arXiv subjects

Y. Togawa

Publications and source records attributed to Y. Togawa.

14 recordsLinked to original sources

arXiv2023

Presence of a chiral soliton lattice in the chiral helimagnet MnTa$_{3}$S$_{6}$

Chiral helimagnetism was investigated in transition-metal intercalated dichalcogenide single crystals of MnTa$_3$S$_6$. Small-angle neutron scattering (SANS) experiments revealed the presence of harmonic chiral helimagnetic order, which was successfully detected as a pair of satellite peaks in the SANS pattern. The magnetization data are also supportive of the presence of chiral soliton lattice (CSL) phase in MnTa$_3$S$_6$. The observed features are summarized in the phase diagram of MnTa$_3$S$_6$, which is in strong contrast with that observed in other dichalcogenides such as CrNb$_3$S$_6$ and CrTa$_3$S$_6$. The presence of the remanent state provides tunable capability of the number of chiral solitons at zero magnetic field in the CSL system, which may be useful for memory device applications.

cond-mat.mtrl-sci↗

arXiv2023

Surface barrier effect as evidence of chiral soliton lattice formation in chiral dichalcogenide CrTa$_{3}$S$_{6}$ crystals

The formation of chiral magnetic soliton lattice (CSL) is investigated in monoaxial chiral dichalcogenide CrTa$_{3}$S$_{6}$ crystals in terms of a surface barrier, which prevents a penetration of chiral solitons into the system and is an intrinsic origin of hysteresis for the continuous phase transition of nucleation-type, as discussed in the system of quantized vortices in type-II superconductors. The magnetoresistance (MR) was examined with microfabricated platelet samples in different dimensions with regard to the $c$-axis direction of the crystal. The CSL formation was confirmed by the discrete MR changes, reflecting the number of chiral solitons, as well as by the presence of surface barrier, recognized as a fixed ratio of critical magnetic fields during the hysteresis field cycle. We also argue the influence of the surface barrier in the bulk CrTa$_{3}$S$_{6}$ crystals.

cond-mat.mtrl-sci↗

arXiv2022

An emergence of chiral helimagnetism or ferromagnetism governed by Cr intercalation in a dichalcogenide CrNb$_{3}$S$_{6}$

A synthesis of single crystals of chiral dichalcogenides $TM_{3}X_{6}$ ($T$: 3$d$ transition metal, $M$: Nb or Ta, $X$: S or Se) remains an intriguing issue for the investigation of emergent quantum properties such as chiral helimagnetism. In this study, we investigated a correlation between the quantity of Cr intercalation $x$ and magnetic property in single crystals of a chromium (Cr) intercalated chiral disulfide Cr$_x$Nb$_3$S$_6$ in order to optimize the synthesis condition for the intercalation-controlled single crystals. The magnetic properties including a magnetic transition temperature $T_{c}$ take different values depending on the samples. We systematically grew single crystals of Cr$_{x}$Nb$_{3}$S$_{6}$ with $x$ ranged from 0.89 to 1.03 and found that the amount of the Cr intercalation $x$ is an essential factor in controlling the magnetic properties of the grown crystals. The magnetization anomaly, which appears in the temperature dependence as evidence of the formation of chiral magnetic soliton lattice (CSL), was observed only in a narrow region of $x$ from 0.98 to 1.03. The single crystals with $x$ being 0.98 and 0.99 showed the CSL behavior with the highest $T_{c}$ of 133 K. These results indicate that small amount of defects on the sites for $T$ ions dramatically affects the quality of the single crystals in the synthesis of $TM_{3}$S$_{6}$. We also discuss an importance of synthesizing enantiopure single crystals of chiral dichalgogenides in order to observe chiral physical properties unique to chiral compounds such as magneto-chiral effect and chiral-induced spin selectivity.

cond-mat.mtrl-sci↗

arXiv2021

Parallel mode differential phase contrast in transmission electron microscopy, I: Theory and analysis

