Symmetry. Physical Review Letters. ISSN 0031-9007. 119(127203). doi: 10.1103/PhysRevLett.119.127203.
• Shulz, Thomek; Alejos, Oscar; Martinez, Eduardo; Hals, Kjetil Magne Dørheim; Garcia, Karin & Vila, Laurent [Vis alle 14 forfattere av denne artikkelen] (2015). Spin-orbit torques for current parallel and perpendicular to a domain wall. Applied Physics Letters. ISSN 0003-6951. 107(12). doi: 10.1063/1.4931429. Vis sammendrag

  We report field- and current-induced domain wall (DW) depinning experiments in Ta/Co20Fe60B20/MgO nanowires through a Hall cross geometry. While purely field-induced depinning shows no angular dependence on in-plane fields, the effect of the current depends crucially on the internal DW structure, which we manipulate by an external magnetic in-plane field. We show for the first time depinning measurements for a current sent parallel to the DW and compare its depinning efficiency with the conventional case of current flowing perpendicularly to the DW. We find that the maximum efficiency is similar for both current directions within the error bars, which is in line with a dominating damping-like spin-orbit torque (SOT) and indicates that no large additional torques arise for currents parallel to the DW. Finally, we find a varying dependence of the maximum depinning efficiency angle for different DWs and pinning levels. This emphasizes the importance of our full angular scans compared to previously used measurements for just two field directions (parallel and perpendicular to the DW) and shows the sensitivity of the spin-orbit torque to the precise DW structure and pinning sites.

• Hals, Kjetil Magne Dørheim & Brataas, Arne (2015). Spin-motive forces and current-induced torques in ferromagnets. Physical Review B. Condensed Matter and Materials Physics. ISSN 1098-0121. 91(21). doi: 10.1103/PhysRevB.91.214401.
• Ciccarelli, Chiara; Hals, Kjetil Magne Dørheim; Irvine, Andrew; Novak, Vit; Tserkovnyak, Y & Kurebayashi, Hidekazu [Vis alle 8 forfattere av denne artikkelen] (2015). Magnonic charge pumping via spin-orbit coupling. Nature Nanotechnology. ISSN 1748-3387. 10, s. 50–54. doi: 10.1038/nnano.2014.252.
• Yuan, Zhe; Hals, Kjetil Magne Dørheim; Liu, Yi; Starikov, Anton A.; Brataas, Arne & Kelly, Paul K. (2014). Gilbert Damping in Noncollinear Ferromagnets. Physical Review Letters. ISSN 0031-9007. 113(26). doi: 10.1103/PhysRevLett.113.266603.
• Hals, Kjetil Magne Dørheim & Brataas, Arne (2014). Spin-orbit torques and anisotropic magnetization damping in skyrmion crystals. Physical Review B. Condensed Matter and Materials Physics. ISSN 1098-0121. 89(6). doi: 10.1103/PhysRevB.89.064426.
• Hals, Kjetil Magne Dørheim & Berre, Inga (2013). Thermal fracturing of geothermal wells and effects of borehole orientation. I Horne, Roland (Red.), Proceedings of the 38th Workshop on Geothermal Reservoir Engineering 2013. Curran Associates, Inc.. ISSN 9781627485708.
• Hals, Kjetil Magne Dørheim & Brataas, Arne (2013). Phenomenology of current-induced spin-orbit torques. Physical Review B. Condensed Matter and Materials Physics. ISSN 1098-0121. 88(8). doi: 10.1103/PhysRevB.88.085423.
• Hals, Kjetil Magne Dørheim & Brataas, Arne (2013). Spin-transfer torques in helimagnets. Physical Review B. Condensed Matter and Materials Physics. ISSN 1098-0121. 87(17:174409). doi: 10.1103/PhysRevB.87.174409. Vis sammendrag

  We theoretically investigate current-induced magnetization dynamics in chiral-lattice helimagnets. Spin-orbit coupling in noncentrosymmetric crystals induces a reactive spin-transfer torque that has not been previously considered. We demonstrate how the torque is governed by the crystal symmetry and acts as an effective magnetic field along the current direction in cubic B20-type crystals. The effects of the new torque are computed for current-induced dynamics of spin spirals and the Doppler shift of spin waves. In current-induced spin-spiral motion, the new torque tilts the spiral structure. The spin waves of the spiral structure are initially displaced by the new torque, while the dispersion relation is unaffected.

