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The coupling of conduction electrons and magnetic textures leads to quantum transport phenomena described by the language of emergent electromagnetic fields1–3. For magnetic skyrmions, spin-swirling particle-like objects, an emergent magnetic field is produced by their topological winding4–6, resulting in the conduction electrons exhibiting the topological Hall effect (THE)7. When the skyrmion lattice (SkL) acquires a drift velocity under conduction electron flow, an emergent electric field is also generated8,9. The resulting emergent electrodynamics dictate the magnitude of the THE by the relative motion of SkL and conduction electrons. Here we report the emergent electrodynamics induced by SkL motion in Gd2PdSi3, facilitated by its giant THE10,11. With increasing current excitation, we observe the dynamic transition of the SkL motion from the pinned to creep regime and finally to the flow regime, in which the THE is totally suppressed. We argue that the Galilean relativity required for the total cancellation of the THE may be generically recovered in the flow regime, even in complex multiband systems such as the present compound. Moreover, the observed THE voltages are large enough to enable real-time measurement of the SkL velocity–current profile, which shows the inertial-like motion of the SkL in the creep regime, appearing as the current hysteresis of the skyrmion velocity.

Polar metals have recently garnered increasing interest because of their promising functionalities. Here we report the experimental realization of an intrinsic coexisting ferromagnetism, polar distortion and metallicity in quasi-two-dimensional Ca3Co3O8. This material crystallizes with alternating stacking of oxygen tetrahedral CoO4monolayers and octahedral CoO6bilayers. The ferromagnetic metallic state is confined within the quasi-two-dimensional CoO6layers, and the broken inversion symmetry arises simultaneously from the Co displacements. The breaking of both spatial-inversion and time-reversal symmetries, along with their strong coupling, gives rise to an intrinsic magnetochiral anisotropy with exotic magnetic field-free non-reciprocal electrical resistivity. An extraordinarily robust topological Hall effect persists over a broad temperature–magnetic field phase space, arising from dipole-induced Rashba spin–orbit coupling. Our work not only provides a rich platform to explore the coupling between polarity and magnetism in a metallic system, with extensive potential applications, but also defines a novel design strategy to access exotic correlated electronic states.

Atomic-scale observations of a specific local area would be considerably beneficial when exploring new fundamental materials and devices. The development of hardware-type aberration correction1,2in electron microscopy has enabled local structural observations with atomic resolution3–5as well as chemical and vibration analysis6–8. In magnetic imaging, however, atomic-level spin configurations are analysed by electron energy-loss spectroscopy by placing samples in strong magnetic fields9–11, which destroy the nature of the magnetic ordering in the samples. Although magnetic-field-free observations can visualize the intrinsic magnetic fields of an antiferromagnet by unit-cell averaging12, directly observing the magnetic field of an individual atomic layer of a non-uniform structure is challenging. Here we report that the magnetic fields of an individual lattice plane inside materials with a non-uniform structure can be observed under magnetic-field-free conditions by electron holography with a hardware-type aberration corrector assisted by post-digital aberration correction. The magnetic phases of the net magnetic moments of (111) lattice planes formed by opposite spin orderings between Fe3+and Mo5+in a ferrimagnetic double-perovskite oxide (Ba2FeMoO6) were successfully observed. This result opens the door to direct observations of the magnetic lattice in local areas, such as interfaces and grain boundaries, in many materials and devices.

We report an inelastic neutron scattering (INS) study on the low-energy magnetic excitations of Ba2CoGe2O7 single crystals under applied magnetic fields for several fixed temperatures. We have observed a sizable spin gap of ≈0.105 meV for the acoustic mode at 2.2 K under zero magnetic fields, whereas no changes in energy of the optical modes at the magnetic zone center are observed under varying temperatures. However, the spin gap of the acoustic mode at finite fields also evolves under varying temperatures and the gap energy decreases with increasing temperature. Interestingly, the difference between the spin gap at 2.2 and 4.6 K is about ≈0.025 meV for all the applied fields of 0.35, 1, and 5 T. Our results enlighten the discussion in detail on understanding the origin of the spin gap in Ba2CoGe2O7.