In Part I of this diptych, we outline the parallel mode of differential phase contrast (TEM-DPC), which uses real-space distortion of Fresnel images arising from electrostatic or magnetostatic fields to quantify the phase gradient of samples with some degree of structural contrast. We present an analysis methodology and the associated software tools for the TEM-DPC method and, using them together with numerical simulations, compare the technique to the widely used method of phase recovery based on the transport-of-intensity equation (TIE), thereby highlighting the relative advantages and limitations of each. The TEM-DPC technique is particularly suitable for \textit{in-situ} studies of samples with significant structural contrast and, as such, complements the TIE method since structural contrast usually hinders the latter, but is an essential feature that enables the former. In Part II of this work, we apply the theory and methodology presented to the analysis of experimental data to gain insight into two-dimensional magnetic phase transitions.

cond-mat.mtrl-sci↗

arXiv2021

Parallel mode differential phase contrast in transmission electron microscopy, II: K$_2$CuF$_4$ phase transition

In Part I of this diptych, we outlined the theory and an analysis methodology for quantitative phase recovery from real-space distortions of Fresnel images acquired in the parallel mode of transmission electron microscopy (TEM). In that work, the properties of the method, termed TEM-differential phase contrast (TEM-DPC), were highlighted through the use of simulated data. In this work, we explore the use of the TEM-DPC technique with experimental cryo-TEM images of a thin lamella of a low temperature two-dimensional (2-D) ferromagnetic material, K$_2$CuF$_4$, to perform two tasks. First, using images recorded below the ordering temperature, we compare the TEM-DPC method to the transport of intensity one for phase recovery, and discuss the relative advantages the former has for experimental data. Second, by tracking the induction of the sample as it is driven through a phase transition by heating, we extract estimates for the critical temperature and critical exponent of the order parameter. The value of the latter is consistent with the 2-D XY class, raising the prospect that a Kosterlitz--Thoules transition may have occurred.

cond-mat.mtrl-sci↗

arXiv2020

Tensile deformations of the magnetic chiral soliton lattice probed by Lorentz transmission electron microscopy

We consider the case of a chiral soliton lattice subjected to uniaxial elastic strain applied perpendicular to the chiral axis and derive through analytical modelling the phase diagram of magnetic states supported in the presence of an external magnetic field. The strain induced anisotropies give rise to three distinct non-trivial spin textures, depending on the nature of the strain, and we show how these states may be identified by their signatures in Lorentz transmission electron microscopy (TEM). Experimental TEM measurements of the Fresnel contrast in a strained sample of the prototypical monoaxial chrial helimagnet CrNb$_3$S$_6$ are reported and compare well with the modelled contrast. Our results demonstrate an additional degree of freedom that may be used to tailor the magnetic properties of helimagnets for fundamental research and applications in the areas of spintronics and the emerging field of strain manipulated spintronics.

cond-mat.mtrl-sci↗

arXiv2019

Theory of standing spin waves in finite-size chiral spin soliton lattice

We present a theory of standing spin wave (SSW) in a monoaxial chiral helimagnet. Motivated by experimental findings on the magnetic field-dependence of the resonance frequency in thin films of Cr${}$Nb$_{3} $S${}_{6}$[Goncalves et al., Phys. Rev. B95, 104415 (2017)], we examine the SSW over a chiral soliton lattice (CSL) excited by an ac magnetic field applied parallel and perpendicular to the chiral axis. For this purpose, we generalize Kittel-Pincus theories of the SSW in ferromagnetic thin films to the case of non-collinear helimagnet with the surface end spins which are softly pinned by an anisotropy field. Consequently, we found there appear two types of modes. One is a Pincus mode which is composed of a long-period Bloch wave and a short-period ripple originated from the periodic structure of the CSL. Another is a short-period Kittel ripple excited by space-periodic perturbation which exists only in the case where the ac field is applied perpendicular the chiral axis. We demonstrate that the existence of the Pincus mode and the Kittel ripple is consistent with experimentally found double resonance profile.

