Search

Magnons (the quanta of spin waves), unlike electrons, do not suffer from Ohmic losses, which makes them an attractive information carrier in communication and data processing. The possibility of nonreciprocal magnon propagation is of great practical relevance to directional communications, particularly when the nonreciprocity is steerable through magnons themselves in a monologic magnonic device. This work demonstrates explicitly such nonreciprocity in the mode profiles of magnons in Ni79Fe21 thin films. Evidence of nonreciprocal emergence of hybridized dipole-exchange spin waves at two surfaces of Ni79Fe21 nanofilms deposited on a surface oxide silicon substrate is provided by studying magnon transmission and asymmetry via Brillouin light scattering measurements. The dipole-dominated spin wave and exchange-dominated spin wave are found to be localized near the top and bottom surfaces, respectively, and travel along opposite directions. The nonreciprocity and the localization can be tuned by an in-plane magnetic field. Our findings provide a simple and flexible approach to nonreciprocal all-magnon logic devices highly compatible with silicon-based integrated circuit technology.

Spin waves, whose quanta are called magnons, are propagating excitations of magnetic materials. Magnonics is an emerging field of modern condensed matter physics that aims to study and utilize the properties and behaviors of magnons. The topological magnon band is an interesting topic of magnonics, and nontrivial topology is usually accompanied with many exotic phenomena such as emergence of robust edge states and the magnon Hall effect. In this Tutorial, using a honeycomb ferromagnet as a prototypical platform, we pedagogically demonstrate how to compute the magnon spectra and the topological invariants characterizing the topology of the magnon bands. We also briefly discuss some numerical techniques. © 2021 Author(s).

Spin waves (SWs) in coupled Co/FeNi bilayers are probed by the Brillouin light-scattering technique. Two types of SWs, magnetostatic surface spin waves (MSSWs) and perpendicular standing SWs, are identified. The dispersion relations of MSSWs obtained from the Stokes and anti-Stokes measurements display, respectively, positive and negative group velocities. The anti-Stokes branch with positive phase velocities and negative group velocities is called the backward magnetostatic surface mode that originates from the magnetostatic interaction in the bilayer. Experimental data agree well with our theoretical calculations. Our results are used to understand SWs propagation. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Magnetic skyrmion motion under parametric pumping is numerically investigated. Parametric pumping with a perpendicularly oscillating electric field can excite the breathing modes of a skyrmion because an electric field can modify the magnetic anisotropy. Interestingly, using an in-plane static magnetic field to break the rotational symmetry, a perpendicularly oscillating electric field can drive a skyrmion undergoing a wiggling motion along a well-defined trajectory. The skyrmion velocity can reach several meters per second when the frequency of the electric field is close to the frequency of the skyrmion breathing motion. The physics is revealed in a generalized Thiele equation where a net spin current excited by the parametric pumping can drive the skyrmion propagation through angular momentum transfer. Our results open new possibilities for manipulating skyrmions in both metals and insulators with low-power consumption. Moreover, the oscillating skyrmion can act as a microwave generator for future spintronic applications such as a nanotool on a diamond nitrogen-vacancy center.

A magnetic skyrmion is a topological object consisting of a skyrmion core, an outer domain, and a wall that separates the skyrmion core from the outer domain. The skyrmion size and wall width are two fundamental quantities of a skyrmion that depend sensitively on material parameters such as exchange energy, magnetic anisotropy, Dzyaloshinskii–Moriya interaction, and magnetic field. However, quantitative understanding of the two quantities is still very poor. Here we present a general theory on skyrmion size and wall width. The two formulas we obtained agree almost perfectly with simulations and experiments for a wide range of parameters, including most of the existing materials that support skyrmions. © 2018, The Author(s).

