Spin-orbit torques and their associated effective fields from gigahertz to terahertz

Guimarães, Filipe S. M.; Bouaziz, Juba; Dias, Manuel dos Santos; Lounis, Samir

doi:10.1038/s42005-020-0282-x

Cited by 9 publications

(17 citation statements)

References 58 publications

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“…Since previous theoretical models have been developed assuming a restricted setup and evaluated only specific contributions to the torque [22,27], it is difficult to compare magnitudes of different contributions directly. However, first-principles approaches often evaluate the total torque from linear response theory [35][36][37][38][39][40], which also makes it difficult to assess contributions by different mechanisms quantitatively. Thus, it is necessary to develop an unified theory within which different mechanisms of the current-induced torque are classified and can be separately evaluated for a given system.…”

Section: Introductionmentioning

confidence: 99%

Theory of current-induced angular momentum transfer dynamics in spin-orbit coupled systems

Freimuth

Hanke

et al. 2020

Phys. Rev. Research

104

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Motivated by the importance of understanding various competing mechanisms to the current-induced spinorbit torque on magnetization in complex magnets, we develop a theory of current-induced spin-orbital coupled dynamics in magnetic heterostructures. The theory describes angular momentum transfer between different degrees of freedom in solids, e.g., the electron orbital and spin, the crystal lattice, and the magnetic order parameter. Based on the continuity equations for the spin and orbital angular momenta, we derive equations of motion that relate spin and orbital current fluxes and torques describing the transfer of angular momentum between different degrees of freedom, achieved in a steady state under an applied external electric field. We then propose a classification scheme for the mechanisms of the current-induced torque in magnetic bilayers. We evaluate the sources of torque using density functional theory, effectively capturing the impact of the electronic structure on these quantities. We apply our formalism to two different magnetic bilayers, Fe/W(110) and Ni/W(110), which are chosen such that the orbital and spin Hall effects in W have opposite sign and the resulting spin-and orbital-mediated torques can compete with each other. We find that while the spin torque arising from the spin Hall effect of W is the dominant mechanism of the current-induced torque in Fe/W(110), the dominant mechanism in Ni/W(110) is the orbital torque originating in the orbital Hall effect of the nonmagnetic substrate. Thus, the effective spin Hall angles for the total torque are negative and positive in the two systems. Our prediction can be experimentally identified in moderately clean samples, where intrinsic contributions dominate. This clearly demonstrates that our formalism is ideal for studying the angular momentum transfer dynamics in spin-orbit coupled systems as it goes beyond the "spin current picture" by naturally incorporating the spin and orbital degrees of freedom on an equal footing. Our calculations reveal that, in addition to the spin and orbital torque, other contributions such as the interfacial torque and self-induced anomalous torque within the ferromagnet are not negligible in both material systems.

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Section: Introductionmentioning

confidence: 99%

Theory of current-induced angular momentum transfer dynamics in spin-orbit coupled systems

Freimuth

Hanke

et al. 2020

Phys. Rev. Research

104

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“…Theory We utilize a multi-orbital tight-binding Hamiltonian that takes into account the electronelectron interaction through a Hubbard like term and the spin-orbit interaction, as implemented on the TITAN code to investigate dynamics of transport and angular momentum properties in nanostructures [46][47][48][49] . To describe the interaction of a laser pulse with the system, we include a timedependent electric field described by a vector potential A(t) = − E(t)dt.…”

Section: Methodsmentioning

confidence: 99%

Polarisation-dependent single-pulse ultrafast optical switching of an elementary ferromagnet

Hamamera

Guimarães

Dias

et al. 2021

Preprint

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The ultimate control of magnetic states of matter at femtosecond (or even faster) timescales defines one of the most pursued paradigm shifts for future information technology. In this context, ultrafast laser pulses developed into extremely valuable stimuli for the all-optical magnetisation reversal in ferrimagnetic and ferromagnetic alloys and multilayers, while this remains elusive in elementary ferromagnets. Here we demonstrate that a single laser pulse with sub-picosecond duration can lead to the reversal of the magnetisation of bulk nickel, in tandem with the expected demagnetisation. As revealed by realistic time-dependent electronic structure simulations, the central mechanism is ultrafast light-induced torques acting on the magnetisation, which are only effective if the laser pulse is circularly polarised on a plane that contains the initial orientation of the magnetisation. We map the laser pulse parameter space enabling the magnetisation switching and unveil rich intra-atomic orbital-dependent magnetisation dynamics featuring transient inter-orbital non-collinear states. Our findings open further perspectives for the efficient implementation of optically-based spintronic devices.

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“…The odd-number PSSW modes in the Ta/Py/Ta sample cannot be detected in our experimental geometry for the reasons discussed above. The assumed SOT efficiency µ 0 H FL /j = 10 −14 T A −1 m 2 is equivalent to about a µ 0 H FL = 10 mT field and is assumed to be similar to that in magnetotransport measurements 26,46 according to the theoretical predictions 27,28 . We note, however, that the exact verification of the SOT efficiency at terahertz frequencies requires further studies with calibrated Faraday angles using high magnetic fields.…”

Section: Articlementioning

confidence: 99%

“…23 ). Similarly, the SOT is inefficient for electrical fields orthogonal to the in-plane magnetization [26][27][28][29][30] , hence the SWR signal was not detected. We attribute the small ZTS visible in Fig.…”

mentioning

confidence: 99%

“…Here we overcome this obstacle and demonstrate an efficient and coherent excitation of nanometre-wavelength spin-wave modes in Ta/Py/Pt (where Py is Ni 81 Fe 19 ) trilayers using a broadband terahertz pulse with about 1 ps duration. Unlike the above-mentioned excitation schemes, the coupling of the terahertz radiation to the spin waves is realized via the spin-orbit torque (SOT) 26 , which has been predicted to be efficient even at terahertz frequencies 27,28 . We used a single-cycle terahertz pulse to induce a current in the Ta and Pt metallic layers.…”

mentioning

confidence: 99%

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Coupling of terahertz light with nanometre-wavelength magnon modes via spin–orbit torque

et al. 2023

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Spin-based technologies can operate at terahertz frequencies but require manipulation techniques that work at ultrafast timescales to become practical. For instance, devices based on spin waves, also known as magnons, require efficient generation of high-energy exchange spin waves at nanometre wavelengths. To achieve this, a substantial coupling is needed between the magnon modes and an electro-magnetic stimulus such as a coherent terahertz field pulse. However, it has been difficult to excite non-uniform spin waves efficiently using terahertz light because of the large momentum mismatch between the submillimetre-wave radiation and the nanometre-sized spin waves. Here we improve the light–matter interaction by engineering thin films to exploit relativistic spin–orbit torques that are confined to the interfaces of heavy metal/ferromagnet heterostructures. We are able to excite spin-wave modes with frequencies of up to 0.6 THz and wavelengths as short as 6 nm using broadband terahertz radiation. Numerical simulations demonstrate that the coupling of terahertz light to exchange-dominated magnons originates solely from interfacial spin–orbit torques. Our results are of general applicability to other magnetic multilayered structures, and offer the prospect of nanoscale control of high-frequency signals.

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Spin-orbit torques and their associated effective fields from gigahertz to terahertz

Cited by 9 publications

References 58 publications

Theory of current-induced angular momentum transfer dynamics in spin-orbit coupled systems

Theory of current-induced angular momentum transfer dynamics in spin-orbit coupled systems

Polarisation-dependent single-pulse ultrafast optical switching of an elementary ferromagnet

Coupling of terahertz light with nanometre-wavelength magnon modes via spin–orbit torque

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