arXiv subjects

Jun-ichiro Ohe

Publications and source records attributed to Jun-ichiro Ohe.

31 records · Page 2Linked to original sources

arXiv2013

Chiral spin-wave edge modes in dipolar magnetic thin films

Based on a linearized Landau-Lifshitz equation, we show that two-dimensional periodic allay of ferromagnetic particles coupled with magnetic dipole-dipole interactions supports chiral spin-wave edge modes, when subjected under the magnetic field applied perpendicular to the plane. The mode propagates along a one-dimensional boundary of the system in a unidirectional way and it always has a chiral dispersion within a band gap for spin-wave volume modes. Contrary to the well-known Damon-Eshbach surface mode, the sense of the rotation depends not only on the direction of the field but also on the strength of the field; its chiral direction is generally determined by the sum of the so-called Chern integers defined for spin-wave volume modes below the band gap. Using simple tight-binding descriptions, we explain how the magnetic dipolar interaction endows spin-wave volume modes with non-zero Chern integers and how their values will be changed by the field.

cond-mat.mes-hall↗

arXiv2013

Topological chiral magnonic edge mode in a magnonic crystal

Topological phases have been explored in various fields in physics such as spintronics, photonics, liquid helium, correlated electron system and cold-atomic system. This leads to the recent foundation of emerging materials such as topological band insulators, topological photonic crystals and topological superconductors/superfluid. In this paper, we propose a topological magnonic crystal which provides protected chiral edge modes for magnetostatic spin waves. Based on a linearized Landau-Lifshitz equation, we show that a magnonic crystal with the dipolar interaction acquires spin-wave volume-mode band with non-zero Chern integer. We argue that such magnonic systems are accompanied by the same integer numbers of chiral spin-wave edge modes within a band gap for the volume-mode bands. In these edge modes, the spin wave propagates in a unidirectional manner without being scattered backward, which implements novel fault-tolerant spintronic devices.

cond-mat.mes-hall↗

arXiv2012

Ultrafast magnetic vortex core switching driven by topological inverse Faraday effect

We present a theoretical discovery of an unconventional mechanism of inverse Faraday effect (IFE) which acts selectively on topological magnetic structures. The effect, topological inverse Faraday effect (TIFE), is induced by spin Berry's phase of the magnetic structure when a circularly polarized light is applied. Thus a spin-orbit interaction is not necessary unlike in the conventional IFE. We demonstrate by numerical simulation that TIFE realizes ultrafast switching of a magnetic vortex within a switching time of 150 ps without magnetic field.

cond-mat.mes-hall↗

arXiv2011

Continuous Generation of Spinmotive Force in a Patterned Ferromagnetic Film

We study, both experimentally and theoretically, the generation of a dc spinmotive force. By exciting a ferromagnetic resonance of a comb-shaped ferromagnetic thin film, a continuous spinmotive force is generated. Experimental results are well reproduced by theoretical calculations, offering a quantitative and microscopic understanding of this spinmotive force.

cond-mat.mes-hall↗

arXiv2011

Equation-of-motion approach of spin-motive force

We formulate a quantitative theory of an electromotive force of spin origin, i.e., spin-motive force, by the equation-of-motion approach. In a ferromagnetic metal, electrons couple to the local magnetization via the exchange interaction. Electrons feel spin dependent forces due to this interaction, and then the spin-motive force and the anomalous Hall effect appears. We have revealed that the origin of these phenomena is a misalignment between the conduction electron spin and the local magnetization.

cond-mat.mes-hall↗

arXiv2011

Spinmotive Force due to Intrinsic Energy of Ferromagnetic Nanowires

We study, both analytically and numerically, a spinmotive force arising from inherent magnetic energy of a domain wall in a wedged ferromagnetic nanowire. In a spatially-nonuniform nanowire, domain walls are subjected to an effective magnetic field, resulting in spontaneous motion of the walls. The spinmotive force mechanism converts the ferromagnetic exchange and demagnetizing energy of the nanowire into the electrical energy of the conduction electrons through the domain wall motion. The calculations show that this spinmotive force can be several microvolts, which is easily detectable by experiments.

