arXiv subjects

Fumitaka Kagawa

Publications and source records attributed to Fumitaka Kagawa.

25 records · Page 2Linked to original sources

arXiv2021

Quantum transport of topological spin solitonsin a one-dimensional organic ferroelectric

We report the dielectric, magnetic, and ultrasonic properties of a one-dimensional organic salt TTF-QBr$_3$I. These indicate that TTF-QBr$_3$I shows a ferroelectric spin-Peierls (FSP) state in a quantum critical regime. In the FSP state, coupling of charge, spin, and lattice leads to emergent excitation of spin solitons as topological defects. Amazingly, the solitons are highly mobile even at low temperatures, although they are normally stationary because of pinning. Our results suggest that strong quantum fluctuations enhanced near a quantum critical point enable soliton motion governed by athermal relaxation. This indicates the realization of quantum topological transport at ambient pressure.

cond-mat.str-el↗

arXiv2021

Emergent electromagnetic induction beyond room temperature

Emergent electromagnetic induction based on electrodynamics of noncollinear spin states may enable dramatic miniaturization of inductor elements widely used in electric circuits, yet many issues are to be solved toward application. One such problem is how to increase working temperature. We report the large emergent electromagnetic induction achieved around and above room temperature based on short-period ($\leq$ 3 nm) spin-spiral states of a metallic helimagnet ${\rm YMn}_{6}{\rm Sn}_{6}$. The observed inductance value $L$ and its sign are observed to vary to a large extent, depending not only on the spin helix structure controlled by temperature and magnetic field but also on the current density. The present finding on room-temperature operation and possible sign control of $L$ may provide a new step toward realizing microscale quantum inductors.

cond-mat.str-el↗

arXiv2021

Room-temperature antiskyrmions and sawtooth surface textures in a noncentrosymmetric magnet with $S_4$ symmetry

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 due to anisotropic Dzyaloshinskii-Moriya interaction (DMI) in the noncentrosymmetric crystal structure. However, antiskyrmions have thus far only been observed in a few Heusler compounds with $D_{2\mathrm{d}}$ symmetry. Here, we report a new material Fe$_{1.9}$Ni$_{0.9}$Pd$_{0.2}$P in a different symmetry class ($S_4$ symmetry), where antiskyrmions exist over a wide temperature region including room temperature, and transform to skyrmions upon changing magnetic field and lamella thickness. The periodicity of magnetic textures greatly depends on crystal thickness, and domains with anisotropic sawtooth fractals are observed at the surface of thick crystals, which are attributed to the interplay between dipolar interaction and DMI as governed by crystal symmetry. Our findings provide a new arena to study antiskyrmions, and should stimulate further research on topological spin textures and their applications.

cond-mat.str-el↗

arXiv2020

Slow steady flow of a skyrmion lattice in a confined geometry probed by resistance narrow-band noise

Using resistance fluctuation spectroscopy, we observe current-induced narrow-band noise (NBN) in the magnetic skyrmion-lattice phase of micrometer-sized MnSi. The NBN appears only when electric-current density exceeds a threshold value, indicating that the current-driven motion of the skyrmion lattice triggers the NBN. The observed NBN frequency is 10-10$^4$ Hz at $\sim$10$^{9}$ A/m$^{2}$, implying a skyrmion steady flow velocity of 1-100 $μ$m/s, 3-5 orders of magnitude slower than previously reported. The temperature evolution of the NBN frequency suggests that the steady flow entails thermally activated processes, which are most likely due to skyrmion creation and annihilation at the sample edges. This scenario is qualitatively supported by our numerical simulations considering boundary effects, which reveals that the edges limit the steady flow of skyrmions, especially at low temperatures. We discuss a mechanism that dramatically slows the skyrmion steady flow in a microfabricated specimen.

cond-mat.mes-hall↗

arXiv2019

Magnetization-polarization cross-control near room temperature in hexaferrite single crystals

Mutual control of the electricity and magnetism in terms of magnetic (H) and electric (E) fields, the magnetoelectric (ME) effect, offers versatile low power-consumption alternatives to current data storage, logic gate, and spintronic devices. Despite its importance, E-field control over magnetization (M) with significant magnitude was observed only at low temperatures. Here we have successfully stabilized a simultaneously ferrimagnetic and ferroelectric phase in a Y-type hexaferrite single crystal up to T=450K and demonstrated the reversal of large non-volatile M by E field close to room temperature. Manipulation of the magnetic domains by E field is directly visualized at room temperature by using magnetic force microscopy. The present achievement provides an important step towards the application of bulk ME multiferroics.

cond-mat.str-el↗

arXiv2016

Quenching of charge and spin degrees of freedom in condensed matter

Electrons in condensed matter have internal degrees of freedom, such as charge, spin and orbital, leading to various forms of ordered states through phase transitions. However, in individual materials, a charge/spin/orbital ordered state of the lowest temperature is normally uniquely determined in terms of the lowest-energy state, i.e., the ground state. Here, we summarize recent results showing that under rapid cooling, this principle does not necessarily hold, and thus, the cooling rate is a control parameter of the lowest-temperature state beyond the framework of the thermo-equilibrium phase diagram. Although the cooling rate utilized in low-temperature experiments is typically 2*10^-3 - 4*10^-1 K/s, the use of optical/electronic pulses facilitate rapid cooling, such as 10^2-10^3 K/s. Such an unconventionally high cooling rate allows some systems to kinetically avoid a first-order phase transition, resulting in a quenched charge/spin state that differs from the ground state. We also demonstrate that quenched states can be exploited as a non-volatile state variable when designing phase-change memory functions. The present findings suggest that rapid cooling is useful for exploring and controlling the metastable electronic/magnetic state that is potentially hidden behind the ground state.

cond-mat.str-el↗

arXiv2015

Transport Signatures of Fermi Surface Topology Change in BiTeI

We report a quantum magnetotransport signature of a change in Fermi surface topology in the Rashba semiconductor BiTeI with systematic tuning of the Fermi level $E_F$. Beyond the quantum limit, we observe a marked increase/decrease in electrical resistivity when $E_F$ is above/below the Dirac node that we show originates from the Fermi surface topology. This effect represents a measurement of the electron distribution on the low-index ($n=0,-1$) Landau levels and is uniquely enabled by the finite bulk $k_z$ dispersion along the $c$-axis and strong Rashba spin-orbit coupling strength of the system. The Dirac node is independently identified by Shubnikov-de Haas oscillations as a vanishing Fermi surface cross section at $k_z=0$. Additionally we find that the violation of Kohler's rule allows a distinct insight into the temperature evolution of the observed quantum magnetoresistance effects.

cond-mat.mtrl-sci↗