arXiv subjects

Mitsuru Itoh

Publications and source records attributed to Mitsuru Itoh.

At least 19 recordsLinked to original sources

arXiv2026

Direct Imaging of Temperature Evolution of Polar Nanoregions and Chemically Ordered Regions in PMN Relaxor: Evidence for Polar Phase Percolation

Polar nanoregions (PNRs) are central to understanding the exceptional dielectric and piezoelectric properties of relaxor ferroelectrics and are key to advancing dielectrics for high-energy storage. However, direct real-space imaging of their formation and evolution remains a major challenge in condensed matter physics. Here, we report the real-space mappings of both PNRs and chemically ordered regions (CORs) in the prototypical relaxor Pb(Mg1/3Nb2/3)O3 and their temperature dependence using convergent-beam electron diffraction (CBED) combined with four-dimensional scanning transmission electron microscopy (4D-STEM). The results reveal that CORs, with sizes of 2-5 nm, remain static with temperature and act to suppress PNR growth. In contrast, PNRs evolve from isolated 2-5 nm regions at room temperature to interconnected structures ~10 nm in size at low temperatures, indicative of a percolation transition. These observations support the random-field model, in which PNRs emerge from a paraelectric matrix and their growth and collective interactions are constrained by random local fields associated with CORs.

cond-mat.mtrl-sci↗

arXiv2021

Short range magnetic correlation, metamagnetism and coincident dielectric anomaly in Na$_5$Co$_{15.5}$Te$_6$O$_{36}$

Here we explore the structural, magnetic and dielectric properties of Co based compound Na$_5$Co$_{15.5}$Te$_6$O$_{36}$ as a candidate of short-range magnetic correlations driven development of dielectric anomaly above N$\acute{e}$el temperature of ($T_N$=) 50 K. Low temperature neutron powder diffraction (NPD) in zero applied magnetic field clearly indicates that the canted spin structure is responsible for the antiferromagnetic transition and partially occupied Co form short range magnetic correlation with other Co, which further facilitates the structural distortion and consequent development of dielectric anomaly above antiferromagnetic transition. Additionally, the temperature dependent magnetic heat capacity and electron spin resonance measurements reveal the presence of short-range magnetic correlations which coincides with an anomaly in the dielectric constant vs temperature curve. Moreover, significant changes in the lattice parameters are also observed around the same temperature, indicating presence of noticeable spin-lattice coupling. Further, sharp jump in the magnetic field dependent magnetization clearly indicates the presence of metamagnetic transition and magnetic field dependent NPD confirms that rotations of Co spins with applied magnetic field are responsible for this metamagnetic phase transition. As a result, this transition causes the magnetocaloric effect to be developed in the system, which is suitable for the application in low temperature refrigeration.

cond-mat.mtrl-sci↗

arXiv2021

Short-range magnetic correlation and magnetodielectric coupling in multiferroic Pb3TeMn3P2O14

In this paper the structural, magnetic, and dielectric properties of langasite compound Pb$_3$TeMn$_3$P$_2$O$_{14}$ have been investigated as a candidate of short-range magnetic correlations driven development of dielectric anomaly above N$\acute{e}$el temperature of ($T_N$=) 7 K. Presence of dielectric anomaly, structural phase transition and a short range magnetic correlation at the same temperature (at around 100 K) as well as magnetic field dependent capacitance clearly indicate that this compound shows magnetodielectric coupling at around 100 K. In addition, unusual behaviour is observed in two polarization loop at room temperature and liquid nitrogen temperature, where coercive field at liquid nitrogen temperature is larger than room temperature. Further, $P$-$E$ loop at liquid nitrogen temperature with different frequencies also affirm that the coercive field and remnant polarization are firstly reduced (but very small value) but when frequency is further increased to 15 Hz and 100 Hz, both of them are enhanced. Therefore, a transition is observed at around 15 Hz in frequency dependent $P_r$ and $E_C$ curve, which may be usually attributed to the generalized pinning and depinning of the dislocation arrays to polarization.

