Multiferroics are materials with coexisting electric and magnetic orders that are of central importance for fundamental research and technological applications. Unfortunately, intrinsic multiferroics that operate at room temperature remain rare due to an apparent incompatibility between magnetism and ferroelectricity. Here we predict that pure ferroelectrics may support multiferroic-like quasiparticles, termed "multiferrons," that simultaneously carry static magnetic and electric dipoles. The electric dipole moment emerges from the parity-odd anharmonicity of the ferroelectric dynamics, while the magnetic moment has both paramagnetic and diamagnetic origins generated by circularly polarized transverse fluctuations of the ferroelectric polarization. In contrast to the established "dynamical multiferroicity" of circularly polarized phonons, which involves only oscillating electric dipoles, multiferrons exhibit or cause, apart from Zeeman and Einstein-de Haas effects, a linear dc Stark response, giant electric-field-tunable second-harmonic generation in the THz-frequency regime, and a magnetoelectric cross coupling. Multiferrons open a new route toward nonlinear THz optical applications and offer multiferroic functionalities with simple ferroelectrics.

We calculate the magnon dispersion spectra of the two-dimensional zigzag van der Waals antiferromagnet NiPS3 for monolayer, bilayer, and bulk systems as a function of an external magnetic field. We compare the exchange and anisotropy constants of a spin model as calculated by first principles with those obtained experimentally. We can accurately explain the transition from a collinear to a canted ground state for a magnetic field applied normal to the (in-plane) easy-axis and a spin-flop transition when the field is parallel to it. A topologically protected Dirac nodal line is present and robust with respect to both external and anisotropy fields.

We investigate the dipolar-exchange spin wave spectrum in thin ferromagnetic bilayers with inplane magnetization, incorporating interlayer exchange coupling and intra- and interlayer dipolar interactions. In the continuum approximation we analyze the nonreciprocity of propagating magnetic stray fields emitted by spin waves as a function of the relative orientation of the layer magnetizations that are observable by magnetometry of synthetic antiferromagnets or weakly coupled type-A van der Waals antiferromagnetic bilayers as a function of an applied magnetic field.

The exchange interaction at interfaces between superconductors (SCs) and ferromagnets (FMs) has been a central topic in condensed matter physics for many decades, starting with the prediction of exotic phases such as the Fulde-Ferrell-Larkin-Ovchinnikov states and leading to the discovery of triplet superconductivity. This review focuses on new phenomena in SC$|$FM heterostructures caused by the \textit{non-contact dipolar interaction} between magnons, i.e., the quanta of spin wave excitations in the ferromagnet, and the superconducting order. A universal non-relativistic spin-orbit coupling locks the polarization and momentum of their evanescent stray magnetic fields and leads to chiral screening by proximate superconductors. The interaction-induced hybrid quasiparticles are magnon-Meissner collective modes, magnon-cooparon, Josephson plasmonic modes, and nodal magnon-photon polaritons. Superconducting and normal metallic gates modulate and control the magnetodipolar interaction and thereby magnetization and energy transport at interfaces and in thin films.

Harmonic generation, a notable non-linear phenomenon, has promising applications in information processing. For spin-waves in ferromagnetic materials, great progress has been made in the generation higher harmonics, however probing the coherence of these higher harmonics is challenging. Here, using in-situ diamond sensors, we study the coherent harmonic generation of spin waves in a soft ferromagnet. High-order resonance lines are generated via a microwave input and detected by nitrogen-vacancy (NV) centers in nanodiamonds. The phase coherence of the harmonic spin waves is verified by the Rabi oscillations of the NV electron spins. Numerical simulations indicate that the harmonic generation by microwaves below the ferromagnetic resonance frequency is associated with the nonlinear mixing of spin waves by magnetization structures at the film edge. Our finding of geometry-induced magnon harmonic generation constitutes a new way to generate magnon combs with coherent high-order harmonics and may pave the way for magnon-based information processing and quantum sensing applications. Recently, significant progress has been made in higher harmonic generation of spin waves in magnetic materials, but it remains challenging to probe the coherence. Here, Lan, Liu et al observe the higher-harmonic spin-wave generation induced by static edge spin textures in a soft ferromagnetic film, and probe the coherence of higher harmonic generation.

Faster random telegraph noise (RTN) in magnetic tunnel junctions (MTJs) would be beneficial for probabilistic computing applications. However, the interactions between the macrospin and spin waves with finite wave numbers reduce the RTN attempt frequency. We theoretically show that mode-selective heating and cooling by parametric excitation of Kittel mode or propagating spin waves can substantially increase or decrease the RTN frequency, respectively, and propose a nonlinear cooling mechanism that accelerates the switching dynamics. We outline experimental pathways to characterize the nonlinear processes that maximize the operation speed of MTJ-based probabilistic (p-) bits.

