Using a nonperturbative approach, the relaxation rate of orbital dipolar and quadrupolar moments is computed analytically for the t_{2g} states. In the presence of short-range impurities and in the absence of spin-orbit coupling, the orbital relaxation emerges from the competition between momentum scattering and the effect of the crystal field. In the case of weak disorder, the orbital relaxation time is proportional to the momentum scattering time: each scattering event contributes to destroying the orbital moment. In the case of strong disorder, the effect of the crystal field is averaged out, and the orbital relaxation time is inversely proportional to the momentum scattering. We finally find that the dipolar and quadrupolar orbital moments are coupled by the crystal field, resulting in a complex dynamical behavior upon orbital injection.

We theoretically study the direct and inverse spin Hall effects in a superconductor-normal metal-superconductor junction induced by a spin-orbit interaction that is invariant under spatial inversion. We show that a supercurrent induces a spin Hall effect, leading to a static spin accumulation with opposite polarizations at the two edges, analogous to that in normal conductors. For the inverse effect, we consider a spatially inhomogeneous static magnetic field and show that it induces an anomalous phase shift, which, in the presence of higher harmonics, results in a diode effect. Unlike Rashba systems, the present mechanism does not require broken structural inversion symmetry.

Noncollinear antiferromagnets can generate a transverse electrical response known as the anomalous Hall effect, even though they possess almost no net magnetization. The microscopic origin of this behaviour, however, has remained unclear because conventional measurement geometries mix different contributions to the measured response. Here, we show that applying magnetic fields in selected in-plane directions allows us to disentangle the mechanisms underlying the Hall effect in a representative noncollinear antiferromagnet. By suppressing any dipole-related signal, we isolate a purely octupole-driven Hall response that exhibits a characteristic three-fold angular symmetry. At low magnetic fields, we further observe an additional Hall-like contribution that arises from the scalar spin chirality associated with noncoplanar spin textures. Combining symmetry analysis, first-principles calculations, and transport measurements, we reveal that octupole order, dipole moments, and chirality coexist and contribute in distinct field regimes. These findings establish a framework for identifying and controlling complex magnetic order parameters for spintronic applications.

Interfacial orbital transport remains far less understood than its bulk counterpart despite its central role in orbitronic experiments. Here, we theoretically investigate the transmission and conversion of orbital angular momentum across a metallic interface using a model Hamiltonian incorporating crystal-field effects. We show that an injected orbital dipole moment undergoes pronounced oscillations driven by the crystal field and generates characteristic quadrupole moments determined by the orbital orientation relative to the interface. Unlike spin precession, the dipole relaxes toward a finite value away from the interface. We further quantify interfacial orbital memory loss and demonstrate that orbital absorption produces a sizable mechanical torque obtained from the orbital continuity equation.

We investigate spin-and orbital-to-charge conversion phenomena in nonmagnetic materials with broken inversion symmetry, treating the contributions from the Hall effect and the Rashba-Edelstein effect on an equal footing. We develop a general formalism for this interconversion based on macroscopic observables (susceptibility, conductivity, conversion efficiencies). The theory is validated through a case study of ferroelectric alpha-GeTe, where we find that the obtained effective Rashba parameter is smaller than previously reported values for the same material. Incorporating these parameters into a drift-diffusion model, we show that the generated charge current is primarily governed by the Rashba-Edelstein effect, rather than by the spin or orbital Hall effects.

The conversion between spin and orbital currents is at the origin of the orbital torque and its Onsager reciprocal, the orbital pumping. Here, we propose a phenomenological model to describe the orbital torque in magnetic bilayers composed of an orbital source (i.e., a light metal such as Ti, Ru, CuO_{x}, etc.) and a spin-orbit coupled magnet (i.e., typically Ni, (Co/Pt)_{n}, etc.). This approach accounts for spin-to-orbit and orbit-to-spin conversion in the ferromagnet and at the interface. We show that the orbital torque arises from a compromise between orbital current injection from the orbital source to the ferromagnet and spin current backflow from the ferromagnet back to the orbital source. We also discuss the concept of orbital-mixing conductance and introduce the “orbit-spin-mixing” and “spin-orbit-mixing” conductances that govern the orbital torque and orbital pumping, respectively.

We investigate the impact of chiral damping (CD) on current-driven domain-wall (DW) dynamics in antiferromagnets (AFMs). Asymmetric CD between sublattices generates off-diagonal components in the DW mass tensor, thereby coupling translational and rotational modes. When CD is modulated by an ac gate voltage via the Rashba spin-orbit interaction (RSOI), symmetric and asymmetric contributions induce oscillations in the DW velocity and tilt angle, respectively. A perturbative analysis yields explicit expressions for the oscillation amplitudes, in quantitative agreement with numerical simulations. Remarkably, even in the absence of Dzyaloshinskii-Moriya interaction (DMI), asymmetric CD enables chirality switching between Néel- and Bloch-type DWs. Finally, by exploiting the relativistic Lorentz contraction of the DW width at high driving currents, we propose an experimentally viable protocol to qualitatively and quantitatively extract the CD contribution. These results establish clear experimental signatures of CD in antiferromagnetic DW dynamics and demonstrate its potential as a control parameter for magnetic textures.

