All-optical control of antiferromagnetic order is essential for realizing next-generation energy-efficient spintronic and high-speed memory applications. However, the optical writing of antiferromagnetic domains remains a fundamental challenge, because conventional opto-magnetic recording techniques rely on net magnetization, which is absent in antiferromagnets. In certain multiferroic antiferromagnets, the magnetic toroidal moment provides an additional degree of freedom through its inherent magnetoelectric coupling, which manifests as directional asymmetry in light propagation. Here we demonstrate the all-optical writing of antiferromagnetic domains using the inverse optical magnetoelectric effect in ferrotoroidic LiNiPO4, driven solely by reversing the light propagation direction. This directional control arises from a strong coupling between the photon linear momentum and the magnetic toroidal moment, enabling non-volatile, deterministic and repeatable switching between time-reversed domains with arbitrary light polarization. Our findings establish an inverse optical magnetoelectric effect as a distinct mechanism for manipulating antiferromagnetic order, opening a new paradigm in opto-magnetism driven by photon momentum.

We report universal behavior of the differential magnetic susceptibility (DMS) in the conical phase that mediates the spin-reorientation (SR) transition of the kagome ferromagnet Fe_3Sn_2. Within the SR temperature range, the DMS isotherms exhibit extremely narrow crossing regions, forming isosbestic points. Using an isosbestic-invariance analysis, we show that the isotherms collapse onto a single temperature-independent curve, revealing quadratic-in-temperature corrections to the susceptibility. Complementary field-dependent magnetic-force-microscopy measurements uncover evolution of spin textures from stripe-like domains at low fields to isolated bubble-like domains near the isosbestic field (∼ 0.6 T), a behavior not previously reported in bulk Fe_3Sn_2 within the conical state. These findings point to a universal mechanism for the emergence of complex magnetic textures near isosbestic points, driven by the competition between magnetocrystalline anisotropy, dipolar interactions, and external magnetic field.

The all-optical control of magnetization at room temperature broadens the scope of applications of spin degrees-of-freedom in data storage, spintronics, and quantum computing. Topological magnetic spin structures, such as skyrmions, are of particular interest due to their particle-like properties, small size and inherent stability. Controlling skyrmion states without strong magnetic fields or large current densities would create new possibilities for their application. In this work, we utilize femtosecond optical pulses to alter the helicity of the spin configuration in dipolar skyrmions formed in the kagome magnet Fe_3Sn_2 in the absence of an external magnetic field and at room temperature. In situ Lorentz transmission electron microscopy is used to visualize the stochastic, light-induced switching process of chiral Néel caps, while the internal Bloch component of the dipolar skyrmions remain unchanged. In addition to this switching process, we observe the interconversion between type I skyrmionic and type II bubble configurations depending on the external magnetic field and illumination conditions. To corroborate the spin states and the light-induced magnetization dynamics, micromagnetic modelling and simulations of the resulting electron phase shift maps are conducted to elucidate the spin rearrangement induced by individual femtosecond optical pulses.

Reversible ferroelectric domain wall movements beyond the 10 nm range associated with Rayleigh behavior are usually restricted to specific defect-engineered systems. Here, we demonstrate that such long-range movements naturally occur in the improper ferroelectric ErMnO3 during electric-field-cycling. We study the electric-field-driven motion of domain walls, showing that they readily return to their initial position after having traveled distances exceeding 250 nm. By applying switching spectroscopy band-excitation piezoresponse force microscopy, we track the domain wall movement with nanometric spatial precision and analyze the local switching behavior. Phase field simulations show that the reversible long-range motion is intrinsic to the hexagonal manganites, linking it to their improper ferroelectricity and topologically protected structural vortex lines, which serve as anchor point for the ferroelectric domain walls. Our results give new insight into the local dynamics of domain walls in improper ferroelectrics and demonstrate the possibility to reversibly displace domain walls over much larger distances than commonly expected for ferroelectric systems in their pristine state, ensuring predictable device behavior for applications such as tunable capacitors or sensors.

The all-optical control of magnetisation at room temperature broadens the scope of applications of spin degrees-of-freedom in data storage, spintronics, and quantum computing. Topological magnetic spin structures, such as skyrmions, are of particular interest due to their particle-like properties, small size and inherent stability. Controlling skyrmion states without strong magnetic fields or large current densities would create new possibilities for their application. In this work, we utilize femtosecond optical pulses to alter the helicity of the spin configuration in dipolar skyrmions formed in the kagome magnet Fe3Sn2 in the absence of an external magnetic field and at room temperature. In situ Lorentz transmission electron microscopy is used to visualise the light-induced stochastic switching process of chiral Néel caps, while the internal Bloch component of the dipolar skyrmions remains unchanged. In addition to this switching process, we observe the interconversion between type I skyrmionic and type II bubble configurations depending on the external magnetic field and illumination conditions. To corroborate the spin states and the light-induced magnetisation dynamics, micromagnetic modelling and simulations of the resulting electron phase shift maps are conducted to elucidate the spin rearrangement induced by individual femtosecond optical pulses. The all-optical control of magnetization at room temperature expands the potential of spin-based technologies in data storage and quantum computing. Here, the authors use femtosecond optical pulses to manipulate skyrmion states in Fe₃Sn₂ without external magnetic fields, revealing light-induced switching and interconversion processes, with implications for advanced spintronic applications.

