arXiv subjects

Noriki Terada

Publications and source records attributed to Noriki Terada.

16 recordsLinked to original sources

arXiv2025

Control of Covalent Bond Enables Efficient Magnetic Cooling

Magnetic cooling, harnessing the temperature change in matter when exposed to a magnetic field, presents an energy-efficient and climate-friendly alternative to traditional vapor-compression refrigeration systems, with a significantly lower global warming potential. The advancement of this technology would be accelerated if irreversible losses arising from hysteresis in magnetocaloric materials were minimized. Despite extensive efforts to manipulate crystal lattice constants at the unit-cell level, mitigating hysteresis often compromises cooling performance. Herein, we address this persistent challenge by forming Sn(Ge)3/Sn(Ge)3 bonds within the unit cell of the Gd5Ge4 compound. Our approach enables an energetically favorable phase transition, leading to the elimination of thermal hysteresis. Consequently, we achieve a synergistic improvement of two key magnetocaloric figures of merit: a larger magnetic entropy change and a twofold increase in the reversible adiabatic temperature change (from 3.8 to 8 K) in the Gd5Sn2Ge2 compound. Such synergies can be extended over a wide temperature range. This study demonstrates a paradigm shift in mastering hysteresis toward simultaneously achieving exceptional magnetocaloric metrics and opens up promising avenues for gas liquefaction applications in the longstanding pursuit of sustainable energy solutions.

cond-mat.mtrl-sci↗

arXiv2023

Stroboscopic Time-of-Flight Neutron Diffraction in Long Pulsed Magnetic Fields

We present proof-of-principle experiments of stroboscopic time-of-flight (TOF) neutron diffraction in long pulsed magnetic fields. By utilizing electric double-layer capacitors, we developed a long pulsed magnet for neutron diffraction measurements, which generates pulsed magnetic fields with the full widths at the half maximum of more than $10^2$ ms. The field variation is slow enough to be approximated as a steady field within the time scale of a polychromatic neutron pulse passing through a sample placed in a distance of the order of $10^1$ m from the neutron source. This enables us to efficiently explore the reciprocal space using a wide range of neutron wavelength in high magnetic fields. We applied this technique to investigate field-induced magnetic phases in the triangular lattice antiferromagnets CuFe$_{1-x}$Ga$_x$O$_2$ ($x=0, 0.035$).

cond-mat.str-el↗

arXiv2020

Successive incommensurate spin orderings and excitations in multiferroic SrMnGe2O6

Anisotropic multiferroic properties of SrMnGe2O6 pyroxene single crystals were systematically investigated by means of magnetization, heat capacity, pyroelectric current measurement and elastic and inelastic neutron scattering experiments. Single crystal neutron diffraction allows us to unambiguously reveal the presence of two incommensurate magnetic orderings: a non-polar amplitude-modulated collinear sinusoidal magnetic structure emerges at TN1=4.36(2)K followed by a polar elliptical cycloidal spin structure below TN2=4.05(2)K. Pyroelectric current measurements on single crystal confirm the appearance of a spontaneous polarization within the (ac) plane below TN2 associated with the latter magnetic symmetry through extended Dzyaloshinsky-Moriya mechanism. The magnetic phase diagram was calculated considering the three isotropic exchange couplings relevant in this system. The magnetic excitations spectra of SrMnGe2O6 measured by inelastic neutron scattering were successfully modeled using a set of exchange interactions consistent with this phase diagram.

cond-mat.str-el↗

arXiv2019

Rich magnetoelectric phase diagrams of multiferroic single-crystal alpha-NaFeO2

The magnetic and dielectric properties of the multiferroic triangular lattice magnet compound alpha-NaFeO2 were studied by magnetization, specific heat, dielectric permittivity, and pyroelectric current measurements and by neutron diffraction experiments using single crystals grown by a hydrothermal synthesis method. This work produced magnetic field (in the monoclinic ab-plane, B_ab, and along the c*-axis, B_c) versus temperature magnetic phase diagrams, including five and six magnetically ordered phases in B_ab and along B_c, respectively. Comparing the polarization direction to the magnetic structures in the different ferroelectric phases, we conclude that the extended inverse Dzyaloshinskii-Moriya mechanism expressed by the orthogonal components p1 ~ rij x (Si x Sj ) and p2 ~ Si x Sj can explain the polarization directions. Based on calculations incorporating exchange interactions up to fourth-nearest-neighbor (NN) couplings, we infer that competition among antiferromagnetic second NN interactions in the triangular lattice plane, as well as weak interplane antiferromagnetic interactions, are responsible for the rich phase diagrams of alpha-NaFeO2.