Adiabatic charge pumping is one of the most salient features of topological phases of matter1–3. Laughlin’s charge pumping in a quantum Hall system is the prototypical example4. In analogy, three-dimensional topological insulators have been predicted to support charge pumping through their magnetically gapped surface states5–10. But despite its importance as a direct probe of surface Hall conductivity, charge pumping has not been demonstrated in topological-insulator-based systems. Here we report the observation of charge pumping in a thin-film magnetic heterostructure of topological insulators in a geometry that prohibits edge transport. We find that charge pumping occurs between the inner and outer electrodes in response to alternating magnetic fields when the sample is in the quantum anomalous Hall insulator phase. The amount of pumped charge is accounted for by the surface Hall conductivity of half the quantum conductance for each surface, from a comparison with the axion insulator phase that shows no charge pumping. Because charge pumping is closely related to the theoretically predicted topological magnetoelectric effect5–10, our observation may provide clues to its direct observation.

Reservoir computing is a neuromorphic architecture that may offer viable solutions to the growing energy costs of machine learning. In software-based machine learning, computing performance can be readily reconfigured to suit different computational tasks by tuning hyperparameters. This critical functionality is missing in ‘physical’ reservoir computing schemes that exploit nonlinear and history-dependent responses of physical systems for data processing. Here we overcome this issue with a ‘task-adaptive’ approach to physical reservoir computing. By leveraging a thermodynamical phase space to reconfigure key reservoir properties, we optimize computational performance across a diverse task set. We use the spin-wave spectra of the chiral magnet Cu2OSeO3that hosts skyrmion, conical and helical magnetic phases, providing on-demand access to different computational reservoir responses. The task-adaptive approach is applicable to a wide variety of physical systems, which we show in other chiral magnets via above (and near) room-temperature demonstrations in Co8.5Zn8.5Mn3(and FeGe).

Configurational entropy can impact crystallization processes, tipping the scales between structures of nearly equal internal energy. Using alloyed single crystals of Gd2PdSi3in the AlB2-type structure, we explore the formation of complex layer sequences made from alternating, two-dimensional triangular and honeycomb slabs. A four-period and an eight-period stacking sequence are found to be very close in internal energy, the latter being favored by entropy associated with covering the full configuration space of interlayer bonds. Possible consequences of polytype formation on magnetism in Gd2PdSi3are discussed.

The concept of topology has dramatically expanded the research landscape of magnetism, leading to the discovery of numerous magnetic textures with intriguing topological properties. A magnetic skyrmion is an emergent topological magnetic texture with a string-like structure in three dimensions and a disk-like structure in one and two dimensions. Skyrmions in zero dimensions have remained elusive due to challenges from many competing orders. Here, by combining electron holography and micromagnetic simulations, we uncover the real-space magnetic configurations of a skyrmionic vortex structure confined in a B20-type FeGe tetrahedral nanoparticle. An isolated skyrmionic vortex forms at the ground state and this texture shows excellent robustness against temperature without applying a magnetic field. Our findings shed light on zero-dimensional geometrical confinement as a route to engineer and manipulate individual skyrmionic metastructures.

Topological electronic materials, such as topological insulators, are distinct from trivial materials in the topology of their electronic band structures that lead to robust, unconventional topological states, which could bring revolutionary developments in electronics. This Perspective summarizes developments of topological insulators in various electronic applications including spintronics and magnetoelectronics. We group and analyse several important phenomena in spintronics using topological insulators, including spin–orbit torque, the magnetic proximity effect, interplay between antiferromagnetism and topology, and the formation of topological spin textures. We also outline recent developments in magnetoelectronics such as the axion insulator and the topological magnetoelectric effect observed using different topological insulators.

The formation of topological spin textures at the nanoscale has a significant impact on the long-range order and dynamical response of magnetic materials. We study the relaxation mechanisms at the conical-to-helical phase transition in the chiral magnet FeGe. By combining macroscopic ac susceptibility measurement, surface-sensitive magnetic force microscopy, and micromagnetic simulations, we demonstrate how the motion of magnetic topological defects, here edge dislocations, impacts the local formation of a stable helimagnetic spin structure. Although the simulations show that the edge dislocations can move with a velocity up to 100 m/s through the helimagnetic background, their dynamics are observed to disturb the magnetic order on the time scale of minutes due to randomly distributed pinning sites. The results corroborate the substantial impact of dislocation motions on the nanoscale spin structure in chiral magnets, revealing previously hidden effects on the formation of helimagnetic domains and domain walls.

Quantum materials show emergent electronic properties and related functions that are profoundly described by quantum mechanics beyond the semi-classical picture of electrons. Here, key developments and progress in the last two decades are surveyed and future challenges outlined.

