arXiv subjects

R. A. Duine

Publications and source records attributed to R. A. Duine.

At least 19 recordsLinked to original sources

arXiv2026

Spin Inertia as a Driver of Chaotic and High-Speed Ferromagnetic Domain Walls

Ferromagnetic domain walls -transitional regions between magnetic domains- are an essential ingredient for racetrack memory, a device concept that promises to deliver faster and more compact memory storage compared to other non-volatile memory devices. Motivated by recent experiments that have found inertial effects in spin dynamics, we explore its consequences on domain wall motion. We find that the inertial dynamics of the individual magnetic moments induce massive dynamics of the domain wall. We investigate these massive dynamics driven by a magnetic field, spin-transfer torque, and spin-orbit torque. We show that, in the absence of Gilbert damping, the domain wall dynamics become chaotic, resembling that of an electron in a two-dimensional crystal. For finite damping, field-like driving of the inertial domain wall significantly increases its velocity compared to conventional massless dynamics, potentially enabling faster racetrack operations. Additionally, in the limit of low driving, we observe that the domain wall width contracts due to the spin inertia of the ferromagnet.

cond-mat.mes-hall↗

arXiv2025

Thermal Spin Waves from Accelerating Domain Walls via the Unruh Effect

We consider a wire consisting of a conducting ferromagnetic layer and an insulating antiferromagnetic layer that are coupled. The ferromagnet hosts a domain wall, which is dynamically driven by a charge current. We show that for a specific time-dependent current, the domain wall moves according to a Rindler trajectory. This motion excites spin waves in the antiferromagnetic insulator, and their emission spectrum is characterised by an effective temperature analogous to the Unruh temperature, $T_U = \hbar a/2πc k_B$, with a the acceleration of the domain wall, c the maximum antiferromagnetic spin wave velocity, and kB the Boltzmann constant. This thermal signature is a direct consequence of the Unruh effect and could be experimentally observed. Our results establish magnetism as a promising platform for probing relativistic quantum field phenomena. Moreover, since the Unruh effect is inherently linked to entanglement, our proposal provides a route for entangling magnetic domain walls via relativistic effects.

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arXiv2025

RKKY quadratic and biquadratic spin-spin interactions in twisted bilayer graphene

We study the competition between the RKKY quadratic and biquadratic spin-spin interactions of two magnetic impurities in twisted bilayer graphene away from the magic angle. We apply the Bistritzer-MacDonald model of two graphene layers twisted with respect to each other by a small angle. By reducing the model to the Dirac-type one with modified Fermi velocity, we derive expressions for the RKKY quadratic and biquadratic spin interactions using perturbation theory for the free energy. The biquadratic interaction is suppressed by a larger power of the interaction constant and decreases faster with a the distance between impurities comparing to the quadratic one. Nevertheless, due to the different period of oscillations with impurity separation distance, chemical potential, twist angle and temperature, it is possible to fine-tune the system to the regime of dominating biquadratic interaction. The existence of such fine-tuned regime might provide a promising opportunity to observe non-conventional spin ordering.

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arXiv2024

Phonon Amplification via Magnetoelastic Klein Scattering

Materials exhibit various wave-like excitations, among which phonons (lattice vibrations) and magnons (oscillations in ferromagnetic ordering) hold significant promise for future nanoscale technologies. Exploring the interaction between these excitations may pave the way for innovative devices that leverage their complementary strengths. This article presents a set-up designed to amplify an incoming phononic current, potentially enhancing the phonon lifetime. The set-up consists of a nonmagnetic and ferromagnetic insulator. The ferromagnet is polarized opposite to the external magnetic field with spin-orbit torque, which allows for negative-energy magnons. Phonons that are incoming from the nonmagnetic side will interact with the negative-energy magnons via magnetoelastic coupling. The reflected phonon will increase in amplitude as a result of energy conservation. This interaction between negative-energy magnons and phonons is an example of Klein scattering. This work opens new avenues for the development of advanced devices that capitalize on the combined properties of phonons and magnons.

