Session R38: Magnon Spin Current and Transport

Live

When a light wave propagates into the different media with an optical anisotropy, the refracted light wave splits into two depending on the polarization of the light, a phenomenon called birefringence. On the other hand, when a light wave reflects at a boundary, reflected beam is usually single ray and forms the same angle with the incident angle, a famous law of reflection. We report a split of the reflected magnetic spin waves as a result of reflection with the aid of magnon-phonon hybridization. Spin waves, collective precession of magnetic moments propagating as waves, couple with phonons via spin-orbit interaction, which is strong enough to create a new normal state, i.e. magnon-phonon hybridized waves. The hybridized wave inherits the properties of phonons, leading to abnormal reflection dynamics. In this talk, we report the result of time-resolved magneto-optical microscopy of the reflection dynamics of the hybridized waves. Our experimental results and theoretical modeling demonstrate that the mode degree of freedom of phonon is transferred to spin waves via the hybridization.   

Live

Coupled magnonic and photonic resonators with strong coupling rates (> 100 MHz) form a basis of a promising hybrid quantum computing architecture [1-3]. Tunability of magnon resonant frequencies with magnetic field allows one to reconfigure such systems during a quantum computation. Here, we show that dynamic tuning of magnon frequencies at timescales comparable to the magnon-photon energy exchange time allows one to achieve qualitatively new functionalities of such hybrid devices. In particular, we demonstrate theoretically the possibility of realization of several unitary magnon-mediated quantum gates, such as coherent quantum data exchange and entanglement generation. Different gates can be realized in the same physical structure by shaping the profile of the pulsed magnetic field that controls magnon frequency. It should be noted, that decoherence processes in magnonic resonators have a very weak effect on the overall performance of the proposed gates and our estimates show that magnon-mediated operations may have efficiencies better than existing quantum gate designs.
[1] H. Huebl, et al., Phys. Rev. Lett., 111, 127003 (2013).
[2] X. Zhang, et al., Phys. Rev. Lett., 113, 156401 (2014).
[3] Y. Li, et al., Phys. Rev. Lett., 123, 107701 (2019).   

Live

There is an enormous need for faster and more efficient information processing and data transfer. It is of fundamental importance that new physical phenomena are found, harvested and brought to a high level of understanding.
The utilization of magnon Bose-Einstein condensates (BECs) and magnon supercurrents driven by a phase gradient in a spatially extended coherent magnon BEC is a very promising approach for the transfer and processing of spin information. In our experiments, we have found a fingerprint of the supercurrent efflux of condensed magnons subjected to a phase gradient imposed by a local heating of the magnetic sample. Moreover, we revealed that the condensed magnons, being pushed out from the heated area, form compact density humps, which propagate over long distances through the thermally homogeneous magnetic medium. We refer to them as a superposition of Bogoliubov waves with oscillations of both the amplitude and the phase of the magnon BEC’s wave function. In the long-wavelength limit, these waves have a linear dispersion law and can be considered as a magnon second sound potentially featuring viscosity-free propagation.
A further consequence of the magnon BEC is the prediction of interference effects of the BEC wavefunctions. The well-known ac Josephson effect relies on the existence of two weakly coupled macroscopic quantum states. Recently, we discovered the ac Josephson effect in a magnon BEC carried by a room-temperature ferrimagnetic magnetic film. The BEC is formed in a parametrically populated magnon gas around a potential trench created by a dc electric current. The appearance of the Josephson effect is manifested by oscillations of the magnon BEC density in the trench, caused by a coherent phase shift between this BEC and the BEC in the nearby left and right zones.
All these findings advance the physics of room-temperature macroscopic quantum phenomena and will allow for their application for data processing in magnon spintronics devices.   

Live

A Bose-Einstein condensate of magnons (mBEC) is formed in magnetic films under parametric pumping at room temperature [1] or by rapid cooling of a thin film [2]. There is a necessity to develop a simple theoretical model describing the main qualitative features of this macroscopic quantum phenomenon, while taking into account the non-local character of the dipole-dipole interaction determining the properties of the magnon spectra. We developed such an approximate model, and demonstrated that mBEC can be described as a gas of weakly-interacting bosonic quasi-particles with anisotropic effective mass in an infinite potential well formed by the ferromagnetic film. The anisotropy of the effective mass results in the anisotropy of the mBEC dynamical properties: the speed of the Bogolyubov’s waves [2] in mBEC is strongly dependent on the direction of the wave propagation. The developed model allows one to make accurate predictions about the nonlinear properties and stability of mBEC in different experimental conditions, the properties of the mBEC fluctuations, and characteristics of magnon density waves in mBEC.