cond-mat.mtrl-sci↗

arXiv2021

The critical role of stereochemically active lone pair in introducing high temperature ferroelectricity

In this paper a comparative structural, dielectric and magnetic study of two langasite compounds Ba$_3$TeCo$_3$P$_2$O$_{14}$ (absence of lone pair) and Pb$_3$TeCo$_3$P$_2$O$_{14}$ (Pb$^{2+}$ 6$s^2$ lone pair) have been carried out to precisely explore the development of room temperature spontaneous polarization in presence of stereochemically active lone pair. In case of Pb$_3$TeCo$_3$P$_2$O$_{14}$, mixing of both Pb 6$s$ with Pb 6$p$ and O 2$p$ help the lone pair to be stereochemically active. This stereochemically active lone pair brings a large structural distortion within the unit cell and creates a polar geometry, while Ba$_3$TeCo$_3$P$_2$O$_{14}$ compound remains in a nonpolar structure due to the absence of any such effect. Consequently, polarization measurement under varying electric field confirms room temperature ferroelectricity for Pb$_3$TeCo$_3$P$_2$O$_{14}$, which was not the case of Ba$_3$TeCo$_3$P$_2$O$_{14}$. Detailed study was carried out to understand the microscopic mechanism of ferroelectricity which revealed the exciting underlying activity of poler TeO$_6$ octahedral unit as well as Pb-hexagon.

cond-mat.mtrl-sci↗

arXiv2021

Covalency driven modulation of paramagnetism and development of lone pair ferroelectricity in multiferroic Pb$_3$TeMn$_3$P$_2$O$_{14}$

We have investigated the structural, magnetic and dielectric properties of Pb-based langasite compound Pb$_3$TeMn$_3$P$_2$O$_{14}$ both experimentally and theoretically in the light of metal-oxygen covalency, and the consequent generation of multiferroicity. It is known that large covalency between Pb 6$p$ and O 2$p$ plays instrumental role behind stereochemical lone pair activity of Pb. The same happens here but a subtle structural phase transition above room temperature changes the degree of such lone pair activity and the system becomes ferroelectric below 310 K. Interestingly, this structural change also modulates the charge densities on different constituent atoms and consequently the overall magnetic response of the system while maintaining global paramagnetism behavior of the compound intact. This single origin of modulation in polarity and paramagnetism inherently connects both the functionalities and the system exhibits mutiferroicity at room temperature.

cond-mat.mtrl-sci↗

arXiv2020

Sizeable suppression of thermal Hall effect upon isotopic substitution in strontium titanate

We report measurements of the thermal Hall effect in single crystals of both pristine and isotopically substituted strontium titanate. We discovered a two orders of magnitude difference in the thermal Hall conductivity between $SrTi^{16}O_3$ and $^{18}O$-enriched $SrTi^{18}O_3$ samples. In most temperature ranges, the magnitude of thermal Hall conductivity ($κ_{xy}$) in $SrTi^{18}O_3$ is proportional to the magnitude of the longitudinal thermal conductivity ($κ_{xx}$), which suggests a phonon-mediated thermal Hall effect. However, they deviate in the temperature of their maxima, and the thermal Hall angle ratio ($|κ_{xy}/κ_{xx}|$) shows anomalously decreasing behavior below the ferroelectric Curie temperature $T_c$ ~$25 K$. This observation suggests a new underlying mechanism, as the conventional scenario cannot explain such differences within the slight change in phonon spectrum. Notably, the difference in magnitude of thermal Hall conductivity and rapidly decreasing thermal Hall angle ratio in $SrTi^{18}O_3$ is correlated with the strength of quantum critical fluctuations in this displacive ferroelectric. This relation points to a link between the quantum critical physics of strontium titanate and its thermal Hall effect, a possible clue to explain this example of an exotic phenomenon in non-magnetic insulating systems.

cond-mat.str-el↗

arXiv2019

Investigation of Room Temperature Ferroelectricity and Ferrimagnetism in Multiferroic AlxFe2-xO3 Epitaxial Thin Films

