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Successive structural phase transitions with ferroelectricity were found in TlCoCl 3 through dielectric constant measurements and D – E hysteresis loop observations. The sequence of the transitions is identical with that in RbMnBr 3 and TlFeCl 3 , which undergo successive transitions from the prototype crystal structure of CsNiCl 3 . The temperature dependence of the magnetic susceptibility below T N1 indicates the presence of successive magnetic orderings, which has not been observed in the Ising-like antiferromagnets CsCoCl 3 and CsCoBr 3 without structural transitions. Dielectric dispersion attributable to the Ising-spin dynamics on a linear-chain spin system was observed at low temperatures. Several problems to be solved are proposed for understanding structural and magnetic properties of TlCoCl 3 .

The complex phase transitions involving dielectricity and magnetism on distorted triangular lattice antiferromagnets, "KNiCl3-family" crystals, which have the crystal structures derived from the prototype CsNiCl3, were studied through the heat capacity measurements. These crystals are classified into characteristic three groups from the viewpoint of magnetic and structural properties clarified so far. The results of the dielectric constant measurement as a function of temperature are also presented in detail. The present calorimetric study reveaed that the structural successive phase transitions rather than the magnetic transitions in these crystals are recognized more distinctly as the presence of specific heat anomalies at the respective phase transition points.

Thermal conductivity measurements have been carried out for single crystals of the onedimensional (1D) antiferromagnetic (AF) Ising-like spin system ACoX3 (A = Rb, Cs; X = Cl, Br). A shoulder originating from the thermal conductivity due to spin excitations, \k{appa}?spin, has been observed around 60 K in the temperature dependence of the thermal conductivity along the c-axis parallel to spin chains for every ACoX3. To our knowledge, this is the first observation of ?\k{appa}spin in 1D AF Ising-like spin systems. It has been found that the magnitude of ?\k{appa}spin in ACoX3 is a little smaller than those in 1D AF Heisenberg spin systems. The reason is discussed in terms of the Heisenberg-like character in ACoX3., 14 pages, 3 figures, 1 table (in press). Journal of the Physical Society of Japan (2014)

Extensive Monte Carlo simulations are performed to analyze a recent neutron diffraction experiment on a distorted triangular lattice compound RbCoBr$_3$. We consider a spin-lattice model, where both spin and lattice are Ising variables. This model explains well successive magnetic and dielectric transitions observed in the experiment. The exchange interaction parameters and the spin-lattice coupling are estimated. It is found that the spin-lattice coupling is important to explain the slow growth of a ferrimagnetic order. The present simulations were made possible by developing a new Monte Carlo algorithm, which accelerates slow Monte Carlo dynamics of quasi-one-dimensional frustrated systems., Comment: 11 pages, 11 figures

We study the lattice-spin model of RbCoBr$_3$ which is proposed by Shirahata and Nakamura, by mean field approximation. This model is an Ising spin system on a distorted triangular lattice. There are two kinds of frustrated variables, that is, the lattice and spin. We obtain a phase diagram of which phase boundary is drawn continuously in a whole region. Intermediate phases that include a partial disordered state appear. The model has the first-order phase transitions in addition to the second-order phase transitions. We find a three-sublattice ferrimagnetic state in the phase diagram. The three-sublattice ferrimagnetic state does not appear when the lattice is not distorted., Comment: 5 pages, 4 figures, jpsj2.cls, to be published in J. Phys. Soc. Jpn. Vol.75 (2006) No. 2

The temperature dependence of the complex dielectric constant of RbCoBr3 along the hexagonal c-axis was measured in the frequency range from 1 kHz to 1 MHz under magnetic fields of up to 12 T applied parallel to the c-axis. On the basis of the observed dielectric behavior, a probable interplay between lattice and spin systems, and appropriate crystal structures for the three phases below the upper Neel point were deduced. A broad peak of the real part \(\epsilon '_{c}(T)\) at approximately 30 K was found to exhibit frequency dispersion with two kinds of relaxation time. These relaxation times seem to correspond closely to the magnetic relaxation times ascribed to the domain-boundary motion in Ising spin chains. In the presence of magnetic fields above 8 T, this peak split into two subpeaks. This magnetic field dependence of \(\epsilon '_{c}(T)\) reflects the suppression of antiferromagnetic ordering under the magnetic field. These findings imply the correlation between the ferroelectric and antiferromagne...

High-field magnetization measurements were performed on the quasi-one-dimensional Ising-like antiferromagnet TlCoCl3 up to 68 T. The magnetization curve \(M(H)\) at 1.4 K showed a nonlinear increase. The field derivative of the magnetization \(\mathrm{d}M/\mathrm{d}H\) had sharp peaks at the critical magnetic field of 42 T and the saturation magnetic field of 55 T. We measured the temperature variation of the peak position at the critical magnetic field and obtained the magnetic phase diagram. The magnetization curve was well reproduced by a calculation for the quasi-one-dimensional Ising-like system.

Thermal expansion and dielectric measurements of RbCoBr3 were performed to examine the lattice distortion accompanied by structural phase transitions. It was confirmed that RbCoBr3 undergoes structural phase transitions at 90, 37, and 10 K because its linear thermal expansion and the temperature dependence of its linear thermal expansion coefficients showed clear anomalies at these temperatures. The pyroelectric charge measurement showed that a spontaneous electric polarization appears between 37 and 31 K and below 10 K. We propose a successive structural phase transition sequence and crystal symmetries at each phase.

