arXiv subjects

Satoru Hayami

Publications and source records attributed to Satoru Hayami.

At least 163 records · Page 9Linked to original sources

arXiv2019

Magnetic Vortex induced by Nonmagnetic Impurity in Ferromagnets: Magnetic Multipole and Toroidal around the Vacancy

We report our theoretical study on nucleation of a magnetic vortex around a nonmagnetic impurity doped into ferromagnets. The mechanism lies in the asymmetric Dzyaloshinskii-Moriya interaction arising from the breaking of spatial inversion symmetry by the impurity. By using the spin-wave analysis and Monte Carlo simulations, we show that the asymmetric interaction induces a magnetic vortex with nonzero vorticity $l=+1$. The vortex is stabilized even for less frustration in exchange interactions and in the absence of an external magnetic field. We also find that the magnetic vortex is characterized by magnetic multipoles according to its vorticity and helicity. We demonstrate a potential realization of such a magnetic vortex by considering a monolayer ferromagnet on a nonmagnetic substrate, which results in the magnetic monopole and toroidal dipole.

cond-mat.str-el↗

arXiv2019

Atomic-Scale Magnetic Toroidal Dipole under Odd-Parity Hybridization

Magnetic toroidal dipole (MTD) is one of a fundamental constituent to induce magneto-electric effects in the absence of both spatial inversion and time-reversal symmetries. We report on a microscopic investigation of the atomic-scale MTD in solids by taking into account the orbital degree of freedom with a different parity. We construct an effective two-orbital $d$-$f$ tight-binding model on a polar tetragonal system for describing the atomic-scale MTD, which are obtained by incorporating the atomic spin-orbit coupling and odd-parity hybridization. The effective model exhibits two types of the MTDs: in-plane $x, y$ components activated through spontaneous ferromagnetic ordering or external magnetic field and an out-of-plane $z$ component by a spontaneous odd-parity hybridization without spin moments. We show that the intra-orbital (inter-orbital) Coulomb interaction in multi-orbital systems plays an important role in stabilizing the in-plane (out-of-plane) MTD orderings. We also examine the magneto-electric effect under each MTD ordering by calculating a linear response tensor. We show that the odd-parity hybridization enhances the magneto-electric effect for the in-plane MTDs, while it suppresses that for the out-of-plane MTD.

cond-mat.str-el↗

arXiv2019

Spin current generation in organic antiferromagnets

Spin current--a flow of electron spins without a charge current--is an ideal information carrier free from Joule heating for electronic devices. The celebrated spin Hall effect, which arises from the relativistic spin-orbit coupling, enables us to generate and detect spin currents in inorganic materials and semiconductors, taking advantage of their constituent heavy atoms. In contrast, organic materials consisting of molecules with light elements have been believed to be unsuited for spin current generation. Here we show that a class of organic antiferromagnets with checker-plate type molecular arrangements can serve as a spin current generator by applying a thermal gradient or an electric field, even with vanishing spin-orbit coupling. Our findings provide another route to create a spin current distinct from the conventional spin Hall effect and open a new field of spintronics based on organic magnets having advantages of small spin scattering and long lifetime.

cond-mat.str-el↗

arXiv2018

Electric Toroidal Quadrupoles in Spin-Orbit Coupled Metal Cd$_2$Re$_2$O$_7$

We report our theoretical results on the order parameters for the pyrochlore metal Cd$_2$Re$_2$O$_7$, which undergoes enigmatic phase transitions with inversion symmetry breaking. By carefully examining active electronic degrees of freedom based on the lattice symmetry, we propose that two parity-breaking phases at ambient pressure are described by unconventional multipoles, electric toroidal quadrupoles (ETQs) with different components, $x^2-y^2$ and $3z^2-r^2$, in the pyrochlore tetrahedral unit. We elucidate that the ETQs are activated by bond or spin-current order on Re-Re bonds. Our ETQ scenario provides a key to reconcile the experimental contradictions, by measuring ETQ specific phenomena, such as peculiar spin splittings in the electronic band structure, magneto-current effect, and nonreciprocal transport under a magnetic field.

cond-mat.str-el↗

arXiv2018

Effect of magnetic anisotropy on Skyrmions with a high topological number in itinerant magnets