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arXiv2023

Non-linear dynamics near exceptional points of synthetic antiferromagnetic spin-torque oscillators

We consider a synthetic antiferromagnetic spin-torque oscillator with anisotropic interlayer exchange coupling. This system exhibits exceptional points in its linearized dynamics. We find the non-linear dynamics and the dynamical phase diagram of the system both analytically and numerically. Moreover, we show that, near one of the exceptional points, the power of the oscillator depends extremely sensitively on the injected spin current. Our findings may be useful for designing sensitive magnetometers and for other applications of spin-torque oscillators.

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arXiv2022

Optical spin-wave detection beyond the diffraction limit

Spin waves are proposed as information carriers for next-generation computing devices because of their low power consumption. Moreover, their wave-like nature allows for novel computing paradigms. Conventional methods to detect spin waves are based either on electrical induction, limiting the downscaling and efficiency complicating eventual implementation, or on light scattering, where the minimum detectable spin-wave wavelength is set by the wavelength of the laser. In this Article we demonstrate magneto-optical detection of spin waves beyond the diffraction limit using a metallic grating that selectively absorbs laser light. Specifically, we demonstrate the detection of propagating spin waves with a wavelength of 700 nm using a diffraction-limited laser spot with a size of 10 $μ$m in 20 nm thick Py strips. Additionally, we show that this grating is selective to the wavelength of the spin wave, providing wavevector-selective spin-wave detection. This should open up new avenues towards the integration of the burgeoning fields of photonics and magnonics, and aid in the optical detection of spin waves in the short-wavelength exchange regime for fundamental research.

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arXiv2022

Unidirectional magnetic coupling

We show that interlayer Dzyaloshinskii-Moriya interaction in combination with non-local Gilbert damping gives rise to unidirectional magnetic coupling. That is, the coupling between two magnetic layers -- say the left and right layer -- is such that dynamics of the left layer leads to dynamics of the right layer, but not vice versa. We discuss the implications of this result for the magnetic susceptibility of a magnetic bilayer, electrically-actuated spin-current transmission, and unidirectional spin-wave packet generation and propagation. Our results may enable a route towards spin-current and spin-wave diodes and further pave the way to design spintronic devices via reservoir engineering.

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arXiv2022

Enhanced magnon spin current using the bosonic Klein paradox

Efficient manipulation of magnons for information processing is a central topic in spintronics and magnonics. An outstanding challenge for long-distance spin transport with minimal dissipation is to overcome the relaxation of magnons and to amplify the spin current they carry. Here, we propose to amplify magnon currents based on the realization of the bosonic Klein paradox in magnetic nanostructures. This paradox involves the magnon's antiparticle, the antimagnon, of which the existence is usually precluded by magnetic instabilities as it is an excitation at negative energy. We show that, by appropriately tuning the effective dissipation through spin-orbit torques, both magnons and antimagnons are dynamically stabilized. As a result, we find that the reflection coefficient of incident magnons at an interface between two coupled magnets can become larger than one, thereby amplifying the reflected magnon current. Our findings can lead to magnon amplifier devices for spintronic applications. Furthermore, our findings yield a solid-state platform to study the relativistic behavior of bosonic particles, which is an outstanding challenge with fundamental particles.

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arXiv2021

Green's function formalism for nonlocal elliptical magnon transport

We develop a non-equilibrium Green's function formalism to study magnonic spin transport through a strongly anisotropic ferromagnetic insulator contacted by metallic leads. We model the ferromagnetic insulator as a finite-sized one-dimensional spin chain, with metallic contacts at the first and last sites that inject and detect spin in the form of magnons. In the presence of anisotropy, these ferromagnetic magnons become elliptically polarized, and spin conservation is broken. We show that this gives rise to a novel parasitic spin conductance, which becomes dominant at high anisotropy. Moreover, the spin state of the ferromagnet becomes squeezed in the high-anisotropy regime. We show that the squeezing may be globally reduced by the application of a local spin bias.