Spin-orbit-torque magnetic random access memory (SOT-MRAM) is a promising technology for the next generation of data storage devices. The main bottleneck of this technology is the high reversal current density threshold. This outstanding problem is now solved by a new strategy in which the magnitude of the driven current density is fixed while the current direction varies with time. The theoretical limit of minimal reversal current density is only a fraction (the Gilbert damping coefficient) of the threshold current density of the conventional strategy. The Euler-Lagrange equation for the fastest magnetization reversal path and the optimal current pulse is derived for an arbitrary magnetic cell and arbitrary spin-orbit torque. The theoretical limit of minimal reversal current density and current density for a GHz switching rate of the new reversal strategy for CoFeB/Ta SOT-MRAMs are, respectively, of the order of 105 A/cm2 and 106 A/cm2 far below 107 A/cm2 and 108 A/cm2 in the conventional strategy. Furthermore, no external magnetic field is needed for a deterministic reversal in the new strategy. © 2018 American Physical Society.

Conventional magnonic devices use magnetostatic waves whose properties are sensitive to device geometry and the details of magnetization structure, so the design and the scalability of the device or circuitry are difficult. We propose topological magnonics, in which topological exchange spin waves are used as information carriers, that do not suffer from conventional problems of magnonic devices with additional nice features of nanoscale wavelength and high frequency. We show that a perpendicularly magnetized ferromagnet on a honeycomb lattice is generically a topological magnetic material in the sense that topologically protected chiral edge spin waves exist in the band gap as long as a spin-orbit-induced nearest-neighbor pseudodipolar interaction (and/or a next-nearest-neighbor Dzyaloshinskii-Moriya interaction) is present. The edge spin waves propagate unidirectionally along sample edges and domain walls regardless of the system geometry and defects. As a proof of concept, spin-wave diodes, spin-wave beam splitters, and spin-wave interferometers are designed by using sample edges and domain walls to manipulate the propagation of topologically protected chiral spin waves. Since magnetic domain walls can be controlled by magnetic fields or electric current or fields, one can essentially draw, erase, and redraw different spin-wave devices and circuitry on the same magnetic plate so that the proposed devices are reconfigurable and tunable. The topological magnonics opens up an alternative direction towards a robust, reconfigurable and scalable spin-wave circuitry. © 2018 American Physical Society.

Magnetic topological states are investigated theoretically and numerically. It is shown that ferromagnetically interacting spins on a two-dimensional honeycomb lattice with nearest-neighbor interactions, which is governed by the Landau-Lifshitz-Gilbert equation, can be topologically nontrivial with gapped bulk spin waves and topologically protected gapless edge spin waves. These edge spin waves are robust against defects and perturbations, and should be superb channels of processing and manipulating spin waves, in contrast to the normal spin waves that are very sensitive to defects as well as sample geometry. Because of the unidirectional nature of these topologically protected edge spin waves, a spin-wave beam splitter can be made out of a domain wall in a strip. It is shown that an incoming spin-wave beam along one edge splits into two spin-wave beams propagating along two opposite directions on the other edge after passing through a domain wall. © 2017 American Physical Society.

Quantum phase transitions of three-dimensional (3D) Weyl semimetals (WSMs) subject to uncorrelated on-site disorder are investigated through quantum conductance calculations and finite-size scaling of localization length. Contrary to previous claims that a direct transition from a WSM to a diffusive metal (DM) occurs, an intermediate phase of Chern insulator (CI) between the two distinct metallic phases should exist due to internode scattering that is comparable to intranode scattering. The critical exponent of localization length is.. 1.3 for both the WSM-CI and CI-DM transitions, in the same universality class of 3D Gaussian unitary ensemble of the Anderson localization transition. The CI phase is confirmed by quantized nonzero Hall conductances in the bulk insulating phase established by localization length calculations. The disorder-induced various plateau-plateau transitions in both the WSM and CI phases are observed and explained by the self-consistent Born approximation. Furthermore, we clarify that the occurrence of zero density of states at Weyl nodes is not a good criterion for the disordered WSM, and there is no fundamental principle to support the hypothesis of divergence of localization length at the WSM-DM transition.

