arXiv subjects

Masayuki Hagiwara

Publications and source records attributed to Masayuki Hagiwara.

At least 37 records · Page 2Linked to original sources

arXiv2022

Magnetic-field-induced Anderson localization in orbital selective antiferromagnet BaMn$_2$Bi$_2$

We report a metal-insulator transition (MIT) in the half-filled multiorbital antiferromagnet (AF) BaMn$_2$Bi$_2$ that is tunable by a magnetic field perpendicular to the AF sublattices. Instead of an Anderson-Mott mechanism usually expected in strongly correlated systems, we find by scaling analyses that the MIT is driven by an Anderson localization. Electrical and thermoelectrical transport measurements in combination with electronic band calculations reveal a strong orbital-dependent correlation effect, where both weakly and strongly correlated $3d$-derived bands coexist with decoupled charge excitations. Weakly correlated holelike carriers in the $d_{xy}$-derived band dominate the transport properties and exhibit the Anderson localization, whereas other $3d$ bands show clear Mott-like behaviors with their spins ordered into AF sublattices. The tuning role played by the perpendicular magnetic field supports a strong spin-spin coupling between itinerant holelike carriers and the AF fluctuations, which is in sharp contrast to their weak charge coupling.

cond-mat.str-el↗

arXiv2022

Novel quantum phase of the chromium spinel oxide HgCr$_{\rm 2}$O$_{\rm 4}$ in high magnetic fields

In this study, we have performed the magnetocaloric effect and the specific heat measurements of chromium spinel oxide HgCr$_2$O$_4$, wherein the magnetic Cr$^{3+}$ ions form a highly frustrated pyrochlore lattice with significant spin-lattice coupling. In addition to the known magnetic-field-induced phases, our thermodynamic measurements detect a novel quantum phase just before the saturation of the magnetization, which has not been expected from the classical theories of the pyrochlore lattice antiferromagnet with spin-lattice coupling. Based on recent theoretical model calculation, we discuss the possibility of a spin nematic state appearing for this quantum phase.

cond-mat.str-el↗

arXiv2021

Stacking-order effect on spin-orbit torque, spin-Hall magnetoresistance, and magnetic anisotropy in Ni$_{81}$Fe$_{19}$-IrO$_2$ bilayers

The 5d transition-metal oxides have been an intriguing platform to demonstrate efficient charge to spin current conversion due to a unique electronic structure dominated by strong spin-orbit coupling. Here, we report on stacking-order effect of spin-orbit torque (SOT), spin-Hall magnetoresistance, and magnetic anisotropy in bilayer Ni$_{81}$Fe$_{19}$-5d iridium oxide, IrO$_2$. While all the IrO$_2$ and Pt control samples exhibit large dampinglike-SOT generation stemming from the efficient charge to spin current conversion, the magnitude of the SOT is larger in the IrO$_2$ (Pt)-bottom sample than in the IrO$_2$ (Pt)-top one. The fieldlike-SOT has even more significant stack order effect, resulting in an opposite sign in the IrO$_2$ samples in contrast to the same sign in the Pt samples. Furthermore, we observe that the magnetic anisotropy energy density and the anomalous Hall effect are increased in the IrO$_2$ (Pt)-bottom sample, suggesting enhanced interfacial perpendicular magnetic anisotropy. Our findings highlight the significant influence of the stack order on spin transport and magnetotransport properties of Ir oxide/ferromagnet systems, providing useful information on design of SOT devices including 5d transition-metal oxides.

cond-mat.mtrl-sci↗

arXiv2021

Spin-orbit torque generation in NiFe/IrO2 bilayers

The 5d transition-metal oxides have a unique electronic structure dominated by strong spin-orbit coupling and hence they can be an intriguing platform to explore spin-current physics. Here, we report on room-temperature generation of spin-orbit torque (SOT) from a conductive 5d iridium oxide, IrO2. By measuring second-harmonic Hall resistance of Ni81Fe19/IrO2 bilayers, we find both dampinglike and fieldlike SOTs. The former is larger than the latter, enabling easier control of magnetization. We also observe that the dampinglike SOT efficiency has a significant dependence on IrO2 thickness, which is well described by the drift-diffusion model based on the bulk spin Hall effect. We deduce the effective spin Hall angle of +0.093 +- 0.003 and the spin-diffusion length of 1.7 +- 0.2 nm. By comparison with control samples Pt and Ir, we show that the effective spin Hall angle of IrO2 is comparable to that of Pt and seven times higher than that of Ir. The fieldlike SOT efficiency has a negative sign without appreciable dependence on the thickness, in contrast to the dampinglike SOT. This suggests that the fieldlike SOT likely stems from the interface. These experimental findings suggest that the uniqueness of the electronic structure of 5d transition-metal oxides is crucial for highly efficient charge to spin-current conversion.

