The dynamics of spin ordering in magnetic materials is of interest for both fundamental understanding and progress in information-processing and recording technology. Radu et al. study spin dynamics in a ferrimagnetic gadolinium–iron–cobalt (GdFeCo) alloy that is optically excited at a timescale shorter than the characteristic magnetic exchange interaction between the Gd and Fe spins. Using element-specific X-ray magnetic circular dichroism spectroscopy, they show that the Gd and Fe spins switch directions at very different timescales. As a consequence, an unexpected transient ferromagnetic state emerges. These surprising observations, supported by simulations, provide a possible new concept of manipulating magnetic order on a timescale of the exchange interaction. Ferromagnetic or antiferromagnetic spin ordering is governed by the exchange interaction, the strongest force in magnetism1,2,3,4. Understanding spin dynamics in magnetic materials is an issue of crucial importance for progress in information processing and recording technology. Usually the dynamics are studied by observing the collective response of exchange-coupled spins, that is, spin resonances, after an external perturbation by a pulse of magnetic field, current or light. The periods of the corresponding resonances range from one nanosecond for ferromagnets down to one picosecond for antiferromagnets. However, virtually nothing is known about the behaviour of spins in a magnetic material after being excited on a timescale faster than that corresponding to the exchange interaction (10–100 fs), that is, in a non-adiabatic way. Here we use the element-specific technique X-ray magnetic circular dichroism to study spin reversal in GdFeCo that is optically excited on a timescale pertinent to the characteristic time of the exchange interaction between Gd and Fe spins. We unexpectedly find that the ultrafast spin reversal in this material, where spins are coupled antiferromagnetically, occurs by way of a transient ferromagnetic-like state. Following the optical excitation, the net magnetizations of the Gd and Fe sublattices rapidly collapse, switch their direction and rebuild their net magnetic moments at substantially different timescales; the net magnetic moment of the Gd sublattice is found to reverse within 1.5 picoseconds, which is substantially slower than the Fe reversal time of 300 femtoseconds. Consequently, a transient state characterized by a temporary parallel alignment of the net Gd and Fe moments emerges, despite their ground-state antiferromagnetic coupling. These surprising observations, supported by atomistic simulations, provide a concept for the possibility of manipulating magnetic order on the timescale of the exchange interaction.

Transient ferromagnetic-like state mediating ultrafast reversal of antiferromagnetically coupled spins

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

https://link.aps.org/accepted/10.1103/RevModPhys.89.025006

Interface-induced phenomena in magnetism

Bose–Einstein condensation (BEC), a form of matter first postulated in 1924, has famously been demonstrated in dilute atomic gases at ultra-low temperatures. Much effort is now being devoted to exploring solid-state systems in which BEC can occur. In theory semiconductor microcavities, where photons are confined and coupled to electronic excitations leading to the creation of polaritons, could allow BEC at standard cryogenic temperatures. Kasprzak et al. now present experiments in which polaritons are excited in such a microcavity. Above a critical polariton density, spontaneous onset of a macroscopic quantum phase occurs, indicating a solid-state BEC. BEC should also be possible at higher temperatures if coupling of light with solid excitations is sufficiently strong. Demokritov et al. have achieved just that, BEC at room temperature in a gas of magnons, which are a type of magnetic excitation. Bose–Einstein condensation, the formation of a collective quantum state of identical particles, called bosons, is observed at room temperature in a gas of magnons, which are a type of magnetic excitation. Bose–Einstein condensation1,2 is one of the most fascinating phenomena predicted by quantum mechanics. It involves the formation of a collective quantum state composed of identical particles with integer angular momentum (bosons), if the particle density exceeds a critical value. To achieve Bose–Einstein condensation, one can either decrease the temperature or increase the density of bosons. It has been predicted3,4 that a quasi-equilibrium system of bosons could undergo Bose–Einstein condensation even at relatively high temperatures, if the flow rate of energy pumped into the system exceeds a critical value. Here we report the observation of Bose–Einstein condensation in a gas of magnons at room temperature. Magnons are the quanta of magnetic excitations in a magnetically ordered ensemble of magnetic moments. In thermal equilibrium, they can be described by Bose–Einstein statistics with zero chemical potential and a temperature-dependent density. In the experiments presented here, we show that by using a technique of microwave pumping it is possible to excite additional magnons and to create a gas of quasi-equilibrium magnons with a non-zero chemical potential. With increasing pumping intensity, the chemical potential reaches the energy of the lowest magnon state, and a Bose condensate of magnons is formed.

