Kinshiro Hirakawa

Experimental Studies of Triangular Lattice Antiferromagnets with S=1/2: NaTiO2 and LiNiO2

Real triangular lattice antiferromagnet with S =1/2 has been long-awaited since Anderson pointed out that its ground state might be a quantum liquid-like state rather than the Neel state. In this paper we report that NaTiO 2 and LiNiO 2 are promising candidates for it, the former being near Heisenberg-type and the latter near Ising-type. Our magnetization measurements on LiNiO 2 show a striking linear to nonlinear response transition at T N1 =210 K. A pronounced anomaly appears in ESR at another transition T N2 =∼20 K suggesting a ferrimagnet-like ordered state below. However, neither ferromagnetic nor antiferromagnetic reflections could be observed in zero field neutron scattering between 1.4 and 300 K. The susceptibility of NaTiO 2 is low suggesting a strong antiferromagnetic coupling between the spins but no Brass scattering could be observed at 1.4 K. These are suggestive of the remarkable quantum effects.

Magnetic Properties of Potassium Iron Group Fluorides KMF3

Evidence of antiferromagnetism in perovskite-type potassium iron group fluoride KMF 3 was found through the susceptibility measurements made between 80°K and 700°K. The obtained results are as follows:

Formation of One-Dimensional Antiferromagnet in KCuF3 with the Perovskite Structure

KCuF 3 shows a broad peak of the magnetic susceptibility at ∼243°K and has been supposed to be an antiferromagnet. Scatturin et al. , on the other hand, have reported that the neutron diffraction shows no magnetic ordering in KCuF 3 even at temperatures as low as 4.2°K. In the discussion, they implies the existence of linear magnetic chains along the c -axis. In order to make sure of their conjecture, the measurements of susceptibility, its anisotropy and specific heat have been made. These measurements assure us that KCuF 3 is a one-dimensional antiferromagnet, with the exchange interaction along the chain being J / k =-190°K.

Neutron scattering study of two-dimensional Ising nature of K2CoF4

Abstract By neutron elastic and quasi-elastic scattering from K2CoF4, sublattice magnetization and spin correlations are examined quantitatively. The obtained critical exponents β, ν, γ, and η coincide precisely with those from Onsagers exact solution for the two-dimensional Ising model.

Structural Study of Iron Selenides FeSex. I Ordered Arrangement of Defects of Fe Atoms

The ordered arrangements of the defects of iron atoms in FeSe x have been studied for several compositions by X-ray analysis. These defects tend to assume an ordered arrangement as in the case of pyrrhotite FeS 1.14 . New phases are found in the region ( x =8/7∼4/3) which have been considered as the single phase region. If we choose the orthohexagonal and pseudo-orthohexagonal unit cells for x =8/7 and 4/3, respectively, the unit cell dimensions of the superstructure resulting from this ordering are twice as large as that of the fundamental structure along the a - and b -axes, and three times along the c -axis for x =8/7, and on the other hand, twice only along the c -axis for x =4/3.

Neutron Scattering Study of Magnetic Excitations in Pseudo-One-Dimensional Singlet Ground State Ferromagnets CsFeCl3 and RbFeCl3

The magnetic excitation spectra in pseudo-one-dimensional planar ferromagnets CsFeCl 3 and RbFeCl 3 were observed using inelastic neutron scattering technique. The observed excitation spectruun of CsFeCl 3 has a gap at the magnetic zone center, while that of RbFeCl 3 has no gap with a feature of XY system. This prominent difference can be understood from the fact that they are singlet-ground-state systems. As RbFeCl 3 has a 3D LRO due to an exchange induced moment, CsFeCl 3 shows no sign of the LRO within the temperature region we measured. Moreover, from a viewpoint of the temperature dependence, it is noticeable that the excitation spectrum of RbFeCl 3 shows the feature of XY system even in the paramagnetic phase. The one-dimensionality is evaluated to be about 0.05 for CsFeCl 3 and 0.1 for RbFeCl 3 showing that they have unexpectedly poor one-dimensionality.

Neutron Scattering Study of a One-Dimensional Ising-Like Antiferromagnet CsCoCl3. I. Instantaneous Correlation

Instantaneous spin correlations \(\mathscr{S}(\mbi{Q})\) in CsCoCl 3 have been studied by means of quasi-elastic neutron scattering technique and well defined plane-like intensity distribution characteristic for 1D system has been observed. At temperatures higher than 30 K, the observed temperature variation of \(\mathscr{S}(\mbi{Q})\) is in good agreement with the rigorous solution for the 1D Ising antiferromagnet with | J |/ k =75 K. When the temperature is decreased, 3D critical scattering consistent with the previously worked model can be observed at two Neel points T N1 =21.3 K and T N2 =9.2 K.

The Magnetic Properties of Iron Selenide Single Crystals

Large single crystals of Fe 7 Se 8 and Fe 3 Se 4 were prepared, and their magnetic properties below the Curie point were investigated by a torque magneto meter. The results obtained on Fe 7 Se 8 at room temperature show similar characterics to that of natural pyrhotite (Fe 7 S 8 ), i.e. the crystals can be magnetized only in the c -plane. As the temperature is lowered, however, the c -axis becomes the direction of easy magnetization. This change in direction seems to occur gradually. In the case of Fe 3 Se 4 , such a change in direction of easy magnetization does not take place. The obtained saturation magnetizations for Fe 7 Se 8 and Fe 3 Se 4 are respectively 68 and 80 gauss at the liquid air temperature.

Magnetic susceptibilities of the frustrated triangular lattice antiferromagnets CsVCl3 and VX2 (X=Cl, Br and I): appearance of magnetic liquid in the ordered state

Magnetic susceptibilities χ of the frustrated triangular lattice antiferromagnets CsVCl 3 and VX 2 (X=Cl, Br and I) have been measured by means of the conventional Faraday-type as well as the SQUID magnetometers. The χ- T curve of CsVCl 3 shows a typical 1D-like character in agreement with the early work by Niel et al . At T < T N , χ c increases rapidly with decreasing T , whereas χ a stays at a constant value. The χ- T curve in VX 2 which are thought to be 2D-like systems is very flat and shows an anomaly at T N . Surprizingly, however, no anisotropy could be detected below T N . This suggests that the transition is quite different from the conventional one for the non-frustrated antiferromagnetic systems and a magnetic liquid state is formed in the ordered state.

Incommensurate Magnetic Phase Transitions in the Triangular XY-Like Antiferromagnet RbFeCl3

RbFeCl 3 is composed of one-dimensional chains in which the spins interact ferromagnetically with XY-like anisotropy. These chains are coupled antiferromagnetically in the hexagonal basal plane. It undergoes three transitions at T N1 =2.5 K, T N2 =2.35 K and T N3 =1.95 K. In order to understand the nature of these transitions, we performed a neutron scattering study. Besides the previously determined 120°-structure observed at T < T N 3 , we found two different incommensurate structures in the basal plane at T N3 < T < T N2 and T N2 < T < T N1 . Only, the transition at T N3 is the first order, but the other two are the second order. The effect of the field applied parallel to the easy plane has also been studied.