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.

The discovery of chiral phonons has expanded the conventional view of lattice vibrations as passive heat carriers, opening new opportunities for phononic spintronic devices. However, their realization has been largely limited to chiral crystals or to specific regions of momentum space in certain achiral materials. Here, we propose a generic mechanism for generating propagating chiral phonons in an ordinary dielectric through the precession of the electric polarization in an adjacent ferroelectric. The polarization dynamics transfers its intrinsic angular momentum to the lattice via electrostrictive coupling, thereby pumping chirality-selective phonons whose handedness is dictated by that of the polarization precession. For a typical LiNbO_3|Y_3Al_5O_12 bilayer, we find that the pumping efficiency quantified by an interfacial convertance substantially exceeds those of thermally induced chiral-phonon generation in chiral crystals, owing to the strong electrostrictive coupling in ferroelectrics. Our work establishes ferroelectric dynamics as a versatile electrical source of chiral phonons and provides a general route toward electrically programmable chiral-phononic and spintronic functionalities.

The Peltier effect induces a heat current when a charge current passes through a conductor. Since a charge current is conserved at the junction between two different conductors, the difference between the heat flowing in both conductors for the same charge current leads to heating or cooling of the interface, providing an operating mechanism of solid-state heat pumps. Here, we report observation of heat absorption and release signals even in a junction-free, homogeneous metal when placed in proximity to a ferroelectric insulator. Our experiments using active thermographic imaging techniques confirm the prediction of the ferron-drag effect, i.e., the nonlocal excitation of ferrons, the collective excitation of the ferroelectric order, by conduction electrons in the adjacent metal. We reveal the electric-polarization-direction dependence of the temperature change signals and their unexpected increase with the metal thickness beyond the charge screening length, uncovering additional electron-phonon-ferron interactions in the metal/ferroelectric hybrid structure. The discovery of crosstalk between metals and ferroelectrics via remote ferrons could become both a nuisance and an opportunity for highly integrated circuits with ferroelectric barrier materials and revolutionize the design architecture of thermoelectric devices.

Charge order is conventionally viewed as a static modulation of the electronic density, despite growing experimental capabilities to probe and manipulate its nonequilibrium evolution. In contrast to spin-ordered systems, a microscopic framework for charge-order dynamics and its control under external driving remains largely underdeveloped. Here, starting from an extended Hubbard model, we derive an effective pseudospin model in which the charge-ordered state maps onto staggered pseudospin order. The resulting charge-order dynamics is governed by Landau–Lifshitz–Gilbert-like equations for the pseudospins, closely analogous to those of a bipartite antiferromagnet. We show that an external electric field directly controls the staggered pseudospin order and, above a threshold field, drives coherent reversal of the charge-order polarity by destabilizing collective pseudospin excitations. Our results establish the charge pseudospin as a microscopic dynamical degree of freedom for coherent switching and control of charge order, providing a charge-sector analogue of the well-established framework for spin-order dynamics in spintronics.

Transport properties of (quasi)particles in condensed matter depend profoundly on the spatial dimension. Motivated by recent advances in growing ultrathin magnetic films and monolayer van der Waals magnets, we present a theory of magnon transport in magnetic films spanning the crossover from bulk to the two-dimensional (2D) limit. We find a magnon conductivity that diverges logarithmically in magnetically soft but stable (quasi)2D magnets with long-range dipolar interactions. This critical enhancement is absent in bulk systems and may explain the unusually large magnon conductivities recently observed in ultrathin yttrium iron garnet films. Our results reveal an intrinsic mechanism for enhanced magnon transport in low dimensions and highlight the potential for engineering high-efficiency magnon conductors in atomically thin magnets.

Spin transport in magnetic insulators is often treated by assuming that magnons carry a fixed spin angular momentum of $\hbar$, which does not hold in general, however. Here we calculate the magnon spin angular momentum of a layered antiferromagnet as a function of applied magnetic field and wave vector. We show that the triaxial anisotropy and intralayer dipolar interactions in bilayer CrSBr renormalize the magnon spin angular momentum, which diverges upon field-induced magnon softening. This divergence gives rise to a pronounced peak in the thermal spin Seebeck response and provides a clear spin-caloritronic signature of soft magnons.