Multiferroic materials open up the possibility to design novel functionality in electronic devices, with low energy consumption. However, there are very few materials that show multiferroicity at room temperature, which is essential to be practically useful. AlxFe2-xO3 (x-AFO) thin films, belonging to the k-Al2O3 family are interesting because they show room temperature ferrimagnetism and have a polar crystal structure. However, it is difficult to realise its ferroelectric properties at room temperature, due to low resistivity of the films. In this work, we have deposited x-AFO (0.5 <= x <= 1) epitaxial thin films with low leakage, on SrTiO3<111> substrates by Pulsed Laser Deposition. Magnetic measurements confirmed room temperature ferrimagnetism of the films, however the Curie temperature was found to be influenced by deposition conditions. First principle calculations suggested that ferroelectric domain switching occurs through shearing of in-plane oxygen layers, and predicted a high polarization value of 24 uC/cm2. However, actual ferroelectric measurements showed the polarization to be two order less. Presence of multiple in-plane domains which oppose polarization switching of adjacent domains, was found to be the cause for the small observed polarization. Comparing dielectric relaxation studies and ferroelectric characterization showed that oxygen-vacancy defects assist domain wall motion, which in turn facilitates polarization switching.

cond-mat.mtrl-sci↗

arXiv2018

Superconducting transition temperatures in the electronic and magnetic phase diagrams of Sr2VFeAsO3-delta, a superconductor

We elucidate the magnetic phases and superconducting transition temperatures (Tc) in Sr2VFeAsO3-delta (21113V), an iron-based superconductor with a thick-blocking layer fabricated from a perovskite-related transition metal oxide. At low temperatures (T < 37.1 K), 21113V exhibited a superconducting phase in the range 0.031 =< delta =< 0.145 and an antiferromagnetic (AFM) iron sublattice in the range 0.267 =< delta =< 0.664. Mixed-valent vanadium exhibited a dominant AFM phase in 0.031 =< delta =< 0.088, and a partial ferrimagnetic (Ferri.) phase in the range 0.124 =< delta =< 0.664. The Ferri. phase was the most dominant at a delta value of 0.267, showing an AFM phase of Fe at T < 20 K. Increasing the spontaneous magnetic moments reduced the magnetic shielding volume fraction due to the superconducting phase. This result was attributed to the magnetic phase of vanadium, which dominates the superconductivity of Fe in 21113V. The Tc-delta curve showed two maxima. The smaller and larger of Tc maxima occurred at delta = 0.073 and delta = 0.145, respectively; the latter resides on the phase boundary between AFM and the partial Ferri. phases of vanadium. 21113V is a useful platform for verifing new mechanisms of Tc enhancement in iron-based superconductors.

cond-mat.supr-con↗

arXiv2017

Bent Electronic Band Structure Induced by Ferroelectric Polarization

Bent band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction and photovoltaic devices. This report presents experimental evidence for ferroelectric band bending, which was observed in the depth profiles of atomic orbitals of angle-resolved hard x-ray photoemission spectra of ferroelectric BaTiO3 thin films. The ferroelectric bent band structure is separated into three depth regions; the shallowest and deepest regions are slightly modulated by the screening effect at surface and interface, respectively, and the intermediate region exhibits the pure ferroelectric effect. In the pure ferroelectric bent band structure, we found that the binding energy of outer shell electrons shows a larger shift than that of inner shell electrons, and that the difference in energy shift is correlated with the atomic configuration of the soft phonon mode. These findings could lead to a simple understanding of the origin of electric polarization.

cond-mat.mtrl-sci↗

arXiv2016

Transmission of spin waves in ordered FeRh epitaxial thin films

We report on B2-ordering dependence of magnetostatic surface spin waves in ferromagnetic FeRh at room temperature. Spin waves transmit over a distance longer than 21 μm in highly ordered FeRh alloys even with relatively large spin-orbit interaction. The long-range transmission likely arises from the induced Rh moments of the ordered FeRh due to ferromagnetic exchange interaction between Fe and Rh. The results indicate a potential of using FeRh in spintronic and magnonic applications by integrating with other fascinating magnetic characteristics of FeRh such as electric field induced magnetic phase transition.