Topological spin textures have attracted much attention both for fundamental physics and spintronics applications. Among them, antiskyrmions possess a unique spin configuration with Bloch-type and Néel-type domain walls owing to anisotropic Dzyaloshinskii–Moriya interaction in the non-centrosymmetric crystal structure. However, antiskyrmions have thus far only been observed in a few Heusler compounds with D2dsymmetry. Here we report a new material, Fe1.9Ni0.9Pd0.2P, in a different symmetry class (S4), in which antiskyrmions exist over a wide temperature range that includes room temperature, and transform into skyrmions on changing magnetic field and lamella thickness. The periodicity of magnetic textures greatly depends on the crystal thickness, and domains with anisotropic sawtooth fractals were observed at the surface of thick crystals and attributed to the interplay between the dipolar interaction and the Dzyaloshinskii–Moriya interaction as governed by crystal symmetry. Our findings provide an arena in which to study antiskyrmions, and should stimulate further research on topological spin textures and their applications.

The quantum anomalous Hall effect (QAHE)1–6is a transport phenomenon where the Hall resistance is quantized to the von Klitzing constant due to the spontaneous magnetization of a ferromagnetic material even at zero magnetic field. Similar to the quantum Hall effect (QHE) under strong magnetic fields, the quantized Hall resistance of QAHE is supposed to be universal, independent of the details in the experimental realization7,8. However, the quantization accuracy of QAHE reported so far9–11is much poorer than that of QHE. Here we demonstrate a precision of 10 parts per billion of Hall resistance quantization in QAHE. By directly comparing QAHE with QHE, we confirm that the quantization accuracy of QAHE satisfies the required level as a primary standard of electric resistance. We achieve this high accuracy of quantization by using a weak magnetic field supplied by a permanent disc magnet to align the magnetization domains. Our findings establish a milestone for developing a quantum resistance standard without strong magnetic fields.

Exotic topological spin textures such as emergent magnetic monopole/anti-monopoles (hedgehog/anti-hedgehog) in the metastable extended skyrmion-strings attract much attention to the fundamental physics owing to their novel electromagnetic properties. However, the direct imaging of such spin textures is lacking. Here, we report the real-space observation of emergent magnetic monopoles involved in extended skyrmion-strings by Lorentz transmission electron microscopy (TEM) in combination with micromagnetic simulations. The in-plane extended skyrmion-strings are observed directly by Lorentz TEM to accompany the topological hedgehog-like defect, where the skyrmion-string terminates or merges with another skyrmion-string, as well as the surface-related defects where skyrmion-string bends 90° and ends on the surface. We also demonstrate the transformation of a metastabilized lattice of out-of-plane short skyrmion-strings into an in-plane array of extended skyrmion-strings by tuning the magnitude of oblique fields in a room-temperature helimagnet, revealing the stability of such topological spin textures and the possibility to control them.

An inductor, one of the most fundamental circuit elements in modern electronic devices, generates a voltage proportional to the time derivative of the input current1. Conventional inductors typically consist of a helical coil and induce a voltage as a counteraction to time-varying magnetic flux penetrating the coil, following Faraday’s law of electromagnetic induction. The magnitude of this conventional inductance is proportional to the volume of the inductor’s coil, which hinders the miniaturization of inductors2. Here, we demonstrate an inductance of quantum-mechanical origin3, generated by the emergent electric field induced by current-driven dynamics of spin helices in a magnet. In microscale rectangular magnetic devices with nanoscale spin helices, we observe a typical inductance as large as −400 nanohenry, comparable in magnitude to that of a commercial inductor, but in a volume about a million times smaller. The observed inductance is enhanced by nonlinearity in current and shows non-monotonous frequency dependence, both of which result from the current-driven dynamics of the spin-helix structures. The magnitude of the inductance rapidly increases with decreasing device cross-section, in contrast to conventional inductors. Our findings may pave the way to microscale, simple-shaped inductors based on emergent electromagnetism related to the quantum-mechanical Berry phase.

The topological nature of the quantum anomalous Hall effect (QAHE) causes a dissipationless chiral edge current at the sample boundary1,2. Of fundamental interest is whether the chirality of the band structure manifests itself in charge transport properties. Here we report the observation of large non-reciprocal charge transport3in a magnetic topological insulator, Cr-doped (Bi,Sb)2Te3. When the surface massive Dirac band is slightly carrier doped by a gate voltage, the edge state starts to dissipate and exhibits a current-direction-dependent resistance with a directional difference as large as 26%. The polarity of this diode effect depends on the magnetization direction as well as on the carrier type, electrons or holes. The correlation between the non-reciprocal resistance and the Hall resistance indicates that the non-reciprocity originates from the interplay between the chiral edge state and the Dirac surface state.