Orbital effects and orbital torque (OT) were recently discovered as a novel pathway for driving the magnetic moment. However, the development of OT-based magnetic memories suffers from low orbital-to-spin conversion efficiency and incompatibility with magnetic tunnel junctions (MTJs). Here we demonstrate OT-MTJ devices based on Ru/W bilayer, achieving an effective spin-orbit Hall conductivity of -12,600 ħ/2e Ω-1 cm-1. The giant orbital torque originates from the strong orbital effects in the Ru layer and an orbital-to-spin conversion efficiency exceeding 90% in the α-W layer because of the large orbit-spin diffusivity. By harnessing the giant orbital torque, we experimentally achieve switching of OT-MTJs by short pulses down to 28.7 ps, with picosecond-switching dynamics captured. Compared with β-W-based spin-orbit torque-MTJs, the OT-MTJs offer a five to eight-fold reduction in driving voltages, and a highly uniform resistance distribution of <1 Ω across write channels. Our work bridges the critical gap between orbital effects and magnetic memory applications.

Cubic boron nitride (cBN) is a wide-bandgap polymorph of boron nitride whose optical response remains only partially understood due to the coexistence of indirect electronic transitions and strong exciton-phonon coupling. Using first-principles Many-Body Perturbation Theory, we investigate the optical properties of cBN by combining GW quasiparticle corrections with Bethe-Salpeter equation calculations of excitonic effects. Phonon-assisted absorption and emission processes are explicitly included through the exciton-phonon coupling formalism. We find that phonon-mediated optical transitions provide a dominant contribution to both absorption and luminescence spectra, partially reconciling the discrepancy between the theoretical optical gap ( approximate to 11 eV) and experimental emission around 6-7 eV. Our results demonstrate the importance of including exciton-phonon interactions for the correct interpretation of experimental spectra, offering new insights into light emission in wide-bandgap materials.

We theoretically investigate orbital accumulation driven by chiral phonons via orbital-dependent electron-lattice coupling. We derive a formula for the orbital accumulation induced by classical lattice dynamics or nonequilibrium phonons, emphasizing the rectified second-order response of the orbital moment to lattice displacement. We show that chiral phonons primarily couple to orbital quadrupole moments and that static orbital dipole accumulation can be generated at second order in the lattice displacement. Our Letter provides a useful method for generating orbital accumulation without using spin-orbit interactions and suggests a strategy to boost its magnitude by harnessing band structure hot spots associated with orbital degeneracy.

The ever-increasing demand for efficient data storage and processing has fueled the search for novel memory devices. By exploiting the spin-to-charge conversion phenomena, spintronics promises faster and low power solutions alternative to conventional electronics. In this work, a remarkable 34-fold increase in spin-to-charge current conversion is demonstrated when incorporating a 2D epitaxial graphene monolayer between iron and platinum layers by exploring spin-pumping on-chip devices. Furthermore, it is found that the spin conversion is also anisotropic. This enhancement and anisotropy is attributed to the asymmetric Rashba contributions driven by an unbalanced spin accumulation at the differently hybridized top and bottom graphene interfaces, as highlighted by ad-hoc first-principles theory. The improvement in spin-to-charge conversion as well as its anisotropy reveals the importance of interfaces in hybrid 2D-thin film systems, opening up new possibilities for engineering spin conversion in 2D materials, leading to potential advances in memory, logic applications, or unconventional computing.

Altermagnets, a recently identified class of magnetic materials, possess a spin-split Fermi surface that results in the so-called spin splitter effect, enabling the generation of a spin current transverse to the injection direction and whose polarization lies along the Néel vector. In this study, we investigate how magnons interact with electrons in an altermagnetic metal. We find that while the electron-magnon interaction does not perturb the magnon dispersion, a charge current flowing in the material can induce a transverse magnon spin current, analogous to the electronic spin splitter effect. This spin current possesses both electronic and magnonic characteristics, i.e., a chemical potential dependence and a strong temperature dependence. This effect realizes the efficient generation of spin currents via magnons without depending on the material's spin-orbit coupling.

In this study, we investigate the spin and orbital densities induced by magnetization dynamics in a planar bilayer heterostructure. To do this, we employed a theory of adiabatic pumping using the Keldysh formalism and Wigner expansion. We first conduct simulations on a model system to determine the parameters that control the spin and orbital pumping into an adjacent non-magnetic metal. We conclude that, in principle, the orbital pumping can be as significant as spin pumping when the spin-orbit coupling is present in the ferromagnet. We extend the study to realistic heterostructures involving heavy metals (W, Pt, Au) and light metals (Ti, Cu) by using first-principles calculations. We demonstrate that orbital pumping is favored in metals with d states close to the Fermi level, such as Ti, Pt, and W, but is quenched in materials lacking such states, such as Cu and Au. Orbital injection is also favored in materials with strong spin-orbit coupling, leading to large orbital pumping in Ni/(Pt, W) bilayers.