[1] S.O. Demokritov et al., Nature 443, 430 (2006).
[2] M. Schneider et al., Nature Nanotechnology 15, 457(2020).
[3] D.A. Bozhko et al. , Nat. Commun. 10, 2460 (2019).   

Live

Magnon spin Nernst effect (SNE) has recently been proposed as a bulk effect in antiferromagnetic insulators without considering diffusions and boundaries. However, real experiments deal with situations far from the ideal bulk picture in that: 1) the system is finite and shares imperfect boundaries with the detecting leads; 2) both the electrons in the leads and the magnons are diffusive. To properly accommodate these complexities, we formulate a diffusive theory of the SNE of antiferromagnetic magnons under a realistic device geometry. By solving the coupled spin diffusion equations of magnons and electrons with a finite spin transmission across the boundaries, we find that the detectable signal can be much larger than that predicted by the bulk picture. Counterintuitively, the diffusive magnon SNE is even in the applied magnetic field, basing on which we conclude that optical detection is more reliable than electronic detection in real experiments.   

Live

An easy-plane ferromagnetic insulator in a uniform external magnetic field and in contact with a phonon bath and a normal metal bath is studied theoretically in the presence of DC spin current injection via the spin Hall effect in the metal. The Keldysh path integral formalism is used to model the magnon gas driven into a nonequilibrium steady state by mismatched bath temperatures and/or electrical injection, and we analyze the magnon system in the normal state, but close to the instability to Bose-Einstein condensation, within the self-consistent Hartree-Fock approximation. We find that the steady state magnon distribution function generally has a nonthermal form that cannot be described by a single effective chemical potential and effective temperature. In the presence of two bath temperatures, we find that the correlation length of the superfluid order parameter fluctuations exhibits nontrivial finite temperature crossover behaviors that are richer than the standard crossover behaviors obtained for the vacuum-superfluid transition in an equilibrium dilute Bose gas. We study the consequences of these thermal crossovers on the magnon spin conductivity.   

Live

Magnon-mediated spin transport in magnetically ordered insulators is of interest in the field of spintronics as it enables the transport of spin information with ultra-low-dissipation. Although the spin diffusion length, which is important to characterize spin transport, has been studied previously in yttrium iron garnet (YIG) films, its dependence on magnetic anisotropy remains largely unexplored due to the low in-plane magnetic anisotropy of YIG. Here we demonstrate anisotropic magnon spin transport in coherently-strained ultra-thin epitaxial films of magnesium aluminum ferrite (MgAl_0.5Fe_1.5O₄, MAFO) with low Gilbert damping and a thickness of 6 nm. Using nonlocal measurements with the spin polarization injected from Pt bars, we found a ~30 % enhancement of the spin diffusion length for propagation parallel to the easy axes as compared to the hard axes, for both electrically and thermally excited magnons. We correlate this anisotropy to the biaxial magnetic anisotropy in MAFO induced by tetragonal distortion.   

Live

By engineering an anti-parity-time (anti-PT) symmetric cavity magnonics system with precise eigenspace controllability, we observe two different singularities in the same system. One type of singularity, the exceptional point (EP), is produced by tuning the magnon damping. Between two EPs, the maximal coherent superposition of photon and magnon states is robustly sustained by the preserved anti-PT symmetry. The other type of singularity, arising from the dissipative coupling of two antiresonances, is an unconventional bound state in the continuum (BIC). At the settings of BICs, the coupled system exhibits infinite discontinuities in the group delay. We find that both singularities coexist at the equator of the Bloch sphere, which reveals a unique hybrid state that simultaneously exhibits the maximal coherent superposition and slow light capability.   