Ferro-/ferri- and antiferromagnetically ordered phases are typically exclusive in nature, thus, their coexistence in atomic-scale proximity is expected only in heterostructures. Breaking this paradigm and broadening the range of unconventional magnetic states, we report here on the observation of a new, atomic-scale hybrid spin state. This ordering is stabilized in three-dimensional crystals of the polar antiferromagnet Co2Mo3O8 by magnetic fields applied perpendicular to the Co honeycomb layers and possesses a spontaneous in-plane ferromagnetic moment. Our microscopic spin model, capturing the observed field dependence of the longitudinal and transverse magnetization as well as the magnetoelectric/elastic properties, reveals that this novel spin state is composed of an alternating stacking of antiferromagnetic and ferrimagnetic honeycomb layers. The strong intralayer and the weak interlayer exchange couplings together with competing anisotropies at octahedral and tetrahedral Co sites are identified as the key ingredients to stabilize antiferromagnetic and ferrimagnetic layers in such close proximity. We show that the proper balance of magnetic interactions can extend the stability range of this hybrid phase down to zero magnetic field. The possibility to realize a layer-by-layer stacking of such distinct spin orders via suitable combinations of microscopic interactions opens a new dimension toward the nanoscale engineering of magnetic states.

The ability to control magnetism with strain offers innovative pathways for the modulation of magnetic domain configurations and for the manipulation of magnetic states in materials on the nanoscale. Although the effect of strain on magnetic domains has been recognized since the early work of C. Kittel, detailed local observations have been elusive. Here, we use mechanical strain to achieve reversible control of magnetic textures in a kagome-type Fe3Sn2 ferromagnet without the use of an external electric current or magnetic field in situ in a transmission electron microscope at room temperature. We use Fresnel defocus imaging, off-axis electron holography and micromagnetic simulations to show that tensile strain modifies the structures of dipolar skyrmions and switches the magnetization between out-of-plane and in-plane configurations. We also present quantitative measurements of magnetic domain wall structures and their transformations as a function of strain. Our results demonstrate the fundamental importance of anisotropy effects and their interplay with magnetoelastic and magnetocrystalline energies, providing opportunities for the development of strain-controlled devices for spintronic applications.

We successfully synthesized single crystals of a series of C15b Laves-phase compounds, RInCo4 (R = Dy-Tm), with Co-pyrochlore and R-fcc sublattices, and systematically studied their magnetic properties via magnetometry measurements. These itinerant cubic compounds, with Curie temperatures above room temperature, show compensated ferrimagnetism featuring an antiferromagnetic coupling between the two sublattices. From this series, DyInCo4 exhibits the highest TC (=368 K) and a near-room-temperature compensation point Tcp (=295 K). TC does not change drastically with the R atom, whereas Tcp is dependent. Another magnetization anomaly is observed in all the compounds at low temperatures, which may be indicative of changes in the lattice or magnetic structure. The easy axis of the ferrimagnetic moment of DyInCo4, ErInCo4, and TmInCo4 is proposed at T = 5 K to be along the [001], [111], and [110] directions, respectively. However, the simple easy-axis or easy-plane ferrimagnetic picture cannot be applied to HoInCo4. These observations suggest that the R sublattice determines magnetic anisotropy and compensation, while the Co sublattice plays a role in strong magnetic ordering. The high Curie temperature, together with the magnetization compensation point near room temperature, renders these itinerant pyrochlore magnets interesting for spintronic applications.

Magnetic anisotropy is a fundamental property of magnetic materials that plays an essential role in the stability of magnetic domains and skyrmions. In this ferromagnetic resonance study, we report the evolution of magnetic anisotropy by substituting various 4d metals in the antiskyrmion host (Fe, Ni)3P with S4 tetragonal symmetry. In the undoped compound (Fe0.63Ni0.37)3P and in the Ru-doped (Fe0.59Ni0.32Ru0.09)3P, the uniaxial magnetic anisotropy has an easy-plane character, while Pd doping turns the material to an easy-axis-type magnet, as observed in (Fe0.62Ni0.29Pd0.09)3P. In addition to the dominant uniaxial anisotropy, we also quantify the fourfold anisotropy of the plane perpendicular to the tetragonal axis in (Fe0.63Ni0.30Pd0.07)3P. Using analytical calculations and micromagnetic simulations, we discuss how this planar anisotropy competes with the anisotropic Dzyaloshinskii-Moriya interaction in determining the orientation of the magnetic stripes and the antiskyrmions.