All-solid-state nanoscale devices capable of efficiently controlling a heat flow are crucial for advanced thermal management technologies. Here we predict a magnon-driven magnetothermal resistance (mMTR) effect in multilayers of ferromagnets and normal metals, i.e. a thermal resistance that varies when switching between parallel and antiparallel magnetization orientations of the ferromagnetic layers, even in the absence of conduction electrons in the ferromagnets. The mMTR arises from an interfacial temperature drop caused by magnon spin accumulations and can be engineered by the layer thicknesses, spin diffusion lengths, and spin conductances. The mMTR predicted here enables magnetothermal switching in insulator-based systems; we already predict large mMTR ratios up to 40% for superlattices of the electrically insulating magnet yttrium iron garnet and elemental metals.

We address that the stray magnetic field emitted by the excited quantum states of the nitrogen-vacancy (NV) centers is spin-momentum locked, such that the spin transfer to nearby ferromagnetic nanostructures is unidirectional. This may allow the controlled excitation of propagating magnons by NV centers in diamond. A pair of NV spin qubits exchange virtual magnons in a magnetic nanowire in a chiral manner that leads to directional quantum entanglement. A magnon-based “quantum-entanglement isolator" should be a useful device in future quantum information technology.

Ferroelectrics feature spontaneous electric dipolar order reconfigurable via electric fields. Recent theoretical studies of the collective excitations of this electric dipolar order give rise to the hope that "ferron" quasiparticles may complement the magnons of magnetic materials in information and heat management technologies. Yet direct experimental evidence of ferron transport remains elusive. Here we demonstrate efficient ferron injection and detection enabled by ferromagnetic metal contacts, achieving nonlocal signal transmission over micrometer distances in a prototypical ferroelectric PMN-PT. The transmission efficiency can be switched by external magnetic fields that couple to the contacts and gate electric fields that control the ferron excitations. Ferron-based devices open new power saving strategies that employ ferroelectric materials in a future sustainable information society.

We study the spin Hall magnetoresistance (SMR) in noncollinear antiferromagnet Mn_{3}Sn/heavy-metal stacks. The measured SMR exhibits peculiar magnetic field angle and magnitude dependence that sharply deviates from the conventional SMR theory based on the dampinglike spin-transfer torque. An alternative model based on a coherent fieldlike torque reproduces the observations well. Our work reveals a previously unrecognized mechanism of interfacial exchange that indicates a precession of the conduction-electron spins in the collective local exchange fields of the noncollinear antiferromagnetic order. The unraveled physics is essential to understanding and controlling spin transport in unconventional magnetic materials.

Surface waves, the evanescent solutions of the wave equation at planar discontinuities, are of fundamental importance in surface physics, optics, phononics, electronics, and magnetism. Here, we predict that van der Waals antiferromagnets support surface spin waves that are unique by their extreme (nanoscale) confinement and high group velocity with application potential in ultrafast, nanoscale magnonic devices. These excitations reside within the bulk magnon band gap of type-A van der Waals antiferromagnets (antiferromagnetically coupled ferromagnetic monolayers). In contrast to conventional magnetostatic Damon-Eshbach modes, they are pure exchange modes owing to reduced interlayer exchange coupling at the surface layers, and thus persist in ultrathin multilayer stacks and at large wave numbers. We show that they can be efficiently excited by electromagnetic waves, with absorption power comparable to or even exceeding that of bulk modes. Moreover, their emitted magnetic stray fields exhibit pronounced even-odd oscillations with the number of monolayers that should be observable by nitrogen-vacancy-center magnetometry.

We derive analytical expressions for the spin wave frequencies and precession amplitudes in monolayer and antiferromagnetically coupled bilayer CrSBr under in-plane external magnetic fields. The analysis covers the antiferromagnetic, ferromagnetic, and canted phases, demonstrating that the spin wave frequencies in all phases are tunable by the applied magnetic field. We discuss the roles of intra- and interlayer exchange interactions, triaxial anisotropy, and intralayer dynamic dipolar fields in controlling the magnetization dynamics.

Controlling spin currents, that is, the flow of spin angular momentum, in small magnetic devices, is the principal objective of spin electronics, a main contender for future energy-efficient information technologies1,2. A pure spin current has never been measured directly because the associated electric stray fields and/or shifts in the non-equilibrium spin-dependent distribution functions are too small for conventional experimental detection methods optimized for charge transport3,4. Here we report that resonant inelastic X-ray scattering (RIXS) can bridge this gap by measuring the spin current carried by magnons-the quanta of the spin wave excitations of the magnetic order-in the presence of temperature gradients across a magnetic insulator. This is possible due to the sensitivity of the momentum- and energy-resolved RIXS intensity to minute changes in the magnon distribution under non-equilibrium conditions. We use the Boltzmann equation in the relaxation time approximation to extract transport parameters, such as the magnon lifetime at finite momentum, essential for the realization of magnon spintronics.