Magnetic skyrmions are topologically stable spin swirls with a particle-like character and are potentially suitable for the design of high-density information bits. Although most known skyrmion systems arise in non-centrosymmetric systems with a Dzyaloshinskii–Moriya interaction, centrosymmetric magnets with a triangular lattice can also give rise to skyrmion formation, with a geometrically frustrated lattice being considered essential in this case. Until now, it remains an open question if skyrmions can also exist in the absence of both geometrically frustrated lattice and inversion symmetry breaking. Here we discover a square skyrmion lattice state with 1.9 nm diameter skyrmions in the centrosymmetric tetragonal magnet GdRu2Si2without a geometrically frustrated lattice by means of resonant X-ray scattering and Lorentz transmission electron microscopy experiments. A plausible origin of the observed skyrmion formation is four-spin interactions mediated by itinerant electrons in the presence of easy-axis anisotropy. Our results suggest that rare-earth intermetallics with highly symmetric crystal lattices may ubiquitously host nanometric skyrmions of exotic origins.

Control of topological spin textures in magnetic systems may enable future spintronic applications. Magnetic field pulses can switch the vortex polarity1or the winding number of magnetic bubbles2. Thermal energy can reverse the helicity of skyrmions3and induce the transformation between meron and skyrmion by modifying the in-plane anisotropy4,5. Among the various topological spin textures, skyrmions6,7and antiskyrmions8–10are nanometric spin-whirling structures carrying integer topological charges (N) of −1 and +1 (refs. 7,11,12), respectively, and can be observed in real space8,13. They exhibit different dynamical properties under current flow14–18, for example, opposite signs for the topological Hall effect. Here we observe, in real space, transformations among antiskyrmions, non-topological (NT) bubbles and skyrmions (with Nof +1, 0 and −1, respectively) and their lattices in a non-centrosymmetric Heusler magnet, Mn1.4Pt0.9Pd0.1Sn, with D2dsymmetry. Lorentz transmission electron microscopy images under out-of-plane magnetic fields show a square lattice of square-shaped antiskyrmions near the Curie temperature and a triangular lattice of elliptically deformed skyrmions with opposite helicities at lower temperatures. The clockwise and counter-clockwise helicities of the skyrmions originate from Dzyaloshinskii–Moriya interactions with opposite signs along the [100] and [010] directions, respectively. A variation of the in-plane magnetic field induces a topological transformation from antiskyrmions to NT-bubbles and to skyrmions, which is accompanied by a change of the lattice geometry. We also demonstrate control of the helicity of skyrmions by variations of the in-plane magnetic field. These results showcase the control of the topological nature of spin configurations in complex magnetic systems.

Magnetic skyrmions are topological spin textures that have shown promise for future nonvolatile memory devices. Herein, we report on the stability of magnetic skyrmions in alloyed cubic B20 Fe1–xCoxSi nanowires (NWs) determined using off-axis electron holography and magnetotransport measurements. This study presents the real space observation of one-dimensional skyrmion lattice in a NW of Fe1–xCoxSi which shows that the skyrmion phase in a Fe0.75Co0.25Si NW exists at lower applied magnetic fields (200 Oe) with a reduced domain size (28 ± 2 nm) in comparison to bulk and thin film samples. Magnetotransport measurements were used to observe the helimagnetic transition temperature dependence on the cobalt concentration in the Fe1–xCoxSi NWs. Field-dependent magnetoresistance measurements of Fe1–xCoxSi NWs under applied magnetic field parallel to the NW axis and their second derivative plots reveal the critical fields for the magnetic state transition at different temperatures. A representative magnetic phase diagram constructed with the results from transport measurements of a Fe0.81Co0.19Si NW clearly shows expanded stability region for magnetic skyrmions in the Fe1–xCoxSi NWs.