cond-mat.mtrl-sci↗

arXiv2015

Role of Ca off-centering in tuning the ferroelectric phase transitions in Ba(Zr,Ti)O3 system

We here report the substitution effects of the smaller Ca for the bulky Ba in (Ba1-xCax)(Ti1-yZry)O3 perovskite oxides for two systems (Ba1-xCax)TiO3 with y=0 and (Ba1-xCax)(Ti0.9Zr0.1)O3 with y=0.1. Ca off-centering was found to play a critical role in stabilizing the ferroelectric phase and tuning the polarization states in both systems. It was demonstrated that the atomic displacement due to Ca off-centering in the bulky Ba-site in the perovskite structure provides an effective approach to compensate the reduction of ferroelectricity due to the chemical pressure, which allows to keep the Curie point nearly constant in the (Ba1-xCax)TiO3 system and increase the Curie point in the (Ba1-xCax)(Ti0.9Zr0.1)O3 system. It was commonly observed that the Ca off-centering effects lead to the shift of the R-O and O-T phase transitions toward lower temperatures and the ferroelectric stability of the T-phase, resulting in the occurrence of quantum phase transitions with interesting physics phenomena at low temperatures in the (Ba1-xCax)TiO3 system and the great enhancement of electromechanical coupling effects around room temperature in the (Ba1-xCax)(Ti0.9Zr0.1)O3 system over a wide composition range of the Ca-concentration. These finding may be of great interest for the design of the green piezoelectric materials.

cond-mat.mtrl-sci↗

arXiv2013

Phase diagram and piezoelectric response of (Ba1-xCax)(Zr0.1Ti0.9)O3 solid solution

We report the phase diagram of (Ba1-xCax)(Zr0.1Ti0.9)O3 solid solution. It is found that substitution of smaller Ca ions for Ba ions can slightly increase the cubic-tetragonal(T) para-ferroelectric phase transition temperature and strongly decrease the T-orthorhombic (O) and O-rhombohedral (R) transition. This unique ferroelectric phase evolution is attributed to Ca off-centering effects. More importantly, lowering of the T-O or O-R phase transitions allows us to prepare the piezoelectric ceramics with a strain response as high as S/E~800 pm/V (E=10 kV/cm) over a wide range of compositions with x~0.1 - 0. 18 at room temperature, which may be interesting for piezoelectric applications.

cond-mat.mtrl-sci↗

arXiv2013

Pb(Mg1/3Nb2/3)O3 (PMN) Relaxor: Dipole Glass or Nano-Domain Ferroelectric ?

Combining our comprehensive investigations of polarization evolution, soft-mode by Raman scattering and microstructure by TEM, and the results reported in the literatures, we show that prototypical relaxor Pb(Mg1/3Nb2/3)O3 (PMN) is essentially ferroelectric for T<Tc~225 K. Its anomalous dielectric behavior over a broad temperature range results from the reorientation of domains in the crystal. A physic picture of the structure evolution in relaxor is also revealed. It is found that nanometric ferroelectric domains (gennerally called as polar nano-region (PNR)) interact cooperatively to form micrometric domain. Such multiscale inhomogeneities of domain structure in addition to the well-known inhomogeneities of chemical composition and local symmetry are considered to play a crucial role in producing the enigmatic phenomena in relaxor system.

cond-mat.mtrl-sci↗

arXiv2012

Ferroelectricity Driven by Twisting of Silicate Tetrahedral Chains

Conventional perovskite-type ferroelectrics are based on octahedral units of oxygen, and often comprise toxic Pb to achieve robust ferroelectricity. Here, we report the ferroelectricity in a silicate-based compound, Bi2SiO5 (BSO), induced by a structural instability of the corresponding silicate tetrahedral chains. A low-energy phonon mode condenses at ~ 673 K to induce the proper ferroelectric phase transition. Polarization switching was observed in a BSO single crystal with a coercive field of 30 kV/cm and a spontaneous polarization of 0.3 microC/cm2 along a direction normal to the cleavage plane. The in-plane polarization was estimated by first principles calculations to be 23 microC/cm2. The present findings provide a new guideline for designing ferroelectric materials based on SiO4 tetrahedral units, which is ubiquitously found in natural minerals.