Topological states of matter have been a subject of intensive studies in recent years because of their exotic properties such as the topologically protected edge and surface states. The initial studies were exclusively for electron systems. It is now known that topological states can also exist for other particles. Indeed, topologically protected edge states have already been found for phonons and photons. In spite of active searching for topological states in many fields, the studies in magnetism are relatively rare although topological states are apparently important and useful in magnonics. Here we show that the pyrochlore ferromagnets with the Dzyaloshinskii-Moriya interaction are intrinsic magnonic Weyl semimetals. Similar to the electronic Weyl semimetals, the magnon bands in a magnonic Weyl semimetal are nontrivially crossing in pairs at special points (called Weyl nodes) in momentum space. The equal energy contour around the Weyl node energy is made up of the magnon arcs on sample surfaces due to the topologically protected surface states between each pair of Weyl nodes. Additional Weyl nodes and magnon arcs can be generated in lower energy magnon bands when an anisotropic exchange interaction is introduced. Finally, magnonic chiral anomaly is realized under mutually perpendicular inhomogeneous electric and magnetic fields. The magnonic chiral anomaly results in linear electric spin and heat conductances that are experimentally detectable.

Spin current injection and spin accumulation near a ferromagnetic insulator (FI)/nonmagnetic metal (NM) bilayer film under a thermal gradient is investigated theoretically. By using the Fermi golden rule and the Boltzmann equations, we find that FI and NM can exchange spins via interfacial electron-magnon scattering because of the imbalance between magnon emission and absorption caused by either the deviation of the magnon number from the equilibrium Bose-Einstein distribution or the difference in magnon temperature and electron temperature. A temperature gradient in FI and/or a temperature difference across the FI/NM interface generates a spin current which carries angular momenta parallel to the magnetization of FI from the hotter side to the colder one. Interestingly, the spin current induced by a temperature gradient in NM is negligibly small due to the nonmagnetic nature of the nonequilibrium electron distributions. The results agree well with all existing experiments.

In this thesis, the interplay of spin waves and domain walls in magnetic nanostructures is studied in three aspects: (1) how a domain wall can propagate in a magnetic field through emitting spin waves, (2) how an externally generated spin wave can drive a domain wall’s propagation, whose mechanism is called “all-magnonic spin transfer torque”, and (3) how spin wave excitations affect the thermodynamic properties of a domain wall system and drive the domain wall propagation in a temperature gradient. We theoretically study field-induced domain wall motion in an electrically insulating ferromagnet with hard- and easy-axis anisotropies. Domain walls can propagate along a dissipationless wire through spin wave emission locked into the known soliton velocity at low fields. In the presence of damping, the usual Walker rigid-body propagation mode can become unstable for a magnetic field smaller than the Walker breakdown field. We also numerically investigate the properties of spin waves emitted by the domain wall motion, such as frequency and wave number, and their relation with the domain wall motion. For a wire with a low transverse anisotropy and in a field above a critical value, a domain wall emits spin waves to both sides (bow and stern), while it oscillates and propagates at a low average speed. For a wire with a high transverse anisotropy and in a weak field, the domain wall emits mostly stern waves, while the domain wall distorts itself and the domain wall center propagates forward like a drill at a relative high speed. The spin wave transportation through a transverse magnetic domain wall in a magnetic nanowire is studied. It is found that in a 1D nanowire, the spin wave passes through a domain wall without reflection. A magnon, the quantum of the spin wave, carries opposite spins on the two sides of the domain wall. As a result, there is a spin angular momentum transfer from the propagating magnons to the domain wall. This magnonic spin transfer torque can effic

The subject application describes systems and methods that drive magnetization switching through magnonic spin transfer torque. A spin current is provided to a first magnetic layer with a first magnetic state. The spin current facilitates magnetization switching via a magnonic spin transfer torque in a second magnetic layer with a second magnetic state that is separated from the first magnetic layer by an interface. Alternatively, a spin current is provided to a first magnetic domain with a first magnetic state. The spin current facilitates domain wall propagation via a magnonic spin transfer torque. The domain wall is between the first magnetic domain and a second magnetic domain in a second magnetic state.

Magnonic spin-transfer torque (STT) is introduced. In comparison with electronic STT, magnonic STT is more efficient in driving domain wall (DW) propagation along a magnetic nanowire because of the total spin wave transmission through a DW. Furthermore, the energy consumption is more than millions times lower than that by electronic STT when the magnonic STT is used to drive a DW to propagate at a practically useful speed.