cond-mat.other↗

arXiv2011

Linear-response theory of spin Seebeck effect in ferromagnetic insulators

We formulate a linear response theory of the spin Seebeck effect, i.e., a spin voltage generation from heat current flowing in a ferromagnet. Our approach focuses on the collective magnetic excitation of spins, i.e., magnons. We show that the linear-response formulation provides us with a qualitative as well as quantitative understanding of the spin Seebeck effect observed in a prototypical magnet, yttrium iron garnet.

cond-mat.mes-hall↗

arXiv2011

Gigantic enhancement of spin Seebeck effect by phonon drag

We investigate both theoretically and experimentally a gigantic enhancement of the spin Seebeck effect in a prototypical magnet LaY$_2$Fe$_5$O$_{12}$ at low temperatures. Our theoretical analysis sheds light on the important role of phonons; the spin Seebeck effect is enormously enhanced by nonequilibrium phonons that drag the low-lying spin excitations. We further argue that this scenario gives a clue to understand the observation of the spin Seebeck effect that is unaccompanied by a global spin current, and predict that the substrate condition affects the observed signal.

cond-mat.mes-hall↗

arXiv2008

Combined approach of density functional theory and quantum Monte Carlo method to electron correlation in dilute magnetic semiconductors

We present a realistic study for electronic and magnetic properties in dilute magnetic semiconductor (Ga,Mn)As. A multi-orbital Haldane-Anderson model parameterized by density-functional calculations is presented and solved with the Hirsch-Fye quantum Monte Carlo algorithm. Results well reproduce experimental results in the dilute limit. When the chemical potential is located between the top of the valence band and an impurity bound state, a long-range ferromagnetic correlations between the impurities, mediated by antiferromagnetic impurity-host couplings, are drastically developed. We observe an anisotropic character in local density of states at the impurity-bound-state energy, which is consistent with the STM measurements. The presented combined approach thus offers a firm starting point for realistic calculations of the various family of dilute magnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2008

Charge current driven by spin dynamics in disordered Rashba spin-orbit system

Pumping of charge current by spin dynamics in the presence of the Rashba spin-orbit interaction is theoretically studied. Considering disordered electron, the exchange coupling and spin-orbit interactions are treated perturbatively. It is found that dominant current induced by the spin dynamics is interpreted as a consequence of the conversion from spin current via the inverse spin Hall effect. We also found that the current has an additional component from a fictitious conservative field. Results are applied to the case of moving domain wall.

cond-mat.mes-hall↗

arXiv2006

Mesoscopic Hall effect driven by chiral spin order

A Hall effect due to spin chirality in mesoscopic systems is predicted. We consider a 4-terminal Hall system including local spins with geometry of a vortex domain wall, where strong spin chirality appears near the center of vortex. The Fermi energy of the conduction electrons is assumed to be comparable to the exchange coupling energy where the adiabatic approximation ceases to be valid. Our results show a Hall effect where a voltage drop and a spin current arise in the transverse direction. The similarity between this Hall effect and the conventional spin Hall effect in systems with spin-orbit interaction is pointed out.

cond-mat.mes-hall↗

arXiv2005

Mesoscopic Stern-Gerlach spin filter by nonuniform spin-orbit interaction

A novel spin filtering in two-dimensional electron system with nonuniform spin-orbit interactions (SOI) is theoretically studied. The strength of SOI is modulated perpendicular to the charge current. A spatial gradient of effective magnetic field due to the nonuniform SOI causes the Stern-Gerlach type spin separation. The direction of the polarization is perpendicular to the current and parallel to the spatial gradient. Almost 100 % spin polarization can be realized even without applying any external magnetic fields and without attaching ferromagnetic contacts. The spin polarization persists even in the presence of randomness.

cond-mat.mes-hall↗

arXiv2005

Dynamics of a domain wall and spin-wave excitations driven by a mesoscopic current

The dynamics of a domain wall driven by a spin-polarized current in a mesoscopic system is studied numerically. Spin-mixing in the states of the conduction electrons is fully taken into account. When the Fermi energy of the electrons is larger than the exchange energy ($E_{\rm F}>J_{\rm sd}$), the spin precession induces spin wave excitations in the local spins which contribute towards the displacement of the domain wall. The resulting average velocity is found to be much smaller than the one obtained in the adiabatic limit. For $E_{\rm F}<J_{\rm sd}$, the results are consistent with the adiabatic approximation except for the region below the critical current where a residual domain wall velocity is found.

cond-mat.mes-hall↗