Magnetic skyrmions have attracted attention because of their emergent electromagnetic properties. In particular, non-equilibrium-state skyrmions, which are protected by topology and hence can exist over a wider temperature–magnetic-field region, show promise for possible practical applications, but their dynamics remain elusive. Here, we report the observation of a magnetic-field-induced dynamical transition from the metastable hexagonal-lattice skyrmion crystal (SkX) at a zero bias-field to an amorphous state via the densely vacancy-populated SkX. With decreasing field, on the other hand, the aggregate transforms from ‘random particles’ to ‘microcrystals’ of skyrmions in a non-equilibrium state, in analogy to colloidal crystallization, and subsequently undergoes a topologically distinct phase separation between the SkX and helical/conical domains accompanied by topological defects. These observations directly demonstrate the aggregation and collapse dynamics of metastable skyrmions and may provide a route to other nontrivial topological phenomena such as the zero-magnetic-field topological Hall effect. A Lorentz transmission electron microscopy study of a thin iron germanium film reveals the aggregation dynamics of skyrmions when pushed out of equilibrium.

Constituent atoms and electrons determine matter properties together, and they can form long-range ordering respectively. Distinguishing and isolating the electronic ordering out from the lattice crystal is a crucial issue in contemporary materials science. However, the intrinsic structure of a long-range electronic ordering is difficult to observe because it can be easily affected by many external factors. Here, we present the observation of electronic multiple ordering (EMO) and its dynamics at the micrometer scale in a manganite thin film. The strong internal couplings among multiple electronic degrees of freedom in the EMO make its morphology robust against external factors and visible via well-defined boundaries along specific axes and cleavage planes, which behave like a multiple-ordered electronic crystal. A strong magnetic field up to 17.6 T is needed to completely melt such EMO at 7 K, and the corresponding formation, motion, and annihilation dynamics are imaged utilizing a home-built high-field magnetic force microscope. The EMO is parasitic within the lattice crystal house, but its dynamics follows its own rules of electronic correlation, therefore becoming distinguishable and isolatable as the electronic ordering. Our work provides a microscopic foundation for the understanding and control of the electronic ordering and the designs of the corresponding devices.

Materials can harbour quantum many-body systems, most typically in the form of strongly correlated electrons in solids, that lead to novel and remarkable functions thanks to emergence—collective behaviours that arise from strong interactions among the elements. These include the Mott transition, high-temperature superconductivity, topological superconductivity, colossal magnetoresistance, giant magnetoelectric effect, and topological insulators. These phenomena will probably be crucial for developing the next-generation quantum technologies that will meet the urgent technological demands for achieving a sustainable and safe society. Dissipationless electronics using topological currents and quantum spins, energy harvesting such as photovoltaics and thermoelectrics, and secure quantum computing and communication are the three major fields of applications working towards this goal. Here, we review the basic principles and the current status of the emergent phenomena and functions in materials from the viewpoint of strong correlation and topology.

Materials can be transformed from one crystalline phase to another by using an electric field to control ion transfer, in a process that can be harnessed in applications such as batteries, smart windows and fuel cells. Increasing the number of transferrable ion species and of accessible crystalline phases could in principle greatly enrich material functionality. However, studies have so far focused mainly on the evolution and control of single ionic species (for example, oxygen, hydrogen or lithium ions). Here we describe the reversible and non-volatile electric-field control of dual-ion (oxygen and hydrogen) phase transformations, with associated electrochromic and magnetoelectric effects. We show that controlling the insertion and extraction of oxygen and hydrogen ions independently of each other can direct reversible phase transformations among three different material phases: the perovskite SrCoO3−δ(ref. 12), the brownmillerite SrCoO2.5(ref. 13), and a hitherto-unexplored phase, HSrCoO2.5. By analysing the distinct optical absorption properties of these phases, we demonstrate selective manipulation of spectral transparency in the visible-light and infrared regions, revealing a dual-band electrochromic effect that could see application in smart windows. Moreover, the starkly different magnetic and electric properties of the three phases—HSrCoO2.5is a weakly ferromagnetic insulator, SrCoO3−δis a ferromagnetic metal, and SrCoO2.5is an antiferromagnetic insulator—enable an unusual form of magnetoelectric coupling, allowing electric-field control of three different magnetic ground states. These findings open up opportunities for the electric-field control of multistate phase transformations with rich functionalities.

Topologically stable matter can have a long lifetime, even if thermodynamically costly, when the thermal agitation is sufficiently low. A magnetic skyrmion lattice (SkL) represents a unique form of long-range magnetic order that is topologically stable, such that a long-lived, metastable SkL can form. Experimental observations of the SkL in bulk crystals, however, have mostly been limited to a finite and narrow temperature region in which the SkL is thermodynamically stable; thus, the benefits of the topological stability remain unclear. Here, we report a metastable SkL created by quenching a thermodynamically stable SkL. Hall-resistivity measurements of MnSi reveal that, although the metastable SkL is short-lived at high temperatures, the lifetime becomes prolonged (≫1 week) at low temperatures. The manipulation of a delicate balance between thermal agitation and the topological stability enables a deterministic creation/annihilation of the metastable SkL by exploiting electric heating and subsequent rapid cooling, thus establishing a facile method to control the formation of a SkL.