cond-mat.mtrl-sci↗

arXiv2021

Effect of uniaxial single-ion anisotropy on a stability of intermediate magnetization plateaus of a spin-1 Heisenberg diamond cluster

Ground-state phase diagrams and magnetization curves of a spin-1 Heisenberg diamond cluster with two different coupling constants and uniaxial single-ion anisotropy are investigated in a presence of the external magnetic field with the help of exact diagonalization methods. It is shown that the spin-1 Heisenberg diamond cluster exhibits several remarkable quantum ground states, which are manifested in zero- and low-temperature magnetization curves as intermediate plateaus at one-quarter, one-half and three-quarters of the saturation magnetization. It is found that the width of the fractional magnetization plateaus depends basically on a relative strength of the coupling constants as well as uniaxial single-ion anisotropy, which may substantially shrink or even cause full breakdown of some intermediate magnetization plateaus. It is evidenced that a relatively weak uniaxial single-ion anisotropy of the easy-axis type considerably improves a theoretical fit of low-temperature magnetization curves of the tetranuclear nickel complex [Ni4(CO3)2(aetpy)8](ClO4)4 in a low-field region without spoiling the previous fit based on the fully isotropic Heisenberg model in a high-field region.

cond-mat.str-el↗

arXiv2021

Large negative magnetoresistance in BaMn$_2$Bi$_2$ antiferromagnet

A very large negative magnetoresistance (LNMR) is observed in the insulating regime of the antiferromagnet BaMn$_2$Bi$_2$ when a magnetic field is applied perpendicular to the direction of the sublattice magnetization. High perpendicular magnetic field eventually suppresses the insulating behavior and allows BaMn$_2$Bi$_2$ to re-enter a metallic state. This effect is seemingly unrelated to any field induced magnetic phase transition, as measurements of magnetic susceptibility and specific heat did not find any anomaly as a function of magnetic fields at temperatures above $2\,\mathrm{K}$. The LNMR appears in both current-in-plane and current-out-of-plane settings, and Hall effects suggest that its origin lies in an extreme sensitivity of conduction processes of holelike carriers to the infinitesimal field-induced canting of the sublattice magnetization. The LNMR-induced metallic state may thus be associated with the breaking of the antiferromagnetic parity-time symmetry by perpendicular magnetic fields and/or the intricate multi-orbital electronic structure of BaMn$_2$Bi$_2$.

cond-mat.str-el↗

arXiv2020

Insights into nature of magnetization plateaus of a nickel complex [Ni4(CO3)2(aetpy)8](ClO4)4 from a spin-1 Heisenberg diamond cluster

Magnetic and magnetocaloric properties of a spin-1 Heisenberg diamond cluster with two different coupling constants are investigated with the help of an exact diagonalization based on the Kambe's method, which employs a local conservation of composite spins formed by spin-1 entities located in opposite corners of a diamond spin cluster. It is shown that the spin-1 Heisenberg diamond cluster exhibits several intriguing quantum ground states, which are manifested in low-temperature magnetization curves as intermediate plateaus at 1/4, 1/2 and 3/4 of the saturation magnetization. Besides, the spin-1 Heisenberg diamond cluster may also exhibit an enhanced magnetocaloric effect, which may be relevant for a low-temperature refrigeration achieved through the adiabatic demagnetization. It is evidenced that the spin-1 Heisenberg diamond cluster with the antiferromagnetic coupling constants J1/kB = 41.4 K and J2/kB = 9.2 K satisfactorily reproduces a low-temperature magnetization curve recorded for the tetranuclear nickel complex [Ni4(CO3)2(aetpy)8](ClO4)4 (aetpy = 2-aminoethyl-pyridine) including a size and position of intermediate plateaus detected at 1/2 and 3/4 of the saturation magnetization. A microscopic nature of fractional magnetization plateaus observed experimentally is clarified and interpreted in terms of valence-bond crystal with either a single or double valence bond. It is suggested that this frustrated magnetic molecule can provide a prospective cryogenic coolant with the maximal isothermal entropy change - Delta S = 10.6 J/(K.kg) in a temperature range below 2.3 K.