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arXiv2021

Theory of the dipole-exchange spin wave spectrum in ferromagnetic films with in-plane magnetization revisited

We present a refinement of the widely accepted spin-wave spectrum that Kalinikos and Slavin computed for magnetic films with an in-plane magnetization (1986). The spin wave spectrum that follows from the diagonal approximation in this theory becomes inaccurate for relatively thick films, as has already been noted by Kreisel et al. (2009). Rather than solving an integrodifferential equation which follows from the magnetostatic Green's function, as done by Kalinikos and Slavin, we impose the exchange and magnetostatic boundary conditions on bulk spin-wave solutions. This boundary problem has an accurate analytical solution which is quantitatively different from the commonly used diagonal theory for magnetic films.

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arXiv2021

Dynamically stable negative-energy states induced by spin-transfer torques

We investigate instabilities of the magnetic ground state in ferromagnetic metals that are induced by uniform electrical currents, and, in particular, go beyond previous analyses by including dipolar interactions. These instabilities arise from spin-transfer torques that lead to Doppler shifted spin waves. For sufficiently large electrical currents, spin-wave excitations have negative energy with respect to the uniform magnetic ground state, while remaining dynamically stable due to dissipative spin-transfer torques. Hence, the uniform magnetic ground state is energetically unstable, but is not able to dynamically reach the new ground state. We estimate this to happen for current densities $ j\gtrsim (1-D/D_c)10^{13} \mathrm{A/m^2} $ in typical thin film experiments, with $ D $ the Dzyaloshinskii-Moriya interaction constant, and $ D_c $ the Dzyaloshinskii-Moriya interaction that is required for spontaneous formation of spirals or skyrmions. These current densities can be made arbitrarily small for ultrathin film thicknesses at the order of nanometers, due to surface- and interlayer effects. From an analogue gravity perspective, the stable negative energy states are an essential ingredient to implement event horizons for magnons -- the quanta of spin waves -- giving rise to e.g. Hawking radiation and can be used to significantly amplify spin waves in a so-called black-hole laser.

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arXiv2021

Statistical-physics-inspired model for intrinsic fluctuations driving supply and demand in markets

We propose a simple statistical-physics-inspired model for the effect of intrinsic fluctuations on supply and demand in markets. The model consists of agents that trade in two types of goods of which the total number is separately conserved. The relative preference of an individual agent for the two types of goods is determined by a utility that is identical for all agents. Market supply and demand curves are computed and compared for various motivated choices of the distribution of goods over the agents. In particular, we compare the "mean-field" case, in which all agents have the same number of goods and that is akin to the economics textbook case, to the case of Boltzmann-Gibbs distributed goods, in which agents have a fluctuating number of goods. We find that the resulting equilibrium prices are not equal for these two approaches, especially when a large fraction of the agents can neither buy nor sell.

physics.soc-ph↗

arXiv2020

Magnon Polarons induced by a magnetic field gradient

In this work, we report the theoretical possibility of generating magnon polaron excitations through a space-varying magnetic field. The spatial dependence of the magnetic field in the Zeeman interaction gives rise to a magnon-phonon coupling when a magnetic field gradient is applied, and such a coupling depends directly on the strength of the gradient. It is also predicted that the direction of the magnetic field gradient allows control over which phonon polarization couples to the magnons in the material. Here we develop the calculations of the magnon-phonon coupling for an arbitrary (anti)ferromagnet, which are later used to numerically study its consequences. These results are compared to the ones obtained with the phenomenological magnetoelastic coupling in YIG, where we show that the magnon polaron bandgap seen in YIG can be also obtained with a magnetic field gradient of $\sim 0.1$T/m which can be achieved with the current experimental techniques. Our results propose a new way of controlling the magnetoelastic coupling in an arbitrary material and open a new route to exploit the magnon-phonon interaction in magnonic and spintronic devices.

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arXiv2020

Geometric phase-control of a spin-torque oscillator

We show that the phase of a spin-torque oscillator generically acquires a geometric contribution upon slow and cyclic variation of the parameters that govern its dynamics. As an example, we compute the geometric phase that results from a cyclic excursion of the magnitude of the external field and the current. We consider thermal phase fluctuations and conclude that the geometric phase should be experimentally observable at room temperature with current setups. We briefly comment on arrays of spin-torque oscillators and possible applications.