The polar magnetic semimetal NdAlSi hosts Weyl fermions that promote helical magnetism, and defines a roadmap for future work on correlated topological matter.

In this study we have investigated dopant effects on a typical charge-orbital ordered state in a single layered manganite La0.5Sr1.5MnO4using magnetization measurement and resonant x-ray scattering technique. We have studied how the charge-orbital ordered state is affected by the 3% substitution of Cr, Fe and Ga ions for Mn ions. It is revealed that the impurity doping suppresses the long-range charge-orbital order, in which the degree of suppression strongly depends on dopant ions, and eventually forms a phase-separated ground state between charge-orbital order and ferromagnetic states.

The neutron Brillouin scattering (NBS) method, which was developed using advanced neutron sources, was extended to an intense pulsed spallation neutron source. The feasibility of this method was demonstrated by observing ferromagnetic spin waves in La0.8Sr0.2MnO3and SrRuO3powders. Gapless spin-wave excitations were observed in La0.8Sr0.2MnO3, which were continuously extrapolated to the lower scattering vector $Q$ from previous results using single crystals. The novel result is a well-defined quadratic $Q$ dependence in the spin-wave dispersion curve with a large energy gap in SrRuO3.

The interfacial atomic mixing in a perovskite type manganese oxide superlattice is studied. A superlattice made of antiferromagnetic insulator Pr0.5Ca0.5MnO3(PCMO) and ferromagnetic metal La0.5Sr0.5MnO3(LSMO) on a (011) surface of perovskite structured substrate is reported to have ferromagnetic PCMO layers, which implies the heterojunction affects the electronic and magnetic structure. We performed a surface x-ray diffraction measurement in order to clarify the origin of the interface effect on the electron system. Our structural observation reveals that the chemical interface between PCMO and LSMO layers has one monolayer of atomic mixing, which accounts for a part of the interfacial effects.

We investigated an effect of spin dilution on the magnetic and thermal properties of delafossite CuCrO2having an $S = 3/2$ antiferromagnetic triangular lattice (ATL) by the substitution of Al3+for Cr3+by measurements of magnetization, specific heat, and neutron scattering. Spin defects introduced into Cr ATL layers blur the anti ferromagnetic transition and induce a dimensional crossover of low-energy magnetic excitation in the residual Cr ATL region due to the reduction of inter-layer magnetic interaction, while accompanying an enhancement of a spin-glass component. The observed $T^{2}$ contribution to the low-$T$ $C_{\text{mag}}$ above $y = 0.15$, as well as in Cu0.85Ag0.15CrO2, comes mainly from a short-range 2D AF excitation originating in a nontrivial magnetic state such as spin-nematic or spin-liquid states.

An　X-ray magnetic diffraction (XMD) is applied to ferromagnetic transition metal oxide YTiO3. The spin magnetic form factors are measured for 81 reciprocal lattice points of h0l, hk0 and 0kl series. Three-dimensional spin density distribution in the real space of YTiO3 is successfully obtained by using Maximum Entropy Method (MEM). This is the first result of spin density in the real space by the XMD.

Hard X-ray and extremely low energy bulk-sensitive photoelectron spectroscopy has been performed in the temperature range of 100--330 K for Fe3O4. In the high temperature phase just above the Verwey transition, the intensity at the Fermi level ($E_{\text{F}}$) is still very weak, but it increases gradually with further increasing the temperature (250 K, 330 K) in consistence with the temperature dependence of the conductivity. The spectral behaviors near $E_{\text{F}}$ with temperature are well explained by the polaronic model, which shows the change from small polaron to large polaron.

We have successfully detected spin chirality in prototypical multiferroic MnWO4wherein the helical magnetic order induces ferroelectricity using a nonresonant circularly polarized X-ray diffraction technique. The intensity of the magnetic reflection from MnWO4shows a clear difference between the right- and left-handed circular polarizations of the incident X-ray beam. The ratio of these intensities is in good agreement with theoretical predictions. The results clearly show that this technique is suitable for the investigation of spin chirality. Finally, we studied the spin chirality of another multiferroic material, DyMnO3. Possible applications of this technique are also discussed.