cond-mat.stat-mech↗

arXiv2020

Tensile behavior of dual-phase titanium alloys under high-intensity proton beam exposure: radiation-induced omega phase transformation in Ti-6Al-4V

A high-intensity proton beam exposure with 181 MeV energy has been conducted at Brookhaven Linac Isotope Producer facility on various material specimens for accelerator targetry applications, including titanium alloys as a beam window material. The radiation damage level of the analyzed capsule was 0.25 dpa at beam center region with an irradiation temperature around 120 degree C. Tensile tests showed increased hardness and a large decrease in ductility for the dual alpha+beta-phase Ti-6Al-4V Grade-5 and Grade-23 extra low interstitial alloys, with the near alpha-phase Ti-3Al-2.5V Grade-9 alloy still exhibiting uniform elongation of a few % after irradiation. Transmission Electron Microscope analyses on Ti-6Al-4V indicated clear evidence of a high-density of defect clusters with size less than 2 nm in each alpha-phase grain. The beta-phase grains did not contain any visible defects such as loops or black dots, while the diffraction patterns clearly indicated omega-phase precipitation in an advanced formation stage. The radiation-induced omega-phase transformation in the beta-phase could lead to greater loss of ductility in Ti-6Al-4V alloys in comparison with Ti-3Al-2.5V alloy with less beta-phase.

physics.ins-det↗

arXiv2020

Emergent spin-$1$ Haldane gap and ferroelectricity in a frustrated spin-$1/2$ ladder

We report experimental and theoretical evidence that Rb$_2$Cu$_2$Mo$_3$O$_{12}$ has a nonmagnetic tetramer ground state of a two-leg ladder comprising antiferromagnetically coupled frustrated spin-$1/2$ chains and exhibits a Haldane spin gap of emergent spin-1 pairs. Three spin excitations split from the spin-1 triplet by a Dzyaloshinskii-Moriya interaction are identified in inelastic neutron-scattering and electron spin resonance spectra. A tiny magnetic field generates ferroelectricity without closing the spin gap, indicating a novel class of ferroelectricity induced by a vector spin chirality order.

cond-mat.str-el↗

arXiv2018

Itinerant antiferromagnetic BaMn$_2$Pn$_2$'s showing both negative and positive magnetoresistances

We report the discovery of a novel giant magnetoresistance (GMR) phenomenon in a family of BaMn$_{2}$Pn$_{2}$ antiferromagnets (Pn stands for P, As, Sb, and Bi) with a parity-time symmetry. The resistivities of these materials are reduced by $60$ times in magnetic fields ($\vec{H}$'s), thus yielding the GMR of about $-98\%$. The GMR changes systematically along with the Pn elements, hinting that its origin is the spin orbit coupling (SOC) and/or $d$-$p$ orbital hybridization. A positive MR component emerging on top of the negative GMR at low temperatures suggests an orbital-sensitive magnetotransport as $\vec{H}$ suppresses the conduction of the electron-like carriers in the $d$-like band but enhances those of hole-like ones in the $d$-$p$ hybridized band. The anisotropy of the GMR reveals that the electrical conductivity is extremely sensitive to the minute changes in the direction of the antiferromagnetic moments induced by the parity-time breaking $\vec{H}$, which seems to be associated with a magnetoelectric effect in the dynamic regime of conduction electrons. We attribute the observed GMR to the non-trivial low energy band of BMPn's, which is governed by the parity-time symmetry and an magnetic hexadecapole ordering.

cond-mat.str-el↗

arXiv2018

High-field Magnetism of the Honeycomb-lattice Antiferromagnet Cu$_2$(pymca)$_3$(ClO$_4$)