• Hals, Kjetil Magne Dørheim & Berre, Inga (2012). Interaction between injection points during hydraulic fracturing. Water Resources Research. ISSN 0043-1397. 48. doi: 10.1029/2012WR012265.
• Hals, Kjetil Magne Dørheim & Brataas, Arne (2011). Magnetization dissipation in the ferromagnetic semiconductor (Ga,Mn)As. Physical Review B. Condensed Matter and Materials Physics. ISSN 1098-0121. 84(10). doi: 10.1103/PhysRevB.84.104404.
• Hals, Kjetil Magne Dørheim; Tserkovnyak, Yaroslav & Brataas, Arne (2011). Phenomenology of Current-Induced Dynamics in Antiferromagnets. Physical Review Letters. ISSN 0031-9007. 106(10). doi: 10.1103/PhysRevLett.106.107206.
• Hals, Kjetil Magne Dørheim; Nguyen, Kiet Anh; Waintal, Xavier & Brataas, Arne (2010). Effective Magnetic Monopoles and Universal Conductance Fluctuations. Physical Review Letters. ISSN 0031-9007. 105(20). doi: 10.1103/PhysRevLett.105.207204.
• Hals, Kjetil Magne Dørheim; Brataas, Arne & Tserkovnyak, Y (2010). Scattering theory of charge-current-induced magnetization dynamics. Europhysics letters. ISSN 0295-5075. 90(4). doi: 10.1209/0295-5075/90/47002.
• Hals, Kjetil Magne Dørheim; Brataas, Arne & Bauer, Gerrit E. W. (2010). Thermopower and thermally induced domain wall motion in (Ga, Mn)As. Solid State Communications. ISSN 0038-1098. 150(11-12), s. 461–465. doi: 10.1016/j.ssc.2010.01.023. Vis sammendrag

  We study two reciprocal thermal effects in the ferromagnetic semiconductor (Ga, Mn)As by scattering theory: domain wall motion induced by a temperature gradient and heat currents pumped by a moving domain wall. The effective out-of-plane thermal spin transfer torque parameter PQβQ, which governs the coupling between heat currents and a magnetic texture, is found to be of the order of unity. Unpinned domain walls are predicted to move at speed 10 m/s in temperature gradients of the order 10 K/μm. The cooling power of a moving domain wall only compensates the heating due to friction losses at ultra-low domain wall velocities of about 0.07 m/s. The Seebeck coefficient is found to be of the order 100–500 μV/K at View the MathML source, in good agreement with recent experiments.

• Hals, Kjetil Magne Dørheim; Nguyen, Kiet Anh & Brataas, Arne (2009). Intrinsic Coupling between Current and Domain Wall Motion in (Ga,Mn)As. Physical Review Letters. ISSN 0031-9007. 102(25). doi: 10.1103/PhysRevLett.102.256601. Vis sammendrag

  We consider current-induced domain wall motion and, the reciprocal process, moving domain wall-induced current. The associated Onsager coefficients are expressed in terms of scattering matrices. Uncommonly, in (Ga,Mn)As, the effective Gilbert damping coefficient αw and the effective out-of-plane spin-transfer torque parameter βw are dominated by spin-orbit interaction in combination with scattering off the domain wall, and not scattering off extrinsic impurities. Numerical calculations give αw∼0.01 and βw∼1 in dirty (Ga,Mn)As. The extraordinarily large βw parameter allows experimental detection of current or voltage induced by domain wall motion in (Ga,Mn)As.

Se alle arbeider i Cristin

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

  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. Vis sammendrag

  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

• Hals, Kjetil Magne Dørheim; Schecter, Michael & Rudner, Mark (2022). Composite topological excitations.
• Hals, Kjetil Magne Dørheim; Salimath, Akshaykumar; Rodrigues, Davi R. & Everschor-Sitte, Karin (2022). Spin-wave driven bidirectional domain wall motion in kagome antiferromagnets. Vis sammendrag

  We predict a mechanism to controllably manipulate domain walls in kagome antiferromagnets via a single linearly polarized spin-wave source. We show by means of atomistic spin dynamics simulations of antiferromagnets with kagome structure that the speed and direction of the domain wall motion can be regulated by only tuning the frequency of the applied spin-wave. Starting from microscopics, we establish an effective action and derive the corresponding equations of motion for the spin-wave-driven domain wall. Our analytical calculations reveal that the coupling of two spin-wave modes inside the domain wall explains the frequency-dependent velocity of the spin texture. Such a highly tunable spin-wave-induced domain wall motion provides a key component toward next-generation fast, energy-efficient, and Joule-heating-free antiferromagnetic insulator devices.