cond-mat.str-el↗

arXiv2019

Magnetic ordering and ferroelectricity in multiferroic 2H-AgFeO2: Comparison between hexagonal and rhombohedral polytypes

Magnetic and dielectric properties of the hexagonal triangular lattice antiferromagnet 2H-AgFeO2 have been studied by neutron diffraction, magnetic susceptibility, specific heat, pyroelectric current, and dielectric constant measurements. The ferroelectric polarization, P ~ 5 μC/m2, has been found to appear below 11 K due to a polar nature of the magnetic ground state of the system. In the temperature range of 11 K < T < 18 K, an incommensurate spin density wave (ICM1) with the nonpolar magnetic point group mmm1' and the k1 = (0,q1_b,0; q1_b = 0.390-0.405)propagation vector takes place. Below 14 K, a proper screw ordering (ICM2) and k2 = (0,q2_b,0; q2_b = 0.385-0.396) appears as a minor phase which coexists with ICM1 and the ground state down to the lowestmeasured temperature 5.5 K. No ferroelectric polarization associated with the ICM2 phase was observed in agreement with its nonpolar point group 2221'. Finally, a spiral order with cycloid and proper screw components (ICM3), and k3 = (q3_a,q3_b,0; q3_a = 0.0467, q3_b = 0.349) emerges below 11 K as the ground state of the system. Based on the deduced magnetic point group 21', we conclude that the ferroelectric polarization in ICM3 is parallel to the c axis and is caused by the inverse Dzyloshinskii-Moriya effect with p1 ~ rij x (Si x Sj ). Unlike the rhombohedral 3R-AgFeO2 polytype, the additional contribution to the macroscopic polarization p2 ~ Si x Sj is not allowed in the present case due to the symmetry constraints imposed by the hexagonal lattice of 2H-AgFeO2.

cond-mat.str-el↗

arXiv2019

Magnetic ordering in pressure-induced phases with giant spin-driven ferroelectricity in multiferroic TbMnO3

In order to clarify the mechanism associated with pressure/magnetic-field-induced giant ferroelectric polarization in TbMnO3, this work investigated changes in magnetic ordering brought about by variations in temperature, magnetic field, and pressure. This was accomplished by means of neutron diffraction analyses under high pressures and high magnetic fields, employing a single crystal. The incommensurate magnetic ordering of a cycloid structure was found to be stable below the reported critical pressure of 4.5 GPa. In contrast, a commensurate E-type spin ordering of Mn spins and a noncollinear configuration of Tb spins with k=(0,1/2,0) appeared above 4.5 GPa. The application of a magnetic field along the a axis (H_{||a}) under pressure induces a k=(0,0,0)antiferromagnetic structure in the case of Tb spins above H_{||a}, enhancing the ferroelectric polarization, while the E-type ordering of Mn spins is stable even above the critical field. From the present experimental findings, we conclude that the E-type ordering of Mn spins induces giant ferroelectric polarization through an exchange striction mechanism. The H_{||a}-induced polarization enhancement can be understood by considering that the polarization, reduced by the polar ordering of Tb moments in a zero field, can be recovered through a field-induced change to nonpolar k=(0,0,0) ordering at H_{||a} ~ 2T.

cond-mat.str-el↗

arXiv2019

Ferroelectricity induced by ferriaxial crystal rotation and spin helicity in a B-site-ordered double-perovskite multiferroic In2NiMnO6

We have performed dielectric measurements and neutron diffraction experiments on the double perovskite In2NiMnO6. A ferroelectric polarization, P ~ 30 μC/m2, is observed in a polycrystalline sample below TN = 26 K where a magnetic phase ransition occurs. The neutron diffraction experiment demonstrates that a complex noncollinear magnetic structure with "cycloidal" and "proper screw" components appears below TN, which has the incommensurate propagation vector k = (ka,0,ks; ka ~ 0.274, ks ~ -0.0893). The established magnetic point group 21' implies that the macroscopic ferroelectric polarization is along the monoclinic b axis. Recent theories based on the inverse Dzyaloshinskii-Moriya effect allow us to specify two distinct contributions to the polarization of In2NiMnO6. One of them is associated with the cycloidal component, p1 ~ rij x (Si x Sj), and the other with the proper screw component, p2 ~ [rij (Si x Sj )]A. The latter is explained by coupling between spin helicity and "ferriaxial" crystal rotation with macroscopic ferroaxial vector A, characteristic of the B-site ordered perovskite systems with out-of-plane octahedral tilting.

cond-mat.str-el↗

arXiv2015

Magnetic structure of the spin-1/2 frustrated quasi-one-dimensional antiferromagnet Cu3Mo2O9: Appearance of a partial disordered state

