Kazukiyo Nagata

Short Range Order Effects on EPR Frequencies in Heisenberg Linear Chain Antiferromagnets

The temperature dependence of paramagnetic resonance frequencies in a short range ordered spin state is studied theoretically and experimentally for two typical one-dimensional Heisenberg antiferromagnets, CsMnCl 3 2H 2 O and (CH 3 ) 4 NMnCl 3 . The present theory with the classical nearest-neighbor Heisenberg model properly predicts the shift of the resonant field from the high temperature limit for both crystals. The observed shift for CsMnCl 3 2H 2 O can be explained only by the dipolar term. For (CH 3 ) 4 NMnCl 3 , however, the effect of the single-ion anisotropy, or the D -term, has to be taken into account in the explanation of the temperature variation.

ESR of ultrafine magnetic particles

Abstract Ultrafine ferrimagnetic particles (MnZn ferrite) dispersed in solid kerosene have been investigated by electron magnetic resonance. Below about 220 K where the particles are embedded in the frozen kerosene, the observed spectra show a broad line with a Gaussian lineshape, which broadens and shifts towards lower fields as the temperature is lowered. The shifts of the resonance fields are proportional to the second- and third-powers of the linewidths for the samples frozen in a finite applied field and a zero field, respectively. A phenomenological theory for the lineshift will be described.

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.

EPR g-shift and anisotropic magnetic susceptibility in K 2 MnF 4

This paper reports the experimental and theoretical study on the temperature dependence of the electron paramagnetic resonance (EPR) field at fixed frequency in a quadratic layer antiferromagnet k 2 MnF 4 . The shift of EPR field (or EPR g-shift) from the value in the infinite temperature limit is theoretically shown to be a function of the parallel and perpendicular susceptibilities. The predictions of the theory are compared with the results of EPR and torque measurements performed on the single crystal of K 2 MnF 4 at temperatures above the Neel point. Excellent agreement is obtained.

Short Range Order Effects on EPR Frequencies in Antiferromagnets with Inequivalent g -Tensors

The temperature dependence of the EPR frequency is calculated for antiferromagnets with inequivalent g -tensors. Contrary to the previous results for the systems with isotropic g -factor, the EPR frequency increases rapidly for every orientations of the magnetic field as the temperature is reduced. The observed temperature dependence of the resonance fields in several copper salts is discussed by use of this result.

EPR Line-Widths of a One-Dimensional Heisenberg Antiferromagnet CsMnCl32H2O

Temperature and angular variation of EPR line-widths of a one-dimensional Heisenberg antiferromagnet CsMnCl 3 2H 2 O have been measured at K-band frequency above its ordering temperature. A minimum in the angular variation appears at room temperature indicating the dominance of long-time, q ∼0 diffusive modes of torque correlation function. However, the line-shape is Lorentzian because of non-negligible magnitude of interchain interactions. As the temperature is lowered, they broaden and the minimum in their angular variation disappears. These phenomena can be understood when one considers the growth of intrachain spin correlation of wave vector Q 0 and the relative importance of short-time, Gaussian decay modes compared with the q ∼0 modes. Line-widths are calculated within Kubo-Tomitas framework and Fishers classical treatment. Good, quantitative agreement is attained particularly at low temperatures.

Short Range Order Effect on the Magnetic Anisotropy in a Heisenberg Linear Chain Antiferromagnet CsMnCl32H2O

Torque measurements were done for CsMnCl 3 2H 2 O in order to investigate the short range order effect on the magnetic anisotropy in a Heisenberg linear chain antiferromagnet. Above 10 K, the anisotropy in the paramagnetic susceptibility has a uniaxial symmetry referred to the direction of chains ( a -axis) corresponding to the essential intrachain spin correlations and, below this temperature, it becomes orthorhombic because of the evolution of three-dimensional magnetic correlations. The magnitude and temperature dependence of the axial anisotropy observed above 10 K could be explained by the classical linear chain model including only a small dipolar term as an anisotropy term.

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.

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.