cond-mat.mtrl-sci↗

arXiv2019

Effect of disorder on the collective excitations of the chiral spin soliton lattice

We assess the impact of magnetic disorder on the spin excitation spectra of the chiral helimagnetic crystal $\mathrm{CrNb_3S_6}$ using microwave resonance spectroscopy. The chiral spin soliton lattice phase (CSL), which is a prototype of a noncollinear spin system that forms periodically over a macroscopic length scale, exhibited three resonance modes over a wide frequency range. We found that the predominance of these modes depended on the degree of magnetic disorder and that disorder can be suppressed by sweeping the external field via an ideal helical state at 0~T. The macroscopic coherence of the CSL can be clearly observed through its collective dynamic behavior. Our study suggests that magnetic disorder can be used as a mechanism to control spin wave excitation in noncollinear spin systems.

cond-mat.mtrl-sci↗

arXiv2018

Laser-Induced Skyrmion Writing and Erasing in an Ultrafast Cryo-Lorentz Transmission Electron Microscopy

We demonstrate that light-induced heat pulses of different duration and energy can write skyrmions in a broad range of temperatures and magnetic field in FeGe. Using a combination of camera-rate and pump-probe cryo-Lorentz Transmission Electron Microscopy, we directly resolve the spatio-temporal evolution of the magnetization ensuing optical excitation. The skyrmion lattice was found to maintain its structural properties during the laser-induced demagnetization, and its recovery to the initial state happened in the sub-μs to μs range, depending on the cooling rate of the system.

cond-mat.str-el↗

arXiv2017

First Determination of Level Structure of an $sd$-Shell Hypernucleus, $\rm {^{19}_ΛF}$

We report on the first observation of $γ$ rays emitted from an $sd$-shell hypernucleus, $\rm ^{19}_ΛF$. The energy spacing between the ground state doublet, $1/2^{+}$ and $3/2^{+}$ states, of $\rm ^{19}_ΛF$ is determined to be $\rm 315.5 \pm 0.4 (stat) ^{+0.6}_{-0.5} (syst)~keV$ by measuring the $γ$-ray energy from the $M1(3/2^{+} \rightarrow 1/2^{+})$ transition. In addition, three $γ$-ray peaks were observed and assigned as $E2(5/2^{+} \rightarrow 1/2^{+})$, $E1(1/2^{-} \rightarrow 1/2^{+})$, and $E1(1/2^{-} \rightarrow 3/2^{+})$ transitions. The excitation energies of the $5/2^{+}$ and $1/2^{-}$ states are determined to be $\rm 895.2 \pm 0.3 (stat) \pm 0.5 (syst)~keV$ and $\rm 1265.6 \pm 1.2 (stat) ^{+0.7}_{-0.5} (syst)~keV$, respectively. It is found that the ground state doublet spacing is well described by theoretical models based on existing $s$- and $p$-shell hypernuclear data.

nucl-ex↗

arXiv2016

Internal structure of hexagonal skyrmion lattices in cubic helimagnets

We have utilised a high spatial resolution imaging method, Differential Phase Contrast (DPC) performed in a scanning transmission electron microscope (STEM), for precise measurement of the magnetic induction distribution in skyrmion states in noncentrosymmetric magnetically ordered materials. Applied to investigate the internal structure of hexagonal skyrmion lattice cells, stabilised by an out-plane applied magnetic field in an FeGe nanowedge specimen, mapping of the in-plane component of magnetic induction has yielded "average" skyrmion profiles and observation of internal six-fold symmetry. With increasing field strength, the diameter of "average" skyrmion cores was observed to decrease accompanied by a non-linear variation of the lattice periodicity. Variations in structure for individual skyrmions were studied utilising an advanced DPC detection scheme with a variety of symmetry lowering distortions being observed. Our observations are consistent with a theoretical phenomenological model, which has predicted the structure of hexagonal skyrmion lattice cells and also that twisting states near to the material surfaces provide a basis for energetic stabilisation of the skyrmion lattice over the conical phase. There was good agreement with experiment for predictions of bulk skyrmion structure and their response (core-size & lattice periodicity variation) to an applied field.