We investigated the crystal and magnetic structures of the spin-1/2 frustrated antiferromagnet Cu3Mo2O9 in which the spin system consists of antiferromagnetic chains and dimers. The space group at room temperature has been reported to be orthorhombic Pnma (No. 62). We infer that the space group above TN = 7.9 K is monoclinic P2_1/m (No. 11) from the observation of reflections forbidden in Pnma in x-ray powder diffraction experiments at room temperature. We determined the magnetic structure of Cu3Mo2O9 in neutron powder diffraction experiments. Magnetic moments on dimer sites lie in the ac planes. The magnitudes are 0.50 - 0.74 mu_B. Moments on chain sites may exist but the magnitudes are very small. The magnetic structure indicates that a partial disordered state is realized. We consider the origin of the magnetic structure, weak ferromagnetism, and electric polarization.

cond-mat.str-el↗

arXiv2015

Multistage ordering and critical singularities in Co1-xZnxAl2O4 (0 < x < 1):Dilution and pressure effects in a magnetically frustrated system

We report comprehensive studies of the crystallographic,magnetic, and thermal properties of a spinel-type magnetically frustrated compound, CoAl2O4, and a magnetically diluted system,Co1-xZnxAl2O4. These studies revealed the effects of dilution and disorder when the tetrahedral magnetic Co ion was replaced by the nonmagnetic Zn ion. Low-temperature anomalies were observed in magnetic susceptibility at x < 0.6. A multicritical point was apparent at T = 3.4 K and x = 0.12, where the antiferromagnetic, spin glass-like, and paramagnetic phases met. At that point, the quenched ferromagnetic component induced by a magnetic field during cooling was sharply enhanced and was observable below x = 0.6. Around x = 0.6, magnetic susceptibility and specific heat were described by temperature power laws, chi ~ C/T ~ T-d, in accord with the site percolation threshold of the diamond lattice. This behavior is reminiscent of a quantum critical singularity. We propose an x-temperature phase diagram in the range 0 < x < 1 for Co1-xZnxAl2O4. The transition temperature of CoAl2O4 determined from magnetic susceptibility measured under hydrostatic pressure increased with increasing pressure.

cond-mat.str-el↗

arXiv2013

Spin-Driven Bond Order in a 1/5-Magnetization Plateau Phase in a Triangular Lattice Antiferromagnet CuFeO2

We have investigated spin-wave excitations in a magnetic-field-induced 1/5-magnetization plateau phase in a triangular lattice antiferromagnet CuFeO2 (CFO), by means of inelastic neutron scattering measurements under applied magnetic fields of up to 13.4 T. Comparing the observed spectra with the calculations in which spin-lattice coupling effects for the nearest neighbor exchange interactions are taken into account, we have determined the Hamiltonian parameters in the field-induced 1/5- plateau phase, which directly show that CFO exhibits a bond order associated with the magnetic structure in this phase.

cond-mat.str-el↗

arXiv2011

Magnons and electromagnons in a spin-lattice-coupled frustrated magnet CuFeO2 as seen via inelastic neutron scattering

We have investigated spin-wave excitations in a four-sublattice (4SL) magnetic ground state of a frustrated magnet CuFeO2, in which `electromagnon' (electric-field-active magnon) excitation has been discovered by recent terahertz time-domain spectroscopy [Seki et al. Phys. Rev. Lett. 105 097207 (2010)]. In previous study, we have identified two spin-wave branches in the 4SL phase by means of inelastic neutron scattering measurements under applied uniaxial pressure. [T. Nakajima et al. J. Phys. Soc. Jpn. 80 014714 (2011) ] In the present study, we have performed high-energy-resolution inelastic neutron scattering measurements in the 4SL phase, resolving fine structures of the lower-energy spin-wave branch near the zone center. Taking account of the spin-driven lattice distortions in the 4SL phase, we have developed a model Hamiltonian to describe the spin-wave excitations. The determined Hamiltonian parameters have successfully reproduced the spin-wave dispersion relations and intensity maps obtained in the inelastic neutron scattering measurements. The results of the spin-wave analysis have also revealed physical pictures of the magnon and electromagnon modes in the 4SL phase, suggesting that collinear and noncollinear characters of the two spin-wave modes are the keys to understand the dynamical coupling between the spins and electric dipole moments in this system.