Topological magnets exhibit fascinating properties like topologically protected surface states or anomalous transport phenomena. While these properties can be significantly altered by manipulating the magnetic state, the experimental verification of such predictions remains challenging. Here, we demonstrate the efficient magnetic field control of the Weyl semimetallic state of the collinear ferromagnet Co_3Sn_2S_2 by magneto-optical spectroscopy. We resolve a redshift of the nodal loop resonance as the magnetization is rotated into the kagome plane by the magnetic field. Our material-specific theory, capturing the observed field-induced spectral reconstruction, shows the creation of 26 Weyl points for one in-plane magnetization direction and predicts the emergence of a gapless nodal loop for the orthogonal in-plane magnetization orientation. These findings demonstrate that while topological band structures are generally considered robust, breaking underlying crystal symmetries with external fields provides an efficient way to manipulate them, even in collinear magnets. This approach opens exciting avenues to control band topology also in materials with more complex magnetic structures and even to study the interplay of real- and momentum-space topological states, e.g. in skyrmion-lattice systems.

Extraordinary physical properties arise at polar interfaces in oxide materials, including the emergence of two-dimensional electron gases, sheet-superconductivity, and multiferroicity. A special type of polar interface are ferroelectric domain walls, where electronic reconstruction phenomena can be driven by bound charges. Great progress has been achieved in the characterization of such domain walls and, over the last decade, their potential for next-generation nanotechnology has become clear. Established tomography techniques, however, are either destructive or offer insufficient spatial resolution, creating a pressing demand for 3D imaging compatible with future fabrication processes. Here, we demonstrate non-destructive tomographic imaging of ferroelectric domain walls using secondary electrons. Utilizing conventional scanning electron microscopy (SEM), we reconstruct the position, orientation, and charge state of hidden domain walls at distances up to several hundreds of nanometers away from the surface. A mathematical model is derived that links the SEM intensity variations at the surface to the local domain wall properties, enabling non-destructive tomography with good noise tolerance on the timescale of seconds. Our SEM-based approach facilitates high-throughput screening of materials with functional domain walls and domain-wall-based devices, which is essential for monitoring during the production of device architectures and quality control in real-time.

Quasimolecular orbitals in cluster Mott insulators provide a route to tailor exchange interactions, which may yield novel quantum phases of matter. We demonstrate the cluster Mott character of the lacunar spinel GaTa4Se8 using resonant inelastic x-ray scattering (RIXS) at the Ta L3 edge. Electrons are fully delocalized over Ta4 tetrahedra, forming quasimolecular Jtet = 3/2 moments. The modulation of the RIXS intensity as function of the transferred momentum q allows us to determine the cluster wave function, which depends on competing intracluster hopping terms that mix states with different character. This mixed wave function is decisive for the macroscopic properties since it affects intercluster hopping and exchange interactions and furthermore renormalizes the effective spin-orbit coupling constant. The versatile wave function, tunable via intracluster hopping, opens a new perspective on the large family of lacunar spinels and cluster Mott insulators in general.

Skyrmion lattices (SkL) in centrosymmetric materials typically have a magnetic period on the nanometer-scale, so that the coupling between magnetic superstructures and the underlying crystal lattice cannot be neglected. We reveal the commensurate locking of a SkL to the atomic lattice in Gd3Ru4Al12 via high-resolution resonant elastic x-ray scattering (REXS). Weak easy-plane magnetic anisotropy, demonstrated here by a combination of ferromagnetic resonance and REXS, penalizes placing a skyrmion core on a site of the atomic lattice. Under these conditions, a commensurate SkL, locked to the crystal lattice, is stable at finite temperatures – but gives way to a competing incommensurate ground state upon cooling. We discuss the role of Umklapp-terms in the Hamiltonian for the formation of this lattice-locked state, its magnetic space group, and the role of slight discommensurations, or (line) defects in the magnetic texture. We also contrast our findings with the case of SkLs in noncentrosymmetric material platforms.