Nanometric topological spin textures, such as skyrmions (Sks) and antiskyrmions (antiSks), have attracted much attention recently. However, most studies have focused on two-dimensional spin textures in films with inherent or synthetic antisymmetric spin-exchange interaction, termed Dzyaloshinskii–Moriya interaction, although three-dimensional (3D) topological spin textures, such as antiSks composed of alternating Bloch- and Néel-type spin spirals, chiral bobbers carrying emergent magnetic monopoles, and deformed Sk strings, are ubiquitous. To elucidate these textures, we have developed a 3D nanometric magnetic imaging technique, tomographic Lorentz transmission electron microscopy (TEM). The approach enables the visualization of the 3D shape of magnetic objects and their 3D vector field mapping. Here we report 3D vector field maps of deformed Sk-strings and antiSk using the technique. This research approach will lead to discoveries and understanding of fertile 3D magnetic structures in a broad class of magnets, providing insight into 3D topological magnetism.

No Abstract

A series of urethane-substituted diacetylene monomers (DA-mUPh) with different alkyl chain length (m= 4, 5, and 6) in side-chain substituents of (CH2)mOCONHC6H5was synthesized. X-ray structural analyses for the DA-mUPh single crystals show that the monomers are packed by hydrogen bonding between the urethane groups, but the arrangement of the diacetylene moieties depends on the alkyl chain length. The reactivity of the DA-mUPh crystals upon γ-irradiation is different depending whether the alkyl chain length (m) is odd or even. The difference can be explained by the distance between the neighboring carbon atoms of the diacetylene groups. The temperature dependence of reflectance spectra for the polydiacetylene crystals has also been investigated. The thermochromic behavior is argued in comparison with the analysis of the differential scanning calorimetry.

Thermal expansion properties are investigated for Ti2O3-based sintered polycrystalline materials by controlling crystal lattice parameters and porous structures. Negative thermal expansion (NTE) is observed in a temperature range between room temperature and 593 K, and found to exceed ?L/L= ?0.7% at high temperatures, in spite of continuous increase of the unit cell volume. The temperature range for the significant NTE is successfully tuned by modifying electron-lattice-coupled phase-transition temperature. The magnitude of the NTE is enhanced upon introducing pores, indicating the importance of microstructural effect. These results demonstrate high potential of Ti2O3-based materials for practical applications.

We have observed novel photo-induced phenomena in conjugated polymers; the reversibly photo-induced phase transitions in single crystals of poly-diacetylene(PDA). Dependence of the phase conversion efficiency on excitation photon energy shows that the photo-generation of charge-carriers (polarons), not excitons, is responsible for the macroscopic phase change.

An overview is presented on photo-induced phase transitions in quasi-one-dimensional molecular systems. The phenomena are characterized by evolution of the local molecular domains induced by photo-excitations into the new macroscopic phase via collective interaction of domain wall excitations.

An abrupt change of some Bragg reflections occurs near the neutral-ionic transition temperature(TN-I) in TTF-chloranil. We performed intensity analyses of (3 k l) Bragg refrections below TN-I and found that donor (TTF+:D) and acceptor (chloranil-:A) molecules form D-A dimers below TN-I

AbstractWe report that bi-directional neutral-ionic (N-I) phase transition in the single crystal of tetrathiafulvalene-chloranil (TTF-CA) can be triggered by pulsed laser excitation with 100 femto-second width. The excitation intensity dependence of the dynamics of N-I domain-wall indicates that cooperative electron-lattice and/or electron-electron interactions play a key role in the driving mechanism of N-I photoinduced phase transition.

The crystal of squaric acid (SQ) shows dielectric property caused by tautomerization coupled with intermolecular proton transfer. We prepared a squaric acid derivative, 1,4-phenylenebis(squaric acid) (PBSQ), and determined its crystal structure. In the unit cell one PBSQ molecule and two water molecules are incorporated. There are one dimensional hydrogen-bonded chains PBSQ molecules. The distance between hydrogen-bonded O···O is very short (2.48Å). The result suggests that PBSA exists as mono deprotonated species accompanied by a counter cation of protonated 2H2O molecules (O···O distance of 2.45Å).

We have measured a spectrum of third-order nonlinear optical susceptibility |x (3)(-3ω;ω,ω,ω)| of σ-conjugated polysilane, poly(di-n-hexylsilane) (PDHS), by means of third-harmonic generation (THG) measurement, in the fundamental photon energy region from 0.56 eV to 2.15 eV. The obtained | x (3)(3ω; ω, ω, ω)| spectrum exhibits three peaks. These peaks are interpreted as the multi-photon resonance enhancement to the one-dimensional exciton states. This exciton model can be applied also to the nonlinear optical response of other conjugated polymers.