Bose–Einstein condensation of quasi-equilibrium magnons at room temperature under pumping

An attractive direction in next-generation information processing is the development of systems employing particles or quasiparticles other than electrons--ideally with low dissipation--as information carriers. One such candidate is the magnon: the quasiparticle associated with the eigen-excitations of magnetic materials known as spin waves. The realization of single-chip all-magnon information systems demands the development of circuits in which magnon currents can be manipulated by magnons themselves. Using a magnonic crystal--an artificial magnetic material--to enhance nonlinear magnon-magnon interactions, we have succeeded in the realization of magnon-by-magnon control, and the development of a magnon transistor. We present a proof of concept three-terminal device fabricated from an electrically insulating magnetic material. We demonstrate that the density of magnons flowing from the transistor's source to its drain can be decreased three orders of magnitude by the injection of magnons into the transistor's gate.

/pdf/magnon-transistor-for-all-magnon-data-processing-18dgl5h4h6.pdf

Magnon transistor for all-magnon data processing

Although electron motion is prohibited in magnetic insulators, the electron spin can be transported by magnons. Such magnons, generated and detected using all-electrical methods, are now shown to travel micrometre distances at room temperature. The transport of spin information has been studied in various materials, such as metals1, semiconductors2 and graphene3. In these materials, spin is transported by the diffusion of conduction electrons4. Here we study the diffusion and relaxation of spin in a magnetic insulator, where the large bandgap prohibits the motion of electrons. Spin can still be transported, however, through the diffusion of non-equilibrium magnons, the quanta of spin-wave excitations in magnetically ordered materials. Here we show experimentally that these magnons can be excited and detected fully electrically5,6,7 in a linear response, and can transport spin angular momentum through the magnetic insulator yttrium iron garnet (YIG) over distances as large as 40 μm. We identify two transport regimes: the diffusion-limited regime for distances shorter than the magnon spin diffusion length, and the relaxation-limited regime for larger distances. With a model similar to the diffusion–relaxation model for electron spin transport in (semi)conducting materials, we extract the magnon spin diffusion length λ = 9.4 ± 0.6 μm in a thin 200 nm YIG film at room temperature.

/pdf/long-distance-transport-of-magnon-spin-information-in-a-226ebl7ijd.pdf

Long-distance transport of magnon spin information in a magnetic insulator at room temperature