The successive magnetic phase transitions of the distorted-triangular-lattice antiferromagnet RbFeBr 3 have been investigated by single-crystal neutron diffraction measurements. The intermediate magnetic phase ( T N2 = 2.0 K< T < T N1 = 5.6 K) appears as a partially disordered (PD) spin structure. An inclusive understanding of a distortion model of this crystal with the symmetry P 6 3 c m , which enables the spontaneous electric polarization and the PD spin structure with a weak ferromagnetic moment, was established.

High-field magnetization measurements were performed on a distorted triangular-lattice antiferromagnet, RbCoBr 3 . The magnetization curve shows anomalies characterized by two peaks of d M /d H . The temperature dependences of these peak positions approach the magnetic phase transition points at H = 0. The results suggest that an intermediate magnetic phase, which is known as the partial disordered phase, may appear even in a finite magnetic field.

The structural changes of TlCoCl 3 around phase IV (68 < T < 75 K) were studied by single-crystal neutron diffraction measurement. The present investigation revealed that a small but definite Bragg peak at (1/6, 1/6, 2) appeared only in phase IV. A precursor phenomenon of the phase V ( T < 68 K) structure in the phase IV temperatures was also identified as the coexistence of a slightly broadened (1/4, 1/4, 2) Bragg peak. We confirmed that intermediate phase IV is a stable, nontransient, single phase with \(2\sqrt{3}a \times 2\sqrt{3}a \times c\) unit cells. A possible sequence of structural phase transitions was proposed.

Powder and single-crystal neutron diffraction measurements were performed on RbCoBr 3 . The magnetic phase transitions of this compound are well explained by the Ising spin model for a distorted triangular lattice. Magnetic peaks of powder diffraction patterns at 34 and 20 K closely fit a partial disordered phase and a ferrimagnetic phase, respectively. The temperature dependences of the ( h , h ,1) scan profile and magnetic Bragg peak heights at (1,1,1), (1/3,1/3,1), and (2/3,2/3,1) were measured for a single-crystal sample. We conclude that the ferrimagnetic phase only exists below 30.5 K. The partial disordered phase is identified between 30.5 (= T N2 ) and 37.2 K (= T N1 ). We found a characteristic linear increase in the (1,1,1) peak height below T N2 with decreasing temperature. These results show that a lattice distortion couples to the spin system. Numerical simulations are also performed to estimate magnetic interaction parameters.

We report magnetic Raman scattering in a distorted-triangular-lattice Ising-like antiferromagnet TlCoCl 3 . Below the Neel temperature, we observe several new Raman peaks from the quantized domain-wall pair excitations (Zeeman-ladder excitations). From their sequencies, we obtain the magnetic parameters of this system. We find that the orthorhombic lattice distortion causes a reduction of exchange interactions between the magnetic chains shifting to the opposite sides. The origin is discussed within a framework of superexchange interaction.

In order to discuss the condensation of the K 4 -mode in KNiCl 3 -family compounds, the temperature dependences of the elastic compliances of KNiCl 3 , RbMnBr 3 , RbFeBr 3 , and RbCoBr 3 were measu...

Through magnetic and dielectric measurements, a simultaneous phase transition of spin and lattice systems observed previously at 37.2 K of RbCoBr 3 have been investigated by doping with nonmagnetic Mg impurities. A mutual reduction in the ferroelectric Curie temperature T C and the Neel temperature T N due to the enhancement of Mg doping confirms the occurrence of the synergistic ordering of both. A broad peak of the real part of dielectric constant along the c axis, e ′ c ( T ), at approximately 32 K of the pure system splits into two sharp peaks in mixed systems. This suggests the presence of successive structural phase transitions at around this temperature. A probable sequence of crystal and magnetic structures was discussed for the successive phase transitions of RbCo 1- x Mg x Br 3 . The interplay between the spin and lattice variables of RbCoBr 3 was interpreted by taking account of spin-dependent polarizability and spin-dependent spontaneous polarization. A dielectric anomaly observed at 9.0 K in ...

Polarized neutron inelastic scattering experiments have been carried out in the quasi-1D Ising-like antiferromagnet TlCoCl$_3$. We observed the longitudinal magnetic fluctuation $S_{zz} (Q, \omega)$ for the spin-wave excitation continuum, which has not been observed in the unpolarized neutron inelastic scattering experiments of the quasi-1D Ising-like antiferromagnets CsCoCl$_3$ and TlCoCl$_3$ so far, together with the transverse magnetic fluctuation $S_{xx} (Q, \omega)$. We compared both obtained intensities of $S_{xx} (Q, \omega)$ and $S_{zz} (Q, \omega)$ with the perturbation theory from the pure Ising limit by Ishimura and Shiba, and a semi-quantitative agreement was found.

A powder and single-crystal neutron diffraction study was performed on the structural and magnetic phase transitions of the distorted-triangular-lattice Ising-like antiferromagnet TlCoCl 3 . From the results of powder sample measurement, the crystal structures were identified as the P 6 3 c m and P b c a structures at the intermediate-temperature (75 < T < 165 K) and lowest-temperature ( T

Neutron inelastic scattering measurements have been performed in order to investigate the magnetic excitations in the quasi-1D Ising-like antiferromagnet TlCoCl$_3$. We observed the magnetic excitation, which corresponds to the spin-wave excitation continuum corresponding to the domain-wall pair excitation in the 1D Ising-like antiferromagnet. According to the Ishimura-Shiba theory, we analyzed the observed spin-wave excitation, and the exchange constant $2J$ and the anistropy $\epsilon$ were estimated as 14.7 meV and 0.14 in TlCoCl$_3$, respectively., Comment: 2 pages, 3 figures, jpsj2.cls, to be published in J. Phys. Soc. Jpn. Vol.75 (2006) No. 1