Thermoelectric effects enable the conversion between heat and electricity without moving parts. While conventionally associated with mobile charges, we report thermoelectricity caused by bound charges in the form of temperature changes measured by multi-harmonic lock-in thermography of a ferroelectric under an ac electric field. The observed temperature gradient depends on the field-induced displacement current, a Peltier effect in a dielectric material. Its coefficient exceeds 100 V around the ferroelectric-paraelectric phase transition, which is several orders of magnitude greater than reported values in conductors. Our findings uncover previously hidden functionalities of ferroelectric materials for thermal management by directional heat transport in ferroelectrics.

Spin Hall magnetoresistance (SMR), the variation in resistance of a heavy metal (HM) with the magnetization orientation of an adjacent ferromagnet (FM), has been extensively studied as a powerful probe of surface magnetic moments in a variety of magnetic materials. While conventional SMR relies on the angular rotation of an assumed rigid magnetization of fixed magnitude, we report a SMR arising from magnetic-field modulation of spin fluctuations in the FM, with its magnetization direction kept fixed parallel to the spin Hall accumulation in the HM. In contrast to conventional SMR, which scales with the magnetization and vanishes near the Curie temperature (Tc), such "longitudinal" SMR, though suppressed at low temperatures, becomes critically enhanced at Tc, reaching a magnitude comparable to conventional SMR amplitudes. Our work highlights the overlooked importance of the magnetization-magnitude degree of freedom in spin transport and opens a promising avenue for electrically detecting enhanced spin fluctuations in magnetic systems.

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.

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.

Noncollinear antiferromagnets (nAFMs) with a small net magnetic moment offer new opportunities for ultrafast spintronic devices, owing to unique physical properties. While in ferromagnets and collinear AFMs the spin current polarization is locked to the magnetization m and N & eacute;el vector n directions, we predict that magnon spin currents injected by metal contacts into nAFMs can be polarized with both n and m components when carried by a coherent superposition of magnon eigenstates. The spin injection efficiency is governed by an interface spin conductance tensor that depends on the noncollinear magnetic texture. While the m component diffuses freely into the nAFM, the n component oscillates as a function of distance from the injector and applied magnetic field, analogous to the Hanle effect of electron spins in metals. Our findings reveal the potential of nAFMs as platforms for the study of tensorial coherent spin transport.

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.

The orbital Hall effect (OHE) has garnered much attention as a promising approach to realize highly efficient "orbitronic" devices with a wide range of materials. However, the existing theories that attempt to explain the experimental evidence focus on the intrinsic effect, neglecting the omnipresent disorder. Here, we formulate the impact of random defect scattering on the orbital Hall effect by a quantum Boltzmann equation and solve it for a generic two-band model including the in-scattering collision integral (vertex correction). In contrast to the common belief that the intrinsic OHE is robust against the disorder, we find that diffuse scattering by an arbitrarily weak disorder affects and can even fully suppress the intrinsic orbital Hall current, depending on the character of orbital states and the disorder.

We discuss spin-wave transport in anisotropic ferromagnets with an emphasis on the zeroes of the band edges as a function of a magnetic field. An associated divergence of the magnon spin should be observable by enhanced magnon conductivities in non-local experiments, especially in two-dimensional ferromagnets.

The duality between electric and magnetic dipoles in electromagnetism only partly applies to condensed matter. In particular, the elementary excitations of the magnetic and ferroelectric orders, namely magnons and ferrons, respectively, have received asymmetric attention from the condensed matter community in the past. In this perspective, we introduce and summarize the current state of the budding field of "ferronics" and speculate about its potential applications in thermal, information, and communication technology.

We demonstrate the potential of van der Waals magnets for spintronic applications by reporting long-distance magnon spin transport in the electrically insulating antiferromagnet chromium thiophosphate (CrPS$_4$) with perpendicular magnetic anisotropy. We inject and detect magnon spins non-locally by Pt contacts and monitor the non-local resistance as a function of an in-plane magnetic field up to 7 Tesla. We observe a non-local resistance over distances up to at least a micron below the Neel temperature (T$_{\rm N}$ = 38 Kelvin) close to magnetic field strengths that saturate the sublattice magnetizations.