Live

In this study, we employ recently developed [M. D. Petrovic et. al., Phys. Rev. Appl. 10, 054038 2018] multiscale time-dependent-quantum transport/classical-micromagnetics formalism (TDNEGF-LLG) to simulate how spin-polarized steady-state current of electrons interacts with a magnon modelled as a classical spin wave. We investigate electronic spin and charge currents generated by the excitation of a single frequency spin wave in a one-dimensional finite-size magnetic nanowire composed of classical local magnetic moments precessing around the anisotropy axis while the phase of the precession of adjacent moments varies harmonically over the wavelength of spin wave with uniform precession cone angle. We show that spin wave hosted in the middle of two metallic leads pumps chiral electronic spin and charge currents into the leads, which can be flipped by reversing the direction of magnonic spin current. The mechanism behind our prediction is nonadiabaticity due to time-retardation effects, which makes the time-dependent non-equilibrium spin density misaligned with the adiabatic direction even without spin-orbit interaction, and in the absence of magnetic or spin-orbit impurities.   

Live

Existing scattering methods often require large samples in order to achieve an appreciable scattering intensity. Mesoscopic samples therefore pose a severe challenge to traditional scattering platforms, motivating the development of alternative table-top scattering techniques better suited to the study of such samples. Toward this end, we study the scattering of spin waves in magnetic thin films for application to our tabletop magnon scattering platform (1). Using NV center magnetometry, our experimental collaborators are able to resolve the amplitude and phase of spin waves scattered from a target material. We have been developing a procedure to invert this signal and yield properties of the scatterer. Properties of interest can be as simple as the shape and magnetic character of the target, or as complex as its magnetic excitation spectrum. This procedure is predicated on a full theory of dipole-exchange spin wave scattering from interesting magnetic and nonmagnetic target materials in the thin film geometry particular to our experiment, which we have also developed.

(1.) T. X. Zhou et al., arXiv:2004.07763.   

Live

We theoretically study the intrinsic magnon Nernst effect in pyrochlore iridate trilayer thin films grown along the [111] direction using a strong coupling approach. The spin configuration of the ground state is the all-in–all-out (AIAO) state resulted from the Dzyaloshinskii-Moriya interaction. In such a state, the system has an inversion symmetry and a conserved spin current is well defined. We then calculate the magnon Nernst response with respect to the lowest magnon band Chern number controlled by the spin-orbit coupling parameters. Our study complements prior work in the magnon thermal Hall effect of thin-film pyrochlore iridates and suggests that the [111] grown thin-film pyrochlore iridates are a promising candidate for thermal spin transport and caloritronic devices.   

Live

Rare-earth iron garnet films with perpendicular magnetic anisotropy (PMA) have been extensively explored in magnon spintronics where spin currents carried by magnons could be used to carry, transport, and process information leading to devices free of Joule heating. In this work, we investigate the spin dynamics and the transport of thermally driven spin currents in valves composed of two layers of TmIG (Tm₃Fe₅O₁₂) thin films with PMA separated by Au. We employed the ferromagnetic resonance technique to probe the magnetic anisotropies and the spin Seebeck effect (SSE) to excite magnon-mediated spin currents. The spin currents were detected through the inverse spin Hall effect as a micro-voltage (V_SSE) at the edges of a platinum electrode deposited on the top of TmIG(30 nm)/Au(8 nm, 12 nm, 16 nm)/TmIG(15 nm). The results show the V_SSE depends on the relative orientation of the TmIG layers assuming its maximum value when the magnetizations are parallel and minimum when antiparallel. These structures could be used to control the transmission coefficient of spin waves in wave-based computing devices.   

On Demand

Antiferromagnets (AFMs) recently demonstrate great potentials in efficient spin transport through magnons, providing a new platform for spintronics applications. Easy-plane AFMs, with two linearly polarized magnon eigenmodes that are orthogonal to each other, own unique advantages for low-energy control of ultrafast magnetic dynamics. However, it is commonly conceived that these magnon modes are less likely to transmit spins due to their vanishing angular momentum. In this work, we experimentally demonstrate that an easy-plane insulating AFM, α-Fe₂O₃ thin film, can conduct efficient spin transmission over micrometer distance. Remarkably, the spin decay length shows an unconventional temperature dependence that cannot be captured by solely considering thermal magnon scatterings. These observations are interpreted by the interference of two linearly polarized propagating magnons, a birefringence-like spin transport behavior. Finally, our devices can realize a bi-stable spin-current switch with a 100% on/off ratio under zero remnant magnetic field, providing additional opportunities for nonvolatile, low-field control of spin transport in AFM-based devices.