We report our numerical results for the effect of magnetic anisotropy on a Skyrmion crystal with a high topological number of two, which was recently discovered in an itinerant electron model [R. Ozawa, S. Hayami, and Y. Motome, Phys. Rev. Lett. 118, 147205 (2017)]. By performing numerical simulations based on the kernel polynomial method and the Langevin dynamics for the Kondo lattice model on a triangular lattice, we find that the topological property remains robust against the single-ion anisotropy, while the magnetic texture is deformed continuously. The resultant spin structure is characterized by three wave numbers (triple-$Q$ state), in which the $xy$ component of spins forms a magnetic vortex crystal and the $z$ component of spins behaves a sinusoidal wave. For larger anisotropy, we show that the system exhibits a phase transition from the Skyrmion crystal to topologically trivial phases with vanishing scalar chirality: a single-$Q$ collinear and double-$Q$ noncoplanar states for the easy-axis and easy-plane anisotropy, respectively. We also examine the effect of the single-ion anisotropy in an external magnetic field, and find that the field range of the Skyrmion crystal is rather insensitive to the anisotropy, in contrast to another Skyrmion crystal with the topological number of one whose field range is considerably extended (reduced) by the easy-axis (easy-plane) anisotropy.

cond-mat.str-el↗

arXiv2018

Classification of atomic-scale multipoles under crystallographic point groups and application to linear response tensors

Four types of atomic-scale multipoles, electric, magnetic, magnetic toroidal, and electric toroidal multipoles, give a complete set to describe arbitrary degrees of freedom for coupled charge, spin, and orbital of electrons. We here present a systematic classification of these multipole degrees of freedom towards the application in condensed matter physics. Starting from the multipole description under the rotation group in real space, we generalize the concept of multipoles in momentum space with the spin degree of freedom. We show how multipoles affect the electronic band structures and linear responses, such as the magneto-electric effect, magneto-current (magneto-gyrotropic) effect, spin conductivity, Piezo-electric effect, and so on. Moreover, we exhibit a complete table to represent the active multipoles under 32 crystallographic point groups. Our comprehensive and systematic analyses will give a foundation to identify enigmatic electronic order parameters and a guide to evaluate peculiar cross-correlated phenomena in condensed matter physics from microscopic point of view.

cond-mat.str-el↗

arXiv2018

Néel- and Bloch-Type Magnetic Vortices in Rashba Metals

We theoretically study noncoplanar spin textures in polar magnetic conductors. Starting from the Kondo lattice model with the Rashba spin-orbit coupling, we derive an effective spin model with generalized Ruderman-Kittel-Kasuya-Yosida interactions including the anisotropic and antisymmetric exchange interactions. By performing simulated annealing for the effective model, we find that a vortex crystal of Néel type is stabilized even in the absence of a magnetic field. Moreover, we demonstrate that a Bloch-type vortex crystal, which is usually associated with the Dresselhaus spin-orbit coupling, can also be realized in our Rashba-based model. A magnetic field turns the vortex crystals into Néel- and Bloch-type skyrmion-like crystals. Our results underscore that the interplay between the spin-orbit coupling and itinerant magnetism brings fertile possibilities of noncoplanar magnetic orderings.

cond-mat.str-el↗

arXiv2017

Manipulating magnetoelectric effect -- Essence learned from Co$_4$Nb$_2$O$_9$

Recent experiments for linear magnetoelectric (ME) response in honeycomb antiferromagnet Co$_4$Nb$_2$O$_9$ revealed that the electric polarization can be manipulated by the in-plane rotating magnetic field in a systematic way. We propose the minimal model by extracting essential ingredients of Co$_4$Nb$_2$O$_9$ to exhibit such ME response. It is the three-orbital model with $xy$-type atomic spin-orbit coupling (SOC) on the single-layer honeycomb structure, and it is shown to reproduce qualitatively the observed field-angle dependence of the electric polarization. The obtained results can be understood by the perturbative calculation with respect to the atomic SOC. These findings could be useful to explore further ME materials having similar manipulability of the electric polarization.

cond-mat.str-el↗

arXiv2017

Microscopic Description of Electric and Magnetic Toroidal Multipoles in Hybrid Orbitals

We present a general formalism of multipole descriptions under the space-time inversion group. We elucidate that two types of atomic toroidal multipoles, i.e., electric and magnetic, are fundamental pieces to express electronic order parameters in addition to ordinary electric and magnetic multipoles. By deriving quantum-mechanical operators for both toroidal multipoles, we show that electric (magnetic) toroidal multipole higher than dipole (monopole) can become a primary order parameter in a hybridized-orbital system. We also demonstrate emergent cross-correlated couplings between electric, magnetic, and elastic degrees of freedom, such as magneto-electric and magneto(electro)-elastic couplings, under toroidal multipole orders.

cond-mat.str-el↗

arXiv2017

Emergent odd-parity multipoles and magnetoelectric effects on a diamond structure: implication to 5$d$ transition metal oxides $A$OsO$_4$ ($A=$ K, Rb, and Cs)