We report a theory of thermal spin pumping into proximity magnets under a transverse-bias-driven heat flow of magnons in magnetic films when the dipolar coupling to the magnetic gate is tuned to be “chiral”. While there is no rectification of the magnon current in the film, we predict that chirality diverts a large percentage (50% for perfect chirality) of it into the gate. This transverse thermal spin pumping effect can be controlled by rotating the film magnetization and may help manage the heat flow in future magnonic circuits.

We analyze the ``ferron" excitations in order-disorder ferroelectrics by a microscopic pseudo-spin model. We demonstrate that analogous to magnons, the quanta of spin waves in magnetic materials, ferrons carry both static and oscillating electric dipole moments, exhibit a Stark effect, and may be parametrically excited by THz radiation. The anti-crossing gap of the ferron-photon hybrid depends strongly on propagation direction and an applied static electric field. We predict ferron diffusion lengths that can reach centimeters, which implies efficient transport of electric polarization by temperature gradients. These properties suggest that ferroelectric materials may be useful for information technology beyond data storage applications.

Recent research has highlighted the potential of ferroelectricity in van der Waals bilayers in providing an unconventional route for improving device performance. Understanding the static and dynamic properties of domain wall (DW) is critical unlocking this potential, as key parameters such as switching field and speed heavily rely on them. In this article, we conduct a theoretical exploration of the fundamental properties of textures in stacking-engineered ferroelectrics using a machine-learning potential model. Our results demonstrate that competition between the switching barrier of stable ferroelectric states and in-plane lattice distortion leads to a DW width of ten nanometers. We also demonstrate that DW motion can drastically reduce the critical ferroelectric switching field of a monodomain by two orders of magnitude and enable domain switching on a picosecond timescale, suggesting the potential for ultrafast and energy-saving non-volatile memory devices. Moreover, twisting the bilayer into a stacking Moiré structure results in a super-paraelectric state, because the ferroelectric order is reversibly broken by DW motion already at ultralow electric fields. These findings offer valuable insights into the behavior and properties of stacking-engineered ferroelectrics, with significant implications for the development of next-generation electronic devices.

We address the photonic spin-orbit coupling known from nano-optics and plasmonics in the microwave regime. The spin S and momentum q of microwaves emitted by an excited magnetic particle are locked by q S = 0 with a fixed chirality n (S x q) = 1 when evanescent along n perpendicular to q . This field excites magnons in a nearby magnetic film in the form of directional beams that rotate with the magnetization direction. The exchange of these magnons between two distant nanomagnets leads to a highly tunable strong coupling and entangles their excited states.

Owing to the unique features of low Gilbert damping, long spin‐diffusion lengths, and zero Ohmic losses, magnetic insulators are promising candidate materials for next‐generation spintronic applications. However, due to the localized magnetic moments and the complex metal–oxide interface between magnetic insulators and heavy metals, spin‐functional Dzyaloshinskii–Moriya interactions or spin Hall and Edelstein effects are weak, which diminishes the performance of these typical building blocks for spintronic devices. Herein, the exchange coupling between metallic and insulating magnets is exploited for efficient electrical manipulation of heavy metal/magnetic insulator heterostructures. By inserting a thin Co layer, the spin‐orbit torque efficiency is enhanced by more than 20 times, which significantly reduces the switching current density. Moreover, field‐free current‐induced magnetization switching caused by a symmetry‐breaking non‐collinear magnetic texture is demonstrated. This work launches magnetic insulators as an alternative platform for low‐power spintronic devices.

Magnets are interesting materials for classical and quantum information technologies. However, the short decoherence and dephasing times that determine the scale and speed of information networks, severely limit the appeal of employing the ferromagnetic resonance. Here we show that the lifetime and coherence of the uniform Kittel mode can be enhanced by 3-magnon interaction-induced mixing with the long-lived magnons at the minima of the dispersion relation. Analytical and numerical calculations based on this model explain recent experimental results and predict experimental signatures of quantum coherence.

We address the photonic spin-orbit coupling known from nano-optics and plasmonics in the microwave regime. The spin 𝐒 and momentum 𝐪 of microwaves emitted by an excited magnetic particle are locked by 𝐪·𝐒=0 with a fixed chirality 𝐧̂·(Ŝ×q̂)=1 when evanescent along 𝐧̂⊥ q. This field excites magnons in a nearby magnetic film in the form of directional beams that rotate with the magnetization direction. The exchange of these magnons between two distant nanomagnets leads to a highly tunable strong coupling and entangles their excited states.