cond-mat.str-el↗

arXiv2010

Incommensurate Orbital Modulation in Multiferroic CuFeO2

CuFeO_2 is one of the multiferroic materials and is the first case that the electric polarization is not explained by the magnetostriction model or the spin-current model. We have studied this material using soft x-ray resonant diffraction and found that superlattice reflection 0, 1 - 2q, 0 appears in the ferroelectric and incommensurate magnetic ordered phase at the Fe L2,3 absorption edges and moreover that the rotation of the x-ray polarization such as from σ to π or from π to σ is allowed at this reflection. These findings definitely provide direct evidence that the 3d orbital state of Fe ions has a long range order in the ferroelectric state and support the spin-dependent d-p hybridization mechanism.

cond-mat.str-el↗

arXiv2010

High-Pressure Suppression of Long Range Magnetic Order in the Triangular Lattice Antiferromagnet CuFeO2

We succeeded in observing pressure-suppressed magnetic long range ordering (LRO) in the triangular lattice antiferromagnet CuFeO$_{2}$, using neutron diffraction experiments under an isotropic pressure. The magnetic LRO of the four-sublattice ground state under ambient pressure in CuFeO$_{2}$ almost disappears at the high pressure of 7.9 GPa, and is replaced by an incommensurate order with temperature-independent wave number of (0.192 0.192 1.5). The incommensurate wave number observed at 7.9 GPa corresponds to that observed just above the temperature at which lattice distortion and magnetic LRO simultaneously occur under ambient pressure. Therefore, the long-range magnetic ordering disappears because the high pressure suppressed the lattice distortion that otherwise relieves spin frustration and leads the spin system to LRO.

cond-mat.str-el↗

arXiv2010

Spin-Wave Spectrum in `Single-Domain' Magnetic Ground State of Triangular Lattice Antiferromagnet CuFeO2

By means of neutron scattering measurements, we have investigated spin-wave excitation in a collinear four-sublattice (4SL) magnetic ground state of a triangular lattice antiferromagnet CuFeO2, which has been of recent interest as a strongly frustrated magnet, a spin-lattice coupled system and a multiferroic. To avoid mixing of spin-wave spectrum from magnetic domains having three different orientations reflecting trigonal symmetry of the crystal structure, we have applied uniaxial pressure on [1-10] direction of a single crystal CuFeO2. By elastic neutron scattering measurements, we have found that only 10 MPa of the uniaxial pressure results in almost 'single domain' state in the 4SL phase. We have thus performed inelastic neutron scattering measurements using the single domain sample, and have identified two distinct spin- wave branches. The dispersion relation of the upper spin-wave branch cannot be explained by the previous theoretical model [R. S. Fishman: J. Appl. Phys. 103 (2008) 07B109]. This implies the importance of the lattice degree of freedom in the spin-wave excitation in this system, because the previous calculation neglected the effect of the spin-driven lattice distortion in the 4SL phase. We have also discussed relationship between the present results and the recently discovered "electromagnon" excitation.

cond-mat.str-el↗

arXiv2009

Resonant Magnetic X-ray Diffraction Study on the Triangular Lattice Antiferromagnet GdPd2Al3

Resonant magnetic x-ray diffraction experiments were carried out on the stacked triangular lattice antiferromagnet GdPd2Al3. The experiments revealed an expected initial collinear c-axis order at TN1 followed by an additional in-plane order at TN2, while at the same time we found that the ground state is a helically ordered state of a very long incommensurate period of approximately 700A. The distribution of K-domains was highly anisotropic, and the domain with the modulation vector normal to the surface of the crystal was ascendant. Low-field magnetization is discussed on the basis of the observed incommensurate magnetic structure.

cond-mat.mtrl-sci↗

arXiv2008

Identification of microscopic spin-polarization coupling in the ferroelectric phase of a magnetoelectric multiferroic CuFe1-xAlxO2

We have performed synchrotron radiation X-ray and neutron diffraction measurements on magnetoelectric multiferroic CuFe1-xAlxO2 (x=0.0155), which has a proper helical magnetic structure with incommensurate propagation wave vector in the ferroelectric phase. The present measurements revealed that the ferroelectric phase is accompanied by lattice modulation with a wave number 2q, where q is the magnetic modulation wave number. We have calculated the Fourier spectrum of the spatial modulations in the local electric polarization using a microscopic model proposed by Arima [T. Arima, J. Phys. Soc. Jpn. 76, 073702 (2007)]. Comparing the experimental results with the calculation results, we found that the origin of the 2q-lattice modulation is not conventional magnetostriction but the variation in the metal-ligand hybridization between the magnetic Fe^3+ ions and ligand O^2- ions. Combining the present results with the results of a previous polarized neutron diffraction study [Nakajima et al., Phys. Rev. B 77 052401 (2008)], we conclude that the microscopic origin of the ferroelectricity in CuFe1-xAlxO2 is the variation in the metal-ligand hybridization with spin-orbit coupling.

cond-mat.str-el↗