We report our theoretical predictions on the linear magnetoelectric (ME) effects originating from odd-parity multipoles associated with spontaneous spin and orbital ordering on a diamond structure. We derive a two-orbital model for $d$ electrons in $e_g$ orbitals by including the effective spin-orbit coupling which arises from the mixing between $e_g$ and $t_{2g}$ orbitals. We show that the model acquires a net antisymmetric spin-orbit coupling once staggered spin and orbital orders occur spontaneously. The staggered orders are accompanied by odd-parity multipoles: magnetic monopole, quadrupoles, and toroidal dipoles. We classify the types of the odd-parity multipoles according to the symmetry of the spin and orbital orders. Furthermore, by computing the ME tensor using the linear response theory, we show that the staggered orders induce a variety of the linear ME responses. We elaborate all possible ME responses for each staggered order, which are useful to identify the order parameter and to detect the odd-parity multipoles by measuring the ME effects. We also elucidate the effect of lowering symmetry by a tetragonal distortion, which leads to richer ME responses. The implications of our results are discussed for $5d$ transition metal oxides, $A$OsO$_4$ ($A=$ K, Rb, and Cs), in which the order parameters are not fully identified.

cond-mat.str-el↗

arXiv2017

Effective bilinear-biquadratic model for noncoplanar ordering in itinerant magnets

Noncollinear and noncoplanar magnetic textures including skyrmions and vortices act as emergent electromagnetic fields and give rise to novel electronic and transport properties. We here report a unified understanding of noncoplanar magnetic orderings emergent from the spin-charge coupling in itinerant magnets. The mechanism has its roots in effective multiple spin interactions beyond the conventional Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism, which are ubiquitously generated in itinerant electron systems with local magnetic moments. Carefully examining the higher-order perturbations in terms of the spin-charge coupling, we construct a minimal effective spin model composed of the bilinear and biquadratic interactions with particular wave numbers dictated by the Fermi surface. We find that our effective model captures the underlying physics of the instability toward noncoplanar multiple-$Q$ states discovered recently in itinerant magnets: a single-$Q$ helical state expected from the RKKY theory is replaced by a double-$Q$ vortex crystal with chirality density waves even for an infinitely small spin-charge coupling on generic lattices [R. Ozawa $\mathit{et \ al.}$, J. Phys. Soc. Jpn. $\mathbf{85}$, 103703 (2016)], and a triple-$Q$ skyrmion crystal with a high topological number of two appears with increasing the spin-charge coupling on a triangular lattice [R. Ozawa, S. Hayami, and Y. Motome, to appear in Phys. Rev. Lett. (arXiv: 1703.03227)]. We find that, by introducing an external magnetic field, our effective model exhibits a plethora of multiple-$Q$ states. Our findings will serve as a guide for exploring further exotic magnetic orderings in itinerant magnets.

cond-mat.str-el↗

arXiv2017

Zero-field Skyrmions with a High Topological Number in Itinerant Magnets

Magnetic skyrmions are swirling spin textures with topologically protected noncoplanarity. Recently, skyrmions with the topological number of unity have been extensively studied in both experiment and theory. We here show that a skyrmion crystal with an unusually high topological number of two is stabilized in itinerant magnets at zero magnetic field. The results are obtained for a minimal Kondo lattice model on a triangular lattice by an unrestricted large-scale numerical simulation and variational calculations. We find that the topological number can be switched by a magnetic field as $2\leftrightarrow 1\leftrightarrow 0$. The skyrmion crystals are formed by the superpositions of three spin density waves induced by the Fermi surface effect, and hence, the size of skyrmions can be controlled by the band structure and electron filling. We also discuss the charge and spin textures of itinerant electrons in the skyrmion crystals which are directly obtained in our numerical simulations.

cond-mat.str-el↗

arXiv2016

Shape of magnetic domain walls formed by coupling to mobile charges

Magnetic domain walls, which are crucially important in both fundamental physics and technical applications, often have a preference in their form due to many different origins, such as the crystalline shape, lattice symmetry, and magnetic anisotropy. We theoretically investigate yet another origin stemming from the coupling to mobile charges in itinerant magnets. Performing a large-scale numerical simulation in a minimal model for itinerant magnets, i.e., the Kondo lattice model with classical localized spins, we show that the shape of magnetic domain walls depends on the electronic band structure and electron filling. While Neel and 120 antiferromagnetic states do not show a strong preference in the shape of domain walls, noncoplanar spin states with scalar chiral ordering have distinct directional preferences of the domain walls depending on the electron filling. We find that the directional preference is rationalized by the wave-number dependence of the effective magnetic interactions induced by the mobile charges, which are set by the band structure and electron filling. We also observe that, in the noncoplanar chiral states, an electric current is induced along the domain walls owing to the spin Berry phase mechanism, with very different spatial distributions depending on whether the bulk state is metallic or insulating.

cond-mat.str-el↗

arXiv2016

Vortices, skyrmions, and chirality waves in frustrated Mott insulators with a quenched periodic array of impurities

