Katsunori Iio

Ferroelectricity in NH4LiSO4

It has been found that in NH 4 LiSO 4 another transition point 186.5°C exists in addition to the already known one 10°C and this substance is ferroelectric between these two transition points. Anomalies of dielectric constant at the transition points are very small as compared with those of proper ferroelectrics. The Curie-Weiss law holds within a narrow temperature range in the phase above the upper transition, where the Curie constant has a very small value of 5.6 K. Moreover the spontaneous polarization shows a curious increase with lowering temperature. Thus NH 4 LiSO 4 can be said to be one of improper ferroelectrics.

Ferroelectric Phase Transition in Hexagonal Antiferromagnet RbFeBr3 with Linear Chains of Face-Sharing Octahedra (FeBr6)4-

A hexagonal antiferromagnet RbFeBr 3 was found to undergo successive structural phase transitions at T I (=109.0 K) and T c (=34.4 K) and to be ferroelectric along the hexagonal c -axis below T c . RbFeBr 3 is a new type of ferroelectric crystal, the crystal structure of which is characterized by linear chains of face-sharing octahedra (FeBr 6 ) 4- along the c -axis (h-h-h packing). The ferroelectricity in RbFeBr 3 is attributed to possible shifts of two-thirds of the chains antiparallel to the remaining one-third along the c -axis, which are possible due to the crystal symmetry of P 6 3 c m below T c .

ESR in Hexagonal ABX3 Type Antiferromagnets. II. KNiCl3: Easy-Plane Anisotropy Case

We report the results of ESR and magnetic measurements on single crystals of KNiCl 3 whose chemical unit cell at room temperature is described by enlarging that of CsNiCl 3 to \(\sqrt{3}a\), \(\sqrt{3}a\), c , containing three –NiCl 3 – chains (two on the A-site and one on the B-site). The intrachain exchange interaction constant was estimated to be J 0 / k B =-15 K. The magnetic anisotropy was found to be the easy-plane type and D / k B =-0.86 K. Three-dimensional ordering occurs at T N =8.2 K. In the ordered state an ESR mode which is unexplainable in terms of the usual two-sublattice model was observed. Introducing two kinds of interchain exchange interactions, J 1 AB and J 1 AA , and taking six sublattices, we investigate the normal modes for the motions of the sublattice magnetizations. The frequency-field diagram calculated with J 1 AB / k B =-1.3×10 -2 K and J 1 AA / k B =-1.1×10 -2 K is in good agreement with the experimental one.

Observations of Short-Range Order by Optical Birefringence in One-Dimensional Antiferromagnets CsNiCl3, RbNiCl3 and CsCoCl3

Magnetic short-range order was observed in one-dimensional S =1 Heisenberg (CsNiCl 3 , RbNiCl 3 ) and \(S{=}\frac{1}{2}\) Ising antiferromagnet (CsCoCl 3 ) up to k T ∼10| J | and 3| J |, respectively, by measuring optical birefringences. An isomorphic diamagnet CsMgCl 3 was used, with corresponding states argument, to extract the magnetic contribution Δ n m from the resultant birefringence Δ n in these magnetic systems. The temperature T max , at which the temperature derivative of the magnetic part Δ n m displays a broad maximutm, is in agreement with that predicted by the rigorous theory for the specific heat in each linear chain system.

Birefringence Study on Structural and Magnetic Phase Transitions of RbMnBr 3

The temperature dependence of birefringence Δ n ( T ) of a triangular lattice antiferromagnet RbMnBr 3 has been measured together with comparative measurements of Δ n s of related compounds CsMgCl 3 , CsMgBr 3 , RbMgBr 3 , CsMnI 3 , CsMnBr 3 . A structural phase transition of RbMnBr 3 at T S2 =220 K with symmetry-breaking in the hexagonal c -plane has been found. The low-temperature structure below T S2 is suggested as orthorhombic, which can be the origin of an incommensurate spin structure of RbMnBr 3 . Critical exponents of magnetic specific heat are determined as α ′ =0.22±0.06, α=0.42±0.16, which are in fair agreement with the values of the Z 2 × S 1 universality class.

Birefringence Study of Ferroelectric Ordering in Oxygen Isotope Exchanged Systems SrTi(16O1-x18Ox)3

An optical birefringence study was performed on the quantum ferroelectric phase transition in the oxygen isotope exchanged system SrTi( 16 O 1- x 18 O x ) 3 . The ferroelectric ordering in (110) c ...

The Successive Magnetic Phase Transition of Hexagonal Antiferromagnet CsMnI3 Observed by Optical Birefringence

Birefringence measurement was made on a triangular lattice antiferromagnet CsMnI 3 at temperatures around the Neel point of 11.42 K. The birefringence for light propagating along the hexagonal c -axis shows that CsMnI 3 has an intermediate ordered phase in the temperature region between 8.20±0.03 K and 11.42 K. Below 8.20 K CsMnI 3 takes the orthorhombic spin structure of the triangular spin arrangement with the spin moments restricted in a plane perpendicular to the basal plane.

Dielectric anomaly involving magnetic phase transition in a hexagonal antiferromagnet RbCoBr3

Abstract The dielectric and magnetic phase transitions in a triangular lattice antiferromagnet RbCoBr3 were studied by means of some dielectric, magnetic and calorimetric measurements. In this compound, which is a typical Ising-like antiferromagnet with antiferromagnetic moment along the hexagonal c-axis, a ferroelectric phase transition involving magnetic ordering with weak ferromagnetic moments confined in the c-plane was found to occur at T N(=37.OK). Those results imply that the magnetic ordering of this system is assisted by a polar lattice distortion, which can release partially the spin frustration due to the antiferromagnetic spin arrangement on a triangular lattice. This co-ordering is certainly ascribed to a probable competition between lattice-system and spin-system. Experimental data on this interesting phenomenon are presented and a possible mechanism is proposed.

Successive Magnetic Phase Transitions in RbVBr3

The magnetic properties of RbVBr 3 with a deformed CsNiCl 3 structure were investigated by means of the susceptibility and torque measurements. RbVBr 3 undergoes the successive transitions, T N1 =28.0 K and T N2 =21.3 K and 19.0 K on heating and cooling, respectively. A large hysteresis was observed for the transition at T N2 . Metastable states appear between the intermediate phase and low-temperature phase. The intermediate phase has the weak ferromagnetic moment of the order of 10 -5 µ B perpendicular to the c-axis, while the low-temperature phase does not. It is compatible with the crystal structure that the weak moment due to the Dzyaloshinsky-Moriya interaction appears in linear chains. It is concluded that in the intermediate phase the spins form a collinear structure, probably ferrimagnetic one in the c-plane and a triangular one in the low-temperature phase.

Magnetic Susceptibility Study on RbCuCl3

Susceptibilities of single crystals of CsCuCl 3 are measured in 4.2 K< T <500 K for H ⊥ c and H // c . Behaviors characteristic to a one-dimensional Heisenberg ferromagnet with weak interchain interactions are observed. The behaviors are analyzed by the high temperature expansion approximation, and intrachain and interchain exchange interaction constants, J 0 and J 1 , are estimated. The results are, J 0 / k =(24 ±3) K and J 1 / J 0 =-0.16 ±0.04. A discontinuity in the susceptibility arising from a Jahn-Teller cooperative phase transition are observed at T t ∼420 K. The discontinuity is due to a change in J 0 at T t . Relation between the phase transition and the change of J 0 is discussed.