cond-mat.mes-hall↗

arXiv2015

Chiral surface twists and skyrmion stability in nanolayers of cubic helimagnets

Lorentz transmission electron microscopy (LTEM) investigations of modulated states in a FeGe wedge and detailed calculations demonstrate that chiral twists arising near the surfaces of noncentrosymmetric ferromagnets (Meynell et al. Phys. Rev. B, 90, 014406 (2014)) provide a stabilization mechanism for skyrmion lattices and helicoids in cubic helimagnet nanolayers. The calculated magnetic phase diagram for free standing cubic helimagnet nanolayers shows that magnetization processes in these compounds fundamentally differ from those in bulk cubic helimagnets and are characterized by the first-order transitions between modulated phases and the formation of specific multidomain states. The paper reports LTEM observations of multidomain patterns in FeGe free-standing nanolayers.

cond-mat.mes-hall↗

arXiv2001

Location-Sensitive Measurement of the Local Fluctuation of Driven Vortex Density in Bi$_2$Sr$_2$CaCu$_2$O$_y$

To investigate the dynamics of driven vortices in superconductors, noise in the local vortex density was investigated in the mixed state of a high-$T_c$ superconductor, Bi$_2$Sr$_2$CaCu$_2$O$_y$, using a two-dimensional electron gas (2DEG) micro-Hall probe array. We studied the cross-correlation function, together with the auto-correlation function, both parallel and perpendicular to the direction of flow of the vortices. The broadband noise (BBN) did not have large spatial correlations. This suggests that the BBN is due to the fluctuation of the local vortex density generated by bulk pinning centers under the area of each probe. On the other hand, the narrow-band noise (NBN) (with the the time scale of the transit time of vortices) had large translational correlations. These definitely shows that the NBN was generated by semimacroscopic imperfections like the surface, and macroscopic line defects {\it etc.}. In relation to the dynamic phase diagram, large BBN was observed when the vortices started moving. The dependence of the spatial correlations on the direction of the array relative to the direction of the driving current suggested that plastic flow was present when the large BBN was observed. The gross features of our data agree well with some of the theoretically proposed dynamical phase diagrams of vortices in superconductors.

cond-mat.supr-con↗

arXiv2000

The Pseudogap in La(2-x)Sr(x)CuO(4): A Raman Viewpoint

We report the results of Raman scattering experiments on single crystals of La(2-x)Sr(x)CuO(4) [La214] as a function of temperature and doping. In underdoped compounds low-energy B1g spectral weight is depleted in association with the opening of a pseudogap on regions of the Fermi surface located near (pi, 0) and (0, pi). The magnitude of the depletion increases with decreasing doping, and in the most underdoped samples, with decreasing temperature. The spectral weight that is lost at low-energies (omega < 800 cm-1) is transferred to the higher energy region normally occupied by multi-magnon scattering. From the normal state B2g spectra we have determined the scattering rate Gamma(omega, T) of qausiparticles located near the diagonal directions in k-space, (pi/2, pi/2) regions. In underdoped compounds, Gamma(omega, T) is suppressed at low temperatures for energies less than Eg(x) ~ 800 cm-1. The observed doping dependence of the two-magnon scattering and the scattering rate suppression thus suggest that the pseudogap is characterized by an energy scale Eg ~ J, where J is the antiferromagnetic super-exchange energy. Comparison with the results from other techniques provides a consistent picture of the pseudogap in La214.

cond-mat.supr-con↗