Optical spectra (ordinary absorption, photoluminescence, electro-absorption, and two-photon absorption spectra) have been investigated for thin films of polysilanes with a variety of backbone conformations such as trans planar, alternating trans-gauche, helical, and disordered forms. The results observed have revealed new features of one-dimensional exciton states characteristic of each conformation of the polymer backbone.

Infrared absorbance (IR) spectra and Raman scattering spectra were measured as a function of pressure for molecular crystals of several 1,3-diketones, which are composed of hydrogen-bonded π -molecular chains. An unconventional proton-related band is commonly observed in the IR spectra around 1800-2000cm-1 in these compounds. We ascribed the observed band to proton vibration localized around kink-(or antikink-) solitons. The intensity of the band rapidly increases with application of hydrostatic pressure, suggesting increase in number of the solitons-like defects of the molecular chains via deformation of the double-well type proton potential.

Infrared reflectance spectra have been investigated on TTF-chloranil single crystals near the neutral-to-ionic phase transition temperature TC. It was confirmed that abrupt changes in the ionicity and the stack dimerization take place simultaneously at TC. The effect of an external electric field and light irradiation upon the dimerized lattice below TC has been investigated by reflectance measurements.

AbstractTwo isomorphous TMTSF cation radical salts with tetrachlorometalate anion, TMTSF·FeCl4(ρrt= 40 Ωcm), and TMTSF·GaCl4(ρrt= 20 Ωcm) were prepared. The crystal containing tetrachloroferrate anion undergoes an antiferromagnetic ordering below Tn= 4K with the easy axis perpendicular to the TMTSF stacking direction. X-ray crystallographic analysis of the crystal reveals that Cl atoms of the anions closely contact with Se atoms of the donor molecules and suggests that the spin ordering is mediated by magnetic interaction between donors and anions.

AbstractA cross-cyclophane twin donor(1) afforded ion-radical salts (l·Br·(CHC12CH2C1)2) through electrocrystallization. Reflecting the molecular structure of the crossed-typed double-layered donor, orthogonal conduction paths were constructed through intermolecular SS contacts at the bridge-head sulfur atoms of the upper or lower layer of donor units, respectively. The salt exhibits a semiconducting behavior (σrt//a= 1.0 × 10−2Scm−1, Ea//a= 0.75 eV, σrt//c= 5.5 × 10×3Scm−1). Judging from the temperature dependence of the magnetic susceptibility, magnetic behavior of the salt was basically paramagnetic, but it showed ferromagnetic interaction at lower temperatures.

A series of polydiacetylenes with alkylurethane substituents R = (CH2)4OCONHCnH2n+1, where n=1-18, have been synthesized and their chromic behaviors have been studied by optical spectra, X-ray and DSC measurements. All of these polydiacetylenes exhibit the reversible thermochromic transitions, unless the samples are heated exceeding the limited temperatures, while the photo-excitation results in irreversible color change, only from blue to red.The length of the terminal alkyl unit in the side groups are found to affect the thermal hystereses from 0 to 60K in width.

Topologically nontrivial spin textures, such as skyrmions and dislocations, display emergent electrodynamics and can be moved by spin currents over macroscopic distances. These unique properties and their nanoscale size make them excellent candidates for the development of next-generation race-track memory and unconventional computing. A major challenge for these applications and the investigation of nanoscale magnetic structures in general is the realization of suitable detection schemes. We study magnetic disclinations, dislocations, and domain walls in FeGe and reveal pronounced responses that distinguish them from the helimagnetic background. A combination of magnetic force microscopy (MFM) and micromagnetic simulations links the response to the local magnetic susceptibility, that is, characteristic changes in the spin texture driven by the MFM tip. On the basis of the findings, which we explain using nonlinear response theory, we propose a read-out scheme using superconducting microcoils, presenting an innovative approach for detecting topological spin textures and domain walls in device-relevant geometries.

We investigated the current-induced dynamics of magnetic skyrmions in Pt/Co/Ir trilayers thin films. Skyrmions segregate in the transverse direction to the current flow via the skyrmion Hall effect, which shows scalability for current density and wire width. We also found the non-local accumulation of nonequilibrium skyrmions under charge current, which has an analogy with the spin accumulation in the nonlocal configuration.