Yusuke Kousaka

Chiral Magnetic Ordering and Commensurate-to-Incommensurate Transition in CuB2O4

We measured the anisotropic magnetization processes of copper metaborate CuB 2 O 4 in detail. We propose an interpretation of the magnetization anomaly observed under the magnetic field applied perpendicular to the tetragonal axis, based on the field- and temperature-induced commensurate-to-incommensurate (C–IC) transition (Freedericksz transition) accompanied by the chiral soliton lattice formation.

Discrete Change in Magnetization by Chiral Soliton Lattice Formation in the Chiral Magnet Cr1/3NbS2

In the chiral magnet Cr1/3NbS2, discrete changes in the magnetization (M) caused by the formation of a chiral soliton lattice (CSL) were observed in magnetization curve measurements using a single crystal of submillimeter thickness. When M is measured with a minimal increment of the magnetic field H, 0.15 Oe, discrete changes in M are observed in the H region that exhibits definite magnetic hysteresis. In particular, enormous discrete changes in M are observed as H decreases from the field above the saturation field, while fine M steps are also found in the intermediate H range independently of the sweeping direction of the field. The former is considered as a type of enormous Barkhausen effect associated with the CSL formation. The latter originates from the change in soliton number during the CSL formation.

Emergence of highly degenerate excited states in the frustrated magnet MgCr2O4.

High degeneracy in ground states leads to the generation of exotic zero-energy modes, a representative example of which is the formation of molecular spin-liquid-like fluctuations in a frustrated magnet. Here we present single-crystal inelastic neutron scattering results for the frustrated magnet MgCr(2)O(4), which show that a common set of finite-energy molecular spin excitation modes is sustained in both the liquid-like phase above magnetic ordering temperature T(N) and an ordered phase with an extremely complex magnetic structure below T(N). Based on this finding, we propose the concept of high degeneracy in excited states, which promotes local resonant elementary excitations. This concept is expected to have ramifications on our understanding of excitations in many complex systems, including not only spin but also atomic liquids, complex order systems, and amorphous systems.

Crystal Structure and Charge-Ordering in La1.5Ca0.5CoO4 Studied by Neutron and Resonant X-ray Scattering

Using neutron and resonant X-ray scattering, we studied crystal structure and charge-ordering pattern in La 1.5 Ca 0.5 CoO 4 . Taking A 2 m m with a – b type twin structure into account, Co 2+ /Co 3+ checkerboard charge ordering pattern was suggested in La 1.5 Ca 0.5 CoO 4 by neutron scattering structure analysis. In order to clarify the charge ordering pattern and the valences of the two Co sites, we performed a resonant X-ray scattering (RXS) measurement near the Co K -edge. We found a resonant signal at (3, 0, 0) reflection which corresponds to the charge ordering with a Co 2+ /Co 3+ checkerboard pattern, and the Co valences of charge ordering were fully charge disproportionated. From the X-ray fluorescence spectra, we found the pre-edge peak structures at E =7.710 and 7.713 keV, corresponding to 1 s → t 2 g and 1 s → e g transitions, respectively. The polarization dependence of the latter one suggests that the anisotropy e g orbital is realized in La 1.5 Ca 0.5 CoO 4 .

Crystallographic Chirality of CsCuCl3 Probed by Resonant Circularly-Polarized Hard X-ray Diffraction

We have observed the circular-polarization-dependent intensities of forbidden (0, 0, 6 n ±2) reflections at the Cu K absorption edge in the chiral crystal CsCuCl 3 . The intensities for the right- and left-handed circularly-polarized incident beams interchange in CsCuCl 3 crystals with opposite chiralities. The result is well explained by a model calculation based on the anisotropic tensor susceptibility of Cu 2+ ions, taking only the electric-dipole ( E 1) process into account. This technique can determine crystallographic chirality by measuring the flipping ratio for only one reflection, in principle.

Understanding the H-T phase diagram of the monoaxial helimagnet

Some unexpected features of the phase diagram of the monoaxial helimagnet in presence of an applied magnetic field perpendicular to the chiral axis are theoretically predicted. A rather general Hamiltonian with long-range Heisenberg exchange and Dzyaloshinskii\char21{}Moriya interactions is considered. The continuum limit simplifies the free energy, which contains only a few parameters which in principle are determined by the many parameters of the Hamiltonian, although in practice they may be tuned to fit the experiments. The phase diagram contains a chiral soliton lattice phase and a forced ferromagnetic phase separated by a line of phase transitions, which are of second order at low

Size dependence of discrete change in magnetization in single crystal of chiral magnet Cr1/3NbS2

T

Unconventional Magnetic Domain Structure in the Ferromagnetic Phase of MnP Single Crystals

and of first order in the vicinity of the zero-field ordering temperature, and are separated by a tricritical point. A highly nonlinear chiral soliton lattice, in which many harmonics contribute appreciably to the spatial modulation of the local magnetic moment, develops only below the tricritical temperature, and in this case, the scaling shows a logarithmic behavior similar to that at

Investigation of structural changes in chiral magnet Cr1∕3NbS2 under application of pressure

T=0

Observation of elastic anomalies driven by coexisting dynamical spin Jahn-Teller effect and dynamical molecular-spin state in the paramagnetic phase of frustrated MgCr2O4

, which is a universal feature of the chiral soliton lattice. Below the tricritical temperature, the normalized soliton density curves are found to be independent of