Hiroaki Kadowaki

Neutron powder diffraction study of the two-dimensional triangular lattice antiferromagnet CuCrO2

An S=3/2 Heisenberg antiferromagnet on a triangular lattice CuCrO2, in which stacking of the triangular lattice of magnetic Cr atoms forms a layered rhombohedral antiferromagnet, is studied by neutron powder diffraction. In the paramagnetic phase the powder diffraction pattern shows asymmetry, which proves a two-dimensional character. In the ordered phase, magnetic Bragg scattering has large width, indicating that the scattering is distributed on a line (1/31/3 zeta ) with peaks where zeta takes integer values. Although the magnetic long-range order is established in the c plane, correlation in the c direction is finite or the modulation vector is distributed on the line. Intensity of magnetic reflections is consistent with the 120 degrees structure in the a-c plane with moment (3.1+or-0.2) mu B.

Double-Q 120 degrees structure in the Heisenberg antiferromagnet on rhombohedrally stacked triangular lattice LiCrO2

The magnetic ordering of the S=2/3 Heisenberg antiferromagnet on a rhombohedrally stacked triangular lattice LiCrO2 is studied by susceptibility, neutron diffraction and polarization analysis measurements using a single crystal. The temperature dependence of the susceptibility strongly suggests that a 120 degrees structure is established in each quasi-two-dimensional layer, which is parallel to the c plane. Assuming the 120 degrees structure in which magnetic moments are confined in a plane including the c axis because of an Ising-type anisotropy, neutron scattering shows that the magnetic ordering is a double-Q structure with non-equivalent wave numbers q=(1/3 1/3 0) and (-2/3 1/3 1/2). It is characterized by an alternating sequence of rotational direction of the 120 degrees structure along the c axis.

Neutron scattering study of antiferromagnetic correlations in the Kondo semiconductor CeNiSn

Spin excitations in the Kondo semiconductor CeNiSn have been studied in a wide Q-range and in the energy range of h(cross) omega =1.2-7 meV by means of single-crystal neutron scattering. The magnetic fluctuation at low temperatures in this energy range is dominated by the easy a-axis component Im chi ad. Below the coherence temperature of 20 K, two dynamic antiferromagnetic correlations develop as excitation peaks at h(cross) omega =2 and 4 meV. The 4 meV excitation appears at Q=(Qa. 1/2 +n, Qc) where Qc and Qc are arbitrary and n is an integer, which indicates that the correlation is quasi-one dimensional along the b-axis. The 2 meV excitation appears around Q=(0,0,I) and (0,1,0), which shows three-dimensional Q-dependence. These two excitations reflect the nature of the Kondo coherent state of CeNiSn.

The ground states and phase transitions in the two-dimensional triangular lattice antiferromagnets

Recent developments of theoretical and experimental studies on triangular lattice antiferromagnets are introduced. Emphasis is placed on two remarkable features. One is the new type of ground state, a quantum liquid, which would appear in the S = 1/2 systems, as first pointed out by Anderson. Another is a new mechanism of spin order which appears even in classical systems due to additional degree of freedom such as chirality. Some experimental examples are shown.

Growth and properties of Li-deficient lithium vanadium dioxide single crystals

Large, high quality LiVO2 single crystals were first grown by a LiBO2-Li2O flux method. The obtained crystals are black hexagonal plates having the formula Li1 − xVO2 (x ≈0.2) with a size reached to 15×15×0.1 mm3. Single crystal X-ray studies revealed that the crystals have a rhombohedral cell with the pseudo-hexagonal lattice constants a0=2.839(2)A and c0=14.83(1)A with an additional superstructure which gives a new trigonal cell constants a0′=4.92A≈√3a0 and c0′=29.66A≈2c0. The superstructure is presumably explained by clustering of V ions in the (001) plane. A marked anisotropy in electrical resistivities was observed whereas no anisotropy was observed in magnetic susceptibilities.

The effect of high pressure on antiferromagnetic correlations in the Kondo semimetal CeNiSn

Magnetic excitation spectra of the Kondo semimetal CeNiSn have been studied by neutron scattering under hydrostatic pressure up to 11 kbar. The inelastic peak at has been observed at the ambient pressure below and is regarded as a dynamic antiferromagnetic correlation. As pressure is applied, the 4 meV peak shifts to a higher-energy region and becomes weak. Since the pseudogap is known to be suppressed under high pressure, this result strongly suggests that the antiferromagnetic correlation is closely related to pseudogap formation.

Neutron-diffraction and high-field-magnetization study of the magnetic phase transition in the triangular-lattice antiferromagnet ABX3

Abstract The neutron-diffraction and magnetization measurements between 1.4 and 20 K under static magnetic fields up to 12 T have revealed a variety of magnetic phase transitions in the stacked triangular-lattice antiferromagnets ABX 3 , including a novel phase transition which belongs to a new universality class linked to chiral degeneracy. The magnetic field versus temperature phase diagrams are classified into two types depending on their magnetic anisotropies.

Magnetic excitations in CeNiSn under high magnetic field

Abstract The magnetic excitation spectrum of CeNiSn has been studied by inelastic neutron scattering under high magnetic field up to 10T. The well known modes of characteristic energies 2 and 4 meV were studied. No Zeeman splitting was observed for either of them. The mode initially peaked at 2 meV shifts continuously to higher energy. The characteristic feature of the 4 meV mode is the broadening observed for the highest applied field of 10 T whereas there is no appreciable change in the spin dynamics between 0 and 6 T.

An inelastic neutron scattering study of the Kondo semiconductor CeNiSn in high magnetic field

We have used inelastic neutron scattering to determine the spin dynamics in the Kondo insulator CeNiSn under magnetic fields up to 10 T applied parallel to the easy a axis. The well known excitations peaked at meV located at the Brillouin zone centre and at meV located at the Brillouin zone border were studied comparatively. At the scattering vectors (0 1 0) and (0 0 1) the peak shifts continuously from 2 to 2.5 meV with the field. No Zeeman splitting could be detected at the accuracy of our measurements. For the `4 meV excitation studied at the scattering vector (0 0.5 1), a field as large as 10 T begins to fill in the gap whereas there is no appreciable change in the spin dynamics between 0 and 6 T. The main feature is the broadening of the response. We compare our experimental results to recent theoretical developments.

The effect of Co doping on antiferromagnetic correlations in the Kondo semi-metal CeNiSn.

Single-crystal neutron scattering experiments have been performed to study magnetic excitations in [Formula: see text], in which the pseudo-gap of CeNiSn is suppressed by the doping. In CeNiSn there are two inelastic excitation peaks at [Formula: see text] and 4 meV, which correspond to dynamic antiferromagnetic correlations. In [Formula: see text] the 2 meV peak is smeared out, whereas the 4 meV peak becomes very weak and broad, but preserves the same quasi-one-dimensional character as that for CeNiSn. These results suggest the strong relation between the antiferromagnetic correlations and the pseudo-gap formation.