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arXiv2019

Hannay Angles in Magnetic Dynamics

We consider, within the framework developed by Hannay for classical integrable systems [Journal of Physics A: Mathematical and General {\bf 18}, 221 (1985)], the geometric phases that occur in semi-classical magnetic dynamics. Such geometric phases are generically referred to as Hannay angles, and, in the context of magnetic dynamics, may arise as a result of both adiabatically-varying ellipticity and axis of magnetization precession. We elucidate both effects and their interplay for single-domain magnetic dynamics within a simple model with time-dependent anisotropies and external field. Subsequently, we consider spin waves and rederive, from our classical approach, some known results on what is commonly referred to as the magnon Berry phase. As an aside, these results are used to give an interpretation for geometric phases that occur in superfluids. Finally, we develop a Green's function formalism for elliptical magnons. Within this formalism, we consider magnon transport in a mesoscopic ring and show how it is influenced by interference effects that are tuned by the Hannay angle that results from a varying ellipticity. Our results may inform the field of magnonics that seeks to utilize spin waves in applications.

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arXiv2019

Magnon spin Hall magnetoresistance of a gapped quantum paramagnet

Motivated by recent experimental work, we consider spin transport between a normal metal and a gapped quantum paramagnet. We model the latter as the magnonic Mott-insulating phase of an easy-plane ferromagnetic insulator. We evaluate the spin current mediated by the interface exchange coupling between the ferromagnet and the adjacent normal metal. For the strongly interacting magnons that we consider, this spin current gives rise to a spin Hall magnetoresistance that strongly depends on the magnitude of the magnetic field, rather than its direction. This Letter may motivate electrical detection of the phases of quantum magnets and the incorporation of such materials into spintronic devices.

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arXiv2019

Nonlocal Spin Transport as a Probe of Viscous Magnon Fluids

Magnons in ferromagnets behave as a viscous fluid over a length scale, the momentum-relaxation length, below which momentum-conserving scattering processes dominate. We show theoretically that in this hydrodynamic regime viscous effects lead to a sign change in the magnon chemical potential, which can be detected as a sign change in the nonlocal resistance measured in spin transport experiments. This sign change is observable when the injector-detector distance becomes comparable to the momentum-relaxation length. Taking into account momentum- and spin-relaxation processes, we consider the quasiconservation laws for momentum and spin in a magnon fluid. The resulting equations are solved for nonlocal spin transport devices in which spin is injected and detected via metallic leads. Because of the finite viscosity we also find a backflow of magnons close to the injector lead. Our work shows that nonlocal magnon spin transport devices are an attractive platform to develop and study magnon-fluid dynamics.

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arXiv2019

Magnon contribution to unidirectional spin Hall magnetoresistance

We develop a model for the magnonic contribution to the unidirectional spin Hall magnetoresistance (USMR) of heavy metal/ferromagnetic insulator bilayer films. We show that diffusive transport of Holstein-Primakoff magnons leads to an accumulation of spin near the bilayer interface, giving rise to a magnoresistance which is not invariant under inversion of the current direction. Unlike the electronic contribution described by Zhang and Vignale [Phys. Rev. B 94, 140411 (2016)], which requires an electrically conductive ferromagnet, the magnonic contribution can occur in ferromagnetic insulators such as yttrium iron garnet. We show that the magnonic USMR is, to leading order, cubic in the spin Hall angle of the heavy metal, as opposed to the linear relation found for the electronic contribution. We estimate that the maximal magnonic USMR in Pt|YIG bilayers is on the order of $10^{-8}$, but may reach values of up to $10^{-5}$ if the magnon gap is suppressed, and can thus become comparable to the electronic contribution in, e.g., Pt|Co. We show that the magnonic USMR at a finite magnon gap may be enhanced by an order of magnitude if the magnon diffusion length is decreased to a specific optimal value that depends on various system parameters.

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