We predict the giant ferroelectric control of interfacial properties of Ni/HfO2, namely, (i) the magnetocrystalline anisotropy and (ii) the inverse spin and orbital Rashba effects. The reversible control of magnetic properties using electric gating is a promising route to low-energy consumption magnetic devices, including memories and logic gates. Synthetic multiferroics, composed of a ferroelectric in proximity to a magnet, stand out as a promising platform for such devices. Using a combination of ab initio simulations and transport calculations, we demonstrate that reversing the electric polarization modulates the interface magnetocrystalline anisotropy from in-plane to out-of-plane. This modulation compares favorably with recent reports obtained upon electromigration induced by ionic gating. In addition, we find that the current-driven spin and orbital densities at the interface can be modulated by about 50% and 30%, respectively. This giant modulation of the spin-charge and orbit-charge conversion efficiencies opens appealing avenues for voltage-controlled spin-and orbitronics devices.

The symmetry of a material fundamentally governs its spin transport properties. While unconventional spin transport phenomena have been predominantly explored in low-symmetry systems (e.g., C_1v symmetry), high-symmetry crystals–which constitute the majority of industry-compatible materials–are generally expected to exhibit only conventional spin-transport behavior. Here, we report the coexistence of two unconventional spin transport effects, the crystal spin-orbit torque (CSOT) and the crystal in-plane Hall effect (CIHE), in a CoPt single ferromagnetic layer with C_3v symmetry. Leveraging the CSOT, we achieve nearly 100

Magnetic two-dimensional (2D) crystals were isolated about a decade ago, triggering a tremendous research activity worldwide. This colloquium raises a stiff question: what is really new about them? At first sight, they seem to be purer implementations of 2D spin models than traditional systems such as ultra-thin films. Yet, they partly realized their promises so far, and whether they give fresh perspectives on long-standing predictions in statistical physics is still an open question. Undoubtedly, they are uniquely amenable to electric-field effect, susceptible to mechanical deformation, and sensitive to moirés, for example. They represent interesting platforms for exploring, challenging, or simply revisiting a wide range of phenomena in condensed matter magnetism. This colloquium intends to offer a critical, yet not necessarily skeptical, overview of the field, clarifying what we believe could be unique with 2D magnets, related quasi-2D van der Waals magnets, and their heterostructures.

We report strong spin-orbit torques (SOTs) generated by noncollinear antiferromagnets Mn3Ni0.35Cu0.65N, over a wide temperature range. The SOT efficiency peaks up to 0.3 at the Néel temperature (TN), substantially higher than that of commonly studied nonmagnets, such as Pt. The sign and magnitude of the SOTs measured in our experiments are corroborated by density functional theory, confirming the dominance of the orbital Hall effect over the spin Hall effect in the nonmagnetic phase above TN. In contrast, the strong temperature-dependent SOTs observed around and below TN can be explained by recently developed mechanisms involving chirality-induced and extrinsic scattering-driven spin and orbital currents, considering the effect of spin fluctuations at finite temperatures. Our work not only reports a large magnitude of SOT but also sheds light on a new possible origin where orbital currents can be harnessed by leveraging the chirality of noncollinear antiferromagnets, which holds promise for magnetic memory applications.

Despite the tremendous interest raised by the recent realization of magnetic Weyl semimetals and the observation of giant anomalous Hall signals, most of the theories used to interpret experimental data overlook the influence of magnetic fluctuations, which are ubiquitous in such materials and can massively impact topological and transport properties. In this work, we predict that in such magnetic topological systems, the interaction between electrons and magnons substantially destabilizes the Weyl nodes, leading to a topological phase transition below the Curie temperature. Remarkably, the sensitivity of the Weyl nodes to electron-magnon interaction depends on their spin chirality. We find that Weyl nodes with a trivial chirality are more sensitive to electron-magnon interactions than Weyl nodes presenting an inverted chirality, demonstrating the resilience of the latter compared to the former. Our results open perspectives for the interpretation of the transport signatures of Weyl semimetals, especially close to the Curie temperature.

The process of spin-charge interconversion is critical in modern spintronics. Nonetheless, experiments conducted on a wide variety of magnetic heterostructures consistently report that charge-to-spin and spin-to-charge conversion efficiencies can be vastly different, especially in the case of topological insulators (TI). This discrepancy between the two "reciprocal" effects remains unexplained, hampering the development of spin devices based on spin-charge conversion. In this study, we investigate both spin-charge and charge-spin interconversion processes in TI Bi2Te3/Py and Pt/Py bilayers experimentally using spin-torque ferromagnetic resonance and spin pumping techniques. We find that the measured charge-to-spin conversion efficiency (C-S) in TI/Py is 26 times larger than the measured spin-to-charge conversion efficiency (S-C), whereas C-S and S-C are comparable in the case of Pt/Py. Using a theoretical model enforcing Onsager reciprocity, we show that spin-to-charge and charge-to-spin conversions in bilayers are genuinely inequivalent, and explain our results as arising from the distinct spin current leakage that takes place during the interconversion. This work clarifies previous conflicting reports on spin-charge interconversion processes and highlights the potential of interface engineering to achieve efficient spin transport in TI-based ferromagnetic heterostructures, paving the way for highly efficient spintronic devices.