We report on the experimental results of magnetic susceptibility, specific heat, electron spin resonance (ESR), and high-field magnetization measurements on a polycrystalline sample of the spin-$1/2$ distorted honeycomb-lattice antiferromagnet Cu$_2$(pymca)$_3$(ClO$_4$). Magnetic susceptibility shows a broad peak at about 25~K, which is typical of a low dimensional antiferromagnet, and no long range magnetic order is observed down to 0.6~K in the specific heat measurements. Magnetization curve up to 70~T at 1.4~K shows triple stepwise jumps. Assuming three different exchange bonds $J_{\rm A}$, $J_{\rm B}$ and $J_{\rm C}$ from the structure, the calculated magnetization curve reproduces the observed one when $J_{\rm A}/k_{\rm B} = 43.7~{\rm K}$, $J_{\rm B}/J_{\rm A} = 1$ and $J_{\rm C}/J_{\rm A} = 0.2$ except the magnetization near 70~T, where the observed magnetization indicates another step while the calculated magnetization becomes saturated. The relationship between magnetization plateaus and exchange bonds is discussed based on the numerical calculations.

cond-mat.str-el↗

arXiv2018

Magnetoelectric behavior from cluster multipoles in square cupolas: Study of Sr(TiO)Cu$_4$(PO$_4$)$_4$ in comparison with Ba and Pb isostructurals

We report our combined experimental and theoretical study of magnetoelectric properties of an antiferromagnet Sr(TiO)Cu$_4$(PO$_4$)$_4$, in comparison with the isostructurals Ba(TiO)Cu$_4$(PO$_4$)$_4$ and Pb(TiO)Cu$_4$(PO$_4$)$_4$. The family of compounds commonly possesses a low-symmetric magnetic unit called the square cupola, which is a source of magnetoelectric responses associated with the magnetic multipoles activated under simultaneous breaking of spatial inversion and time reversal symmetries. Measuring the full magnetization curves and the magnetic-field profiles of dielectric constant for Sr(TiO)Cu$_4$(PO$_4$)$_4$ and comparing them with the theoretical analyses by the cluster mean-field theory, we find that the effective $S=1/2$ spin model, which was used for the previous studies for Ba(TiO)Cu$_4$(PO$_4$)$_4$ and Pb(TiO)Cu$_4$(PO$_4$)$_4$, well explains the experimental results by tuning the model parameters. Furthermore, elaborating the phase diagram of the model, we find that the square cupolas could host a variety of magnetic multipoles, i.e., monopole, toroidal moment, and quadrupole tensor, depending on the parameters that could be modulated by deformations of the magnetic square cupolas. Our results not only provide a microscopic understanding of the series of the square cupola compounds, but also stimulate further exploration of the magnetoelectric behavior arising from cluster multipoles harboring in low-symmetric magnetic units.

cond-mat.str-el↗

arXiv2018

Spin-Orbital Entangled Liquid State in the Copper Oxide Ba$_3$CuSb$_2$O$_9$

Structure with orbital degeneracy is unstable toward spontaneous distortion. Such orbital correlation usually has a much higher energy scale than spins, and therefore, magnetic transition takes place at a much lower temperature, almost independently from orbital ordering. However, when the energy scales of orbitals and spins meet, there is a possibility of spin-orbital entanglement that would stabilize novel ground state such as spin-orbital liquid and random singlet state. Here we review on such a novel spin-orbital magnetism found in the hexagonal perovskite oxide Ba$_3$CuSb$_2$O$_9$, which hosts a self-organized honeycomblike short-range order of a strong Jahn-Teller ion Cu$^{2+}$. Comprehensive structural and magnetic measurements have revealed that the system has neither magnetic nor Jahn-Teller transition down to the lowest temperatures, and Cu spins and orbitals retain the hexagonal symmetry and paramagnetic state. Various macroscopic and microscopic measurements all indicate that spins and orbitals remain fluctuating down to low temperatures without freezing, forming a spin-orbital entangled liquid state.

cond-mat.str-el↗

arXiv2018

Magnetic Structural Unit with Convex Geometry: a Building Block Hosting an Exchange-striction-driven Magnetoelectric Coupling

We perform a combined experimental and theoretical study of a magnetic-field ($B$) induced evolution of magnetic and ferroelectric properties in an antiferromagnetic material Pb(TiO)Cu$_4$(PO$_4$)$_4$, whose structure is characterized by a staggered array of Cu$_4$O$_{12}$ magnetic units with convex geometry known as square cupola. Our experiments show a $B$-induced phase transition from a previously reported low-$B$ linear magnetoelectric phase to a new high-$B$ magnetoelectric phase, which accompanies a 90$^\circ$ flop of electric polarization and gigantic magnetodielectric effect. Moreover, we observe a $B$-induced sign reversal of ferroelectric polarization in the high-$B$ phase. Our model and first-principles calculations reveal that the observed complex magnetoelectric behavior is well explained in terms of a $B$-dependent electric polarization generated in each Cu$_4$O$_{12}$ unit by the so-called exchange striction mechanism. The present study demonstrates that the materials design based on the magnetic structural unit with convex geometry deserves to be explored for developing strong magnetoelectric couplings.