cond-mat.mtrl-sci↗

arXiv2012

51V-NMR study of antiferromagnetic state and spin dynamics in the quasi-one-dimensional BaCo2V2O8

We report on our 51V-NMR study of static and dynamical magnetic properties in the quasi-one-dimensional antiferromagnet BaCo2V2O8. Although the NMR spectrum shows well-defined antiferromagnetic (AF) order in the Néel ground state, the AF characteristic from the NMR spectrum is incomplete between 3.5 K and TN= 5.4 K, which could be affected by quantum spin fluctuations. The AF NMR spectrum indicates two V sites experiencing different magnetic field magnitudes, HA1=2.1 kOe and HA2=3.8 kOe. These internal fields could be explained by accounting for the classical and the pseudo-dipolar fields from Co2+ spins with a proposed magnetic structure based on the neutron diffraction measurements. In the paramagnetic state, the nuclear spin relaxation is dominated by AF spin fluctuations through the dipolar-type coupling between V and surrounding Co2+ ions. The linear relation between the nuclear spin-lattice relaxation rate 1/T1T and the magnetic susceptibility chi indicates that the Q component of magnetic susceptibility chi(Q) is roughly proportional to chi, where Q is the AF wave number. A change in slope of 1/T1T with respect to χaround 150 K suggests a change in the AF spin fluctuation spectrum.

cond-mat.str-el↗

arXiv2011

Ferroelectricity in Silver Perovskite Oxides

There are two silver perovskite oxides: AgNbO3 and AgTaO3. AgNbO3 has a noncentrosymmetric group of Pmc21 at room temperature with a ferri-electric ordering of polarization. Such a ferri-electric state with small polarization can be changed into a ferroelectric state with very large polarization by a high electric field or by a chemical modification. The induced ferroelectric phase shows promising electromechanical response for applications in piezoelectric devices. In contrast, AgTaO3 is a quantum paraelectric, but ferroelectricity also can be induced through chemical substitution. The findings of good ferroelectric and piezoelectric performance in the silver perovskites are hoped to trigger further theoretical and experimental investigations on these systems.

cond-mat.mtrl-sci↗

arXiv2011

High-Tc BaTiO3 ferroelectric films with frozen negative pressure states

We report that an energetic plasma process is extremely effective in enlarging the unit cell volume and ferroelectric distortion of the ferroelectric oxides, resulting in a significant increase in its Tc. We demonstrate experimentally that c-axis oriented BaTiO3 films can be deposited directly on quartz glass and Si substrates using such a process and that the material shows an approximately 5% expansion of its unit cell volume and approximately 4 times the ferroelectric tetragonal distortion of the bulk crystals. Such a frozen negative pressure results in a Tc value that is approximately 580 K higher than that of bulk single crystals, providing a wide range of operating temperatures for the devices. The present results suggest an approach to producing ferroelectric oxides with unique properties that might be extended to ferromagnetic or superconductor oxides and demonstrate a route to a lead-free ferroelectric oxide for capacitive, ferroelectric memory, and electro-optical devices.

cond-mat.mtrl-sci↗

arXiv2011

Ferroelectricity of Li-doped silver niobate (Ag,Li)NbO3

Phase evolution in (Ag1-xLix)NbO3 (ALN) solid solution was investigated by X-ray diffraction technique, dielectric and polarization measurements. It is shown that small substitution of Ag with Li gives rise to an orthorhombic-rhombohedral structural transformation in ABO3-perovskite silver niobate at room temperature. Structural refinements indicate that both A- and B-site displacements contribute to the spontaneous polarization of the ferroelectric phase with symmetry R3c. Increasing Li-concentration enhances the ferroelectric rhombohedral distortion, resulting in the increase of the para-ferroelectric phase transition temperature and the polarization of the solid solutions.

cond-mat.mtrl-sci↗