Hemozoin is a natural biomarker formed during the hemoglobin metabolism of Plasmodium parasites, the causative agents of malaria. The rotating-crystal magneto-optical detection (RMOD) has been developed for its rapid and sensitive detection both in cell cultures and patient samples. In the current article we demonstrate that, besides quantifying the overall concentration of hemozoin produced by the parasites, RMOD can also track the size distribution of the hemozoin crystals. We establish the relations between the magneto-optical signal, the mean parasite age and the median crystal size throughout one erythrocytic cycle of Plasmodium falciparum parasites, where the latter two are determined by optical and scanning electron microscopy, respectively. The significant correlation between the magneto-optical signal and the stage distribution of the parasites indicates that the RMOD method can be utilized for species-specific malaria diagnosis and for the quick assessment of drug efficacy.

Distinguishing different antiferromagnetic domains by electrical probes is a challenging task, which in itinerant compounds can be achieved, e.g., via the anisotropic magnetoresistance. Here, we demonstrate that in insulators, the anisotropic magnetocapacitance can be exploited for the same purpose. We studied the magnetic field dependence of the dielectric response in BiFeO3, one of the few room-temperature multiferroics. We observed a sizeable dielectric anisotropy upon the rotation of the modulation vector of the antiferromagnetic cycloid in the plane normal to the rhombohedral axis. Importantly, this anisotropy is characteristic of the cycloidal mono-domain state even in zero magnetic field, thus facilitating the determination of the antiferromagnetic domain population. This approach can be utilized to electrically distinguish between antiferromagnetic domains even in complex magnets, such as modulated spin structures, via the magnetodielectric coupling. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/4.0/).

We explore the impact of a magnetic field on the ferroelectric domain pattern in polycrystalline hexagonal ErMnO3 at cryogenic temperatures. Utilizing piezoelectric force microscopy measurements at 1.65 K, we observe modifications of the topologically protected ferroelectric domain structure induced by the magnetic field. These alterations likely result from strain induced by the magnetic field, facilitated by intergranular coupling in polycrystalline multiferroics. Our findings give insights into the interplay between electric and magnetic properties at the local scale and represent a so far unexplored pathway for manipulating topologically protected ferroelectric vortex patterns in hexagonal manganites.

Direct electron detectors in scanning transmission electron microscopy give unprecedented possibilities for structure analysis at the nanoscale. In electronic and quantum materials, this new capability gives access to, for example, emergent chiral structures and symmetry-breaking distortions that underpin functional properties. Quantifying nanoscale structural features with statistical significance, however, is complicated by the subtleties of dynamic diffraction and coexisting contrast mechanisms, which often results in low signal-to-noise and the superposition of multiple signals that are challenging to deconvolute. Here we apply scanning electron diffraction to explore local polar distortions in the uniaxial ferroelectric Er(Mn,Ti)O$_3$. Using a custom-designed convolutional autoencoder with bespoke regularization, we demonstrate that subtle variations in the scattering signatures of ferroelectric domains, domain walls, and vortex textures can readily be disentangled with statistical significance and separated from extrinsic contributions due to, e.g., variations in specimen thickness or bending. The work demonstrates a pathway to quantitatively measure symmetry-breaking distortions across large areas, mapping structural changes at interfaces and topological structures with nanoscale spatial resolution.

Quasimolecular orbitals in cluster Mott insulators provide a route to tailor exchange interactions, which may yield novel quantum phases of matter. We demonstrate the cluster Mott character of the lacunar spinel GaTa$_4$Se$_8$ using resonant inelastic x-ray scattering (RIXS) at the Ta $L_3$ edge. Electrons are fully delocalized over Ta$_4$ tetrahedra, forming quasimolecular $J_{\rm tet}$=3/2 moments. The modulation of the RIXS intensity as function of the transferred momentum q allows us to determine the cluster wavefunction, which depends on competing intracluster hopping terms that mix states with different character. This mixed wavefunction is decisive for the macroscopic properties since it affects intercluster hopping and exchange interactions and furthermore renormalizes the effective spin-orbit coupling constant. The versatile wavefunction, tunable via intracluster hopping, opens a new perspective on the large family of lacunar spinels and cluster Mott insulators in general.

The lack of both time-reversal and spatial inversion symmetry in polar magnets is a prerequisite for the occurrence of optical magnetoelectric effects such as nonreciprocal directional dichroism with the potential for the realization of optical diodes. In particular, antiferromagnetic materials with magnetic excitations in the THz range such as Fe2Mo3O8 are promising candidates for next-generation spintronic applications. In a combined experimental and theoretical effort we investigated the THz excitations of the polar honeycomb antiferromagnet Fe2Mo3O8 in external magnetic fields and their nonreciprocal directional dichroism, together with the temperature dependence of the electronic transitions in the mid- and near-infrared frequency range. Using an advanced single-ion approach for the Fe ions, we are able to describe optical excitations from the THz to the near-infrared frequency range quantitatively and successfully model the observed nonreciprocal directional dichroism in the THz regime.