Isotropic Ferromagnet Magnetized to Saturation Ferromagnetism Elementary Magnetic Moments Paramagnetism Weiss Theory Exchange Interaction Equation of Motion of Magnetization High-Frequency Magnetic Susceptibility Solution of the Linearized Equation of Motion Peculiarities of the Susceptibility Tensor High-Frequency Permeability Allowance for Magnetic Losses Dissipative Terms and Dissipation Parameters Susceptibility Tensor Components Uniform Oscillations in a Small Ellipsoid Internal and External Magnetic Fields Eigenoscillations Forced Oscillations Anisotropic Ferromagnet Landau-Lifshitz Equation Generalization of Equation of Motion Methods of Analysis of Ferromagnetic Resonance in Anisotropic Ferromagnet Magnetocrystalline Anisotropy Origins of Magnetocrystalline Anisotropy Phenomenological Description Equilibrium Orientations of Magnetization Ferromagnetic Resonance in a Single Crystal Sphere of Uniaxial Ferromagnet Sphere of a Cubic Ferromagnet Simultaneous Allowance for Different Kinds of Anisotropy Ferromagnetic Resonance in a Polycrystal Independent-Grain and Strongly-Coupled Grain Approximations Influence of Porosity Antiferromagnets and Ferrites Antiferromagnetism and Ferrimagnetism Crystal and Magnetic Structures Equations of Motion and Energy Terms Ground States and Small Oscillations Antiferromagnetic Resonance Antiferromagnet with an Easy Axis of Anisotropy: Steady States Oscillations in Antiparallel State Oscillations in Noncollinear State Oscillations in Transverse and Arbitrarily Oriented Fields Antiferromagnet with Easy Plane of Anisotropy Magnetic Oscillations in Ferrimagnets Ground States of Two-Sublattice Ferrimagnet Oscillations in Antiparallel Ground State Oscillations in Noncollinear Ground State Damped and Forced Oscillations Fundamentals of Electrodynamics of Gyrotropic Media Equations General Equations and Boundary Conditions Equations for Bigyotropic Media Uniform Plane Waves General Relations Longitudinal Magnetization Transverse Magnetization Nonreciprocity Energy Relations Equation of Energy Balance Energy Losses Perturbation Method Gyrotropic Perturbation of a Waveguide Gyrotropic Perturbation of a Resonator Quasistatic Approximation Resonator with Walls of Real Metal Waveguides and Resonators with Gyrotropic Media. Microwave Ferrite Devices Waveguide with Longitudinally Magnetized Medium Circular Waveguide Circular Waveguide with Ferrite Rod Faraday Ferrite Devices Waveguide with Transversely Magnetized Ferrite Rectangular Waveguide Filled with Ferrite Rectangular Waveguide with Ferrite Plates Microwave Ferrite Devices Resonators with Gyrotropic Media Eigenoscillations and Forced Oscillations Waveguide Resonators Ferrite Resonators Use of Perturbation Method Waveguides and Waveguide Junctions with Ferrite Samples Ferrite Ellipsoid in a Waveguide Coupling of Orthogonal Waveguides. Ferrite Band-Pass Filters General Properties of Nonreciprocal Junctions Magnetostatic Waves and Oscillations Magnetostatic Approximation Nonexchange Magnetostatic Waves in Plates and Rods Volume Waves in Plates Surface Waves Magnetostatic Waves in Waveguides with Finite Cross Section Energy Flow and Losses Magnetostatic Waves in Ferrite Films: Excitation, Applications Magnetostatic Oscillations (Walker's Modes) Metallized Cylinder Sphere and Ellipsoid of Revolution Damping, Excitation, and Coupling Spin Waves Spin Waves in Unbounded Ferromagnet Energy and Effective Field of Exchange Interaction Dispersion Law Magnetization, Field Components, and Damping Spin Waves in Bounded Bodies Exchange Boundary Conditions Standing Spin Waves in Films Propagating Spin Waves in Films Spin Waves in Nonuniform Magnetic Fields Magnons Quantization of Magnetic Oscillations and Waves Thermal Magnons Elements of Microscopic Spin-Wave Theory Diagonalization of the Hamiltonian Discussion of the Dispersion Law Allowance for Dipole-Dipole Interaction and Anisotropy Interaction of Magnons Magnetic Oscillations and Waves in Unsaturated Ferromagnet Oscillations of Domain Walls Domain Walls and Domain Structures Equation of Motion of a Domain Wall Dynamic Susceptibility Ferromagnetic Resonance in Samples with Domain Structure Ellipsoid of Uniaxial Ferromagnet Sphere of Cubic Ferromagnet Nonuniform Modes in Unsaturated Samples Nonlinear Oscillations of Magnetization Ferromagnetic Resonance in Strong Alternating Fields Rigorous Solution of Equation of Motion Approximate Methods Harmonic Generation and Frequency Conversion Frequency Doubling Frequency Mixing Parametric Excitation of Magnetic Oscillations and Waves Nonlinear Coupling of Magnetic Modes Thresholds of Parametric Excitation under Transverse Pumping First-Order and Second-Order Instabilities Threshold Fields Effect of Pumping-Field Polarization Longitudinal and Oblique Pumping Longitudinal Pumping Effect of Nonuniformities Oblique Pumping Instability of Nonuniform Modes and Nonuniform Pumping Parametric Excitation of Magnetostatic Oscillations and Waves Ferrite Parametric Amplifier Nonuniform Pumping Above-Threshold State Reaction of Parametric Spin Waves on Pumping Phase Mechanism Nonlinear Damping Stability of the Above-Threshold State Nonlinear Microwave Ferrite Devices Spin-Spin Relaxation Relaxation Processes in Magnetically Ordered Substances Kinds of Relaxation Processes Methods of Theoretical Study Inherent Spin-Spin Processes Three-Magnon Splitting Three-Magnon Confluence Four-Magnon Scattering Inherent Processes for Uniform Precession Experimental Data Two-Magnon Processes Theory of Two-Magnon Processes Disorder in Distribution of Ions over Lattice Sites Anisotropy-Field Variation and Pores in Polycrystals Surface Roughness Magnetoelastic Coupling Elastic Properties and Magnetoelastic Interaction Elastic Waves and Oscillations Magnetoelastic Energy and Equations of Motion Effect of Elastic Stresses on Ferromagnetic Resonance Magnetoelastic Waves Normal Waves Damping and Excitation Magnetoelastic Waves in Nonuniform Steady Magnetic Field Parametric Excitation of Magnetoelastic Waves Longitudinal Pumping of Magnetoelastic Waves Parametric Excitation Caused by Magnetoelastic Coupling Elastic Pumping Spin-Lattice Relaxation Ionic Anisotropy and Relaxation Anisotropy Caused by Impurity Ions Energy Levels of Ions One-Ion Theory of Ferromagnetic-Resonance Anisotropy Near-Crossing Energy Levels Experimental Data Ion Relaxation Processes Transverse Relaxation Longitudinal (Slow) Relaxation Relaxation of Ionic-Level Populations Experimental Data Interaction of Magnetic Oscillations and Waves with Charge Carriers Effect of Charge Carriers in Semiconductors Damping of Magnetic Oscillations Caused by Conductivity Influence of Interionic Electron Transitions Interaction of Spin Waves with Charge Carriers Ferromagnetic Resonance and Spin Waves in Metals Thin-Film Model Theory without Allowance for Exchange Interaction Influence of Exchange Interaction Antiresonance Processes of Magnetic Relaxation Appendices Units and Constants Demagnetization Factors Dirac Delta Function and Kronecker Delta Symbol Bibliography Subject Index to the Bibliography Index

Alexander G. Gurevich

Papers

Magnetization Oscillations and Waves