Finite-${\mathbf Q}$ magnetic instabilities are rather common in frustrated magnets. When the magnetic susceptibility is maximized at multiple-${\mathbf Q}$ vectors related through lattice symmetry operations, exotic magnetic orderings such as vortex and skyrmion crystals may follow. Here we show that a periodic array of nonmagnetic impurities, which can be realized through charge density wave ordering, leads to a rich phase diagram featuring a plethora of chiral magnetic phases, especially when there is a simple relation between the reciprocal vectors of the impurity superlattice and the magnetic ${\mathbf Q}$-vectors. We also investigate the effect of changing the impurity concentration or disturbing the impurity array with small quenched randomness. Alternative realizations of impurity superlattices are briefly discussed.

cond-mat.str-el↗

arXiv2016

Emergent spin-valley-orbital physics by spontaneous parity breaking

The spin-orbit coupling in the absence of spatial inversion symmetry plays an important role in realizing intriguing electronic states in solids, such as topological insulators and unconventional superconductivity. Usually, the inversion symmetry breaking is inherent in the lattice structures, and hence, it is not easy to control these interesting properties by external parameters. We here theoretically investigate the possibility of generating the spin-orbital entanglement by spontaneous electronic ordering caused by electron correlations. In particular, we focus on the centrosymmetric lattices with local asymmetry at the lattice sites, e.g., zig-zag, honeycomb, and diamond structures. In such systems, conventional staggered orders, such as charge order and antiferromagnetic order, break the inversion symmetry and activate the antisymmetric spin-orbit coupling, which is hidden in a sublattice-dependent form in the paramagnetic state. Considering a minimal two-orbital model on a honeycomb lattice, we scrutinize the explicit form of the antisymmetric spin-orbit coupling for all the possible staggered charge, spin, orbital, and spin-orbital orders. We show that the complete table is useful for understanding of spin-valley-orbital physics, such as spin and valley splitting in the electronic band structure and generalized magnetoelectric responses in not only spin but also orbital and spin-orbital channels, reflecting in peculiar magnetic, elastic, and optical properties in solids.

cond-mat.str-el↗

arXiv2016

Bubble and Skyrmion Crystals in Frustrated Magnets with Easy-Axis Anisotropy

We clarify the conditions for the emergence of multiple-${\bf Q}$ structures out of lattice and easy-axis spin anisotropy in frustrated magnets. By considering magnets whose exchange interaction has multiple global minima in momentum space, we find that both types of anisotropy stabilize triple-${\bf Q}$ orderings. Moderate anisotropy leads to a magnetic field-induced skyrmion crystal, which evolves into a bubble crystal for increasing spatial or spin anisotropy. The bubble crystal exhibits a quasi-continuous (devil's staircase) temperature dependent ordering wave-vector, characteristic of the competition between frustrated exchange and strong easy-axis anisotropy.

cond-mat.mes-hall↗

arXiv2016

Asymmetric Magnon Excitation by Spontaneous Toroidal Ordering

Effects of spontaneous toroidal ordering on magnetic excitation are theoretically investigated for a localized spin model that includes a staggered Dzyaloshinsky-Moriya interaction and anisotropic exchange interactions, which arise from the antisymmetric spin-orbit coupling and the multi-orbital correlation effect. We show that the model exhibits a Neel-type antiferromagnetic order, which simultaneously accompanies a ferroic toroidal order. We find that the occurrence of toroidal order modulates the magnon dispersion in an asymmetric way with respect to the wave number: a toroidal dipole order on the zigzag chain leads to a band-bottom shift, while a toroidal octupole order on the honeycomb lattice gives rise to a valley splitting. These asymmetric magnon excitations could be a source of unusual magnetic responses, such as nonreciprocal magnon transport. A variety of modulations are discussed while changing the lattice and magnetic symmetries. Implications of candidate materials for asymmetric magnon excitations are presented.

cond-mat.str-el↗

arXiv2016

Engineering chiral density waves and topological band structures by multiple-$Q$ superpositions of collinear up-up-down-down orders

Magnetic orders characterized by multiple ordering vectors harbor noncollinear and noncoplanar spin textures and can be a source of unusual electronic properties through the spin Berry phase mechanism. We theoretically show that such multiple-$Q$ states are stabilized in itinerant magnets in the form of superpositions of collinear up-up-down-down (UUDD) spin states, which accompany the density waves of vector and scalar chirality. The result is drawn by examining the ground state of the Kondo lattice model with classical localized moments, especially when the Fermi surface is tuned to be partially nested by the symmetry-related commensurate vectors. We unveil the instability toward the multiple-$Q$ UUDD states with chirality density waves, using the perturbative theory, variational calculations, and large-scale Langevin dynamics simulations. We also show that the chirality density waves can induce rich nontrivial topology of electronic structures, such as the massless Dirac semimetal, Chern insulator with quantized topological Hall response, and peculiar edge states which depend on the phase of chirality density waves at the edges.

cond-mat.str-el↗