Fumitaka Matsubara

Mixture of Two Anisotropic Antiferromagnets with Different Easy Axes

Thermal and magnetic properties of a mixture of two anisotropic antiferro-magnets with different easy axes have been analyzed both in mean field approximation and in the method of distribution function. A new phase called OAF phase is found within a certain range of concentration. In OAF phase, the spin of each species of ion on a sublattice has each own axis of sublattice magnetization tilting oblique to the easy axes of the pure systems. As temperature increases, OAF phase makes a transition into the antiferromagnetic phase and then to the paramagnetic phase successively. It is expected that these transitions are detected by measurements of specific heats for a powder sample and of susceptibilities for a single crystal. It is also pointed out that magnetization processes show some characteristic features of OAF phase.

Magnetic Ordering in a Partially Frustrated Triangular Antiferromagnet RbFeBr3

Specific heat of quasi-one-dimensional RbFeBr 3 has been measured in the temperature range 1.1∼10 K in applied magnetic fields. Two anomalies with slight field dependences have been found at T N =5.61±0.02 K and T t =2.00±0.04 K in zero field. Below 120 K, the spin frustration is partially released by a lattice distorsion. Analysis by molecular field approximation for magnetic ordering process in the triangular spin ( S =1) system with frustration partially relased by lattice modulation is presented. The partial release of the frustration enables us to predict from the analysis that the high temperature anomaly corresponds to the transition from paramagnetic phase with mutually uncorrelated magnetic chains to a partially disordered phase in which one third of magnetic chains remains uncorrelated with the others and the low temperature one to the transition from the partially disorded phase to the triangular phase in the c -plane.

Magnetic Ordering in a Hexagonal Antiferromagnet

The magnetic ordering in a hexagonal antiferromagnetic system with both the exchange anisotropy energy and the single-ion anisotropy energy is studied by using the molecular field approximation. It is shown that the magnetic moments in an ordered phase appearing at T ≤ T N form either a collinear structure with the moments parallel or antiparallel to the hexagonal axis or a triangular structure in the basal plane, and that an arrangement of the moments at T ∼0 shows a variety of structures. Thus, the system exhibits a variety of magnetic ordering processes as the temperature lowers. Our results may give a systematic explanation of anomalous ordering processes observed in recent experiments.

Magnetic Properties of Solid Solution CoxFe1-xCl22H2O

The magnetic properties and the phase diagram of the solid solution Co x Ni 1- x Cl 2 6H 2 O are analyzed by using a model Hamiltonian with the mean field approximation. Our results are qualitatively in agreement with experimental ones. It is shown that the sublattice magnetization lies in a plane including both easy axes of two pure substances ( ca -plane) and its direction varies smoothly over the whole range of the concentration x , not within a limited range of x as observed in K 2 Mn x Fe 1- x F 4 and Co x Fe 1- x Cl 2 2H 2 O. The feature is due to off diagonal terms in the Hamiltonian which come from the non-orthogonality of the easy axes of two pure substances CoCl 2 6H 2 O and NiCl 2 6H 2 O (∼65°). The characteristic features of the magnetization processes are also analyzed considering the fact that the second easy axis of CoCl 2 6H 2 O does not lie within the ca -plane.

Nature of the Ordered Phase in a Hexagonal Ising Antiferromagnet

A Monte Carlo study of a hexagonal Ising Antiferromagnet reveals the occurrence of a new type of modulated phase in which a spin structure described by using three sublattices is randomly modulated in the lattice. The order parameter of this phase is not individual sublattice magnetizations but a symmetric function of them which corresponds to the magnetic structure factor F (1/3, 1/3, 0). As the temperature is decreased, this phase gradually changes into a usual homogeneous ferrimagnetic phase. This process is discussed in detail. It is suggested that this modulated phase occurs in CsCoCl 3 and CsCoBr 3 .

Theory of One-Dimensional Random Mixture of Ising Spins

A general theory to obtain the magnetic properties of the one-dimensional random mixture of plural kinds of magnetic atoms (Ising spins) in the finite magnetic field is formulated in terms of distribution functions which give the probabilities of finding powers of average values of spins situated on the end site of the chain. The distribution functions are determined from the simultaneous functional equation of order n, where n is the number of the kind of magnetic atoms. Using this theory, we study the magnetic properties of the following systems: (1) The regular system composed of only one kind of magnetic atoms, (2) the simple random system composed of one kind of magnetic atoms and non-magnetic atoms, (3) the random binary mixture composed of two kinds of magnetic atoms of S=l/2, (4) the random binary mixture of two kinds of magnetic atoms of S=l/2 and S=l. An Ising chain with random exchange integrals is also formulated in terms of a distribution function, and the magnetic properties are investigated when S=l/2. It is shown that the magnetization curves of those mixtures at low temperatures have various steps when the antiferromagnetic elements are included, and the susceptibilities at H=O and T~O diverge when the ferromagnetic elements are included even if the magnetization vanishes when T=O and H~O. Those are discussed in detail from the random nature of the mixtures.

Stiffness of the Heisenberg Spin-Glass Model at Zero- and Finite-Temperatures in Three Dimensions

We examine the stiffness of the Heisenberg spin-glass (SG) model at both zero temperature ( T =0) and finite temperatures ( T ≠0) in three dimensions. We calculate the excess energies at T =0 which are gained by rotating and reversing all the spins on one surface of the lattice, and find that they increase with the lattice size L . We also calculate the excess free-energies at T ≠0 which correspond to these excess energies, and find that they increase with L at low temperatures, while they decrease with increasing L at high temperatures. These results strongly suggest the occurrence of the SG phase at low temperatures. The SG phase transition temperature is estimated to be T SG ∼0.19 J from the lattice size dependences of these excess free-energies.We examine the stiffness of the Heisenberg spin-glass (SG) model at both zero temperature (T=0) and finite temperatures (

Reentrant spin-glass transition in a dilute magnet.

T \ne 0

Low Temperature Phase of Asymmetric Spin Glass Model in Two Dimensions

) in three dimensions. We calculate the excess energies at T=0 which are gained by rotating and reversing all the spins on one surface of the lattice, and find that they increase with the lattice size

Monte Carlo Simulation of Magnetic Ordering of Quasi-One Dimensional Ising Antiferromagnets CsCoBr 3 and CsCoCl 3

L