• Lund, Mike Alexander; Salimath, Akshaykumar & Hals, Kjetil Magne Dørheim (2022). Spin pumping in noncollinear antiferromagnets.
• Diaz, Sebastian A; Hals, Kjetil Magne Dørheim; Meng, Tobias; Rizzi, Matteo; Nothhelfer, Jonas & Everschor-Sitte, Karin [Vis alle 7 forfattere av denne artikkelen] (2022). Steering Majorana braiding via skyrmion-vortex pairs: a scalable platform. Vis sammendrag

  Majorana zero modes are quasiparticles that hold promise as building blocks for topological quantum computing. However, the litmus test for their detection, the observation of exotic non-abelian statistics revealed by braiding, has so far eluded experimental efforts. Here we take advantage of the fact that skyrmion-vortex pairs in superconductor-ferromagnet heterostructures harboring Majorana zero modes can be easily manipulated in two spatial dimensions. We adiabatically braid the hybrid topological structures and explicitly confirm the non-abelian statistics of the Majorana zero modes numerically using a self-consistent calculation of the superconducting order parameter. Our proposal of controlling skyrmion-vortex pairs provides the necessary leeway toward a scalable topological quantum computing platform. [1] J. Nothhelfer, K. M. D. Hals, K. Everschor-Sitte, and M. Rizzi, European Patent Application EP3751472A1 / International Patent Application WO2020EP66120 (2019).

• Salimath, Akshaykumar; Rodrigues, David; Everschor-Sitte, Karin & Hals, Kjetil Magne Dørheim (2021). Magnon driven bidirectional domain wall motion in kagome antiferromagnets. Vis sammendrag

  We predict a mechanism to controllably manipulate domain walls in kagome antiferromagnets via a single linearly polarized spin-wave source. We show by means of atomistic spin dynamics simulations of antiferromagnets with kagome structure that the speed and direction of the domain wall motion can be regulated by only tuning the frequency of the applied spin wave. Starting from microscopics, we establish an effective action and derive the corresponding equations of motion for the spin-wave-driven domain wall. Our analytical calculations reveal that the coupling of two spin-wave modes inside the domain wall explains the frequency-dependent velocity of the spin texture. Such a highly tunable spin-wave-induced domain wall motion provides a key component toward next-generation fast, energy-efficient, and Joule-heating-free antiferromagnetic insulator devices.

• Lund, Mike Alexander; Everschor-Sitte, Karin & Hals, Kjetil Magne Dørheim (2021). Large Surface Magnetization in Noncentrosymmetric Antiferromagnets. Vis sammendrag

  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

• Mike Alexander, Lund; Hals, Kjetil Magne Dørheim & Karin, Everschor-Sitte (2020). Large Surface Magnetization in Noncentrosymmetric Antiferromagnets.
• Hals, Kjetil Magne Dørheim (2018). s Composite topological excitations in superconductor-ferromagnet heterostructures.
• Hals, Kjetil Magne Dørheim (2018). Composite Topological Excitations in Ferromagnet-Superconductor Heterostructures.
• Brataas, Arne; Hals, Kjetil Magne Dørheim; Waintal, Xavier & Nguyen, Anh Kiet (2011). Magnetic Monopole Fingerprints.
• Hals, Kjetil Magne Dørheim; Brataas, Arne & Tserkovnyak, Y (2011). Phenomenology of Current-Induced Dynamics in Antiferromagnets.
• Brataas, Arne; Hals, Kjetil Magne Dørheim; Waintal, Xavier & Nguyen, Anh Kiet (2011). Magnetic Monopole Fingerprints.
• Brataas, Arne; Hals, Kjetil Magne Dørheim; Waintal, Xavier & Nguyen, Anh Kiet (2011). Magnetic Monopole Fingerprints.
• Hals, Kjetil Magne Dørheim; Brataas, Arne & Tserkovnyak, Y (2011). Phenomenology of Current-Induced Dynamics in Antiferromagnets.
• Brataas, Arne; Hals, Kjetil Magne Dørheim; Waintal, Xavier & Nguyen, Anh Kiet (2011). Magnetic Monopole Fingerprints.
• Brataas, Arne; Hals, Kjetil Magne Dørheim; Waintal, Xavier & Nguyen, Anh Kiet (2011). Magnetic Monopole Fingerprints.

Se alle arbeider i Cristin