cond-mat.mtrl-sci↗

arXiv2017

Direct observation of spin-quadrupolar excitations in Sr$_2$CoGe$_2$O$_7$ by high field ESR

Exotic spin-multipolar ordering in spin transition metal insulators has so far eluded unambiguous experimental observation. A less studied, but perhaps more feasible fingerprint of multipole character emerges in the excitation spectrum in the form of quadrupolar transitions. Such multipolar excitations are desirable as they can be manipulated with the use of light or electric field and can be captured by means of conventional experimental techniques. Here we study single crystals of multiferroic Sr$_2$CoGe$_2$O$_7$, and show that due to its nearly isotropic nature a purely quadrupolar bimagnon mode appears in the electron spin resonance (ESR) spectra. This non-magnetic spin-excitation couples to the electric field of the light and becomes observable for a specific experimental configuration, in full agreement with a theoretical analysis of the selection rules.

cond-mat.str-el↗

arXiv2017

Magnetic properties of quasi-two-dimensional $S$ = 1/2 Heisenberg antiferromagnet with distorted square lattice

We successfully synthesize single crystals of the verdazyl radical $α$-2,3,5-Cl$_3$-V. $Ab$ $initio$ molecular orbital calculations indicate that the two dominant antiferromagnetic interactions, $J_{\rm{1}}$ and $J_{\rm{2}}$ ($α=J_{\rm{2}}/J_{\rm{1}}\simeq 0.56$), form an $S$ = 1/2 distorted square lattice. We explain the magnetic properties based on the $S$ = 1/2 square lattice Heisenberg antiferromagnet using the quantum Monte Carlo method, and examine the effects of the lattice distortion and the interplane interaction contribution. In the low-temperature regions below 6.4 K, we observe anisotropic magnetic behavior accompanied by a phase transition to a magnetically ordered state. The electron spin resonance signals exhibit anisotropic behavior in the temperature dependence of the resonance field and the linewidth. We explain the frequency dependence of the resonance fields in the ordered phase using a mean-field approximation with out-of-plane easy-axis anisotropy, which causes a spin-flop phase transition at approximately 0.4 T for the field perpendicular to the plane. Furthermore, the anisotropic dipole field provides supporting information regarding the presence of the easy-axis anisotropy. These results demonstrate that the lattice distortion, anisotropy, and interplane interaction of this model are sufficiently small that they do not affect the intrinsic behavior of the $S$ = 1 / 2 square lattice Heisenberg antiferromagnet.

cond-mat.str-el↗

arXiv2017

Magnetic Structure and Dielectric State in the Multiferroic Ca$_{2}$CoSi$_{2}$O$_{7}$

Magnetic structure of the multiferroic Ca$_{2}$CoSi$_{2}$O$_{7}$ was determined by neutron diffraction techniques. Combination of the polycrystalline and single-crystal samples experiments revealed a collinear antiferromagnetic structure with the easy axis along $<$100$>$ directions. The dielectric state was discussed in the framework of the spin-dependent $d$-$p$ hybridization mechanism, leading to the realization of the antiferroelectric structure. The origin of the magnetic anisotropy was discussed in comparison with the isostructural Ba$_{2}$CoGe$_{2}$O$_{7}$.

cond-mat.str-el↗

arXiv2017

Absence of magnetic long range order in Ba$_3$ZnRu$_2$O$_9$: A spin-liquid candidate in the $S=3/2$ dimer lattice

We have discovered a novel candidate for a spin liquid state in a ruthenium oxide composed of dimers of $S = $ 3/2 spins of Ru$^{5+}$,Ba$_3$ZnRu$_2$O$_9$. This compound lacks a long range order down to 37 mK, which is a temperature 5000-times lower than the magnetic interaction scale of around 200 K. Partial substitution for Zn can continuously vary the magnetic ground state from an antiferromagnetic order to a spin-gapped state through the liquid state. This indicates that the spin-liquid state emerges from a delicate balance of inter- and intra-dimer interactions, and the spin state of the dimer plays a vital role. This unique feature should realize a new type of quantum magnetism.

cond-mat.str-el↗