Kazuko Kawasaki

Magnetism and Phase Transition in Two-Dimensional Lattices; M(HCOO)2·2H2O (M; Mn, Fe, Ni, Co)

The heat capacities of Fe 2+ and Ni 2+ formate dihydrates were measured. Fe 2+ salt showed a sharp peak at T N =3.75±0.05 K with total magnetic entropy N k /2(ln 5+ln 2). Ni 2+ salt exhibited a distinct peak at T N =15.5±0.2 K and a Schottky type maximum around 3.1 K. It was discussed that M 2+ ions in (200) layer in this series of compounds were magnetically free and that M 2+ ions in (100) layer behaved as magnetically two-dimensional spin system which ordered at T N . It was concluded theoretically, using R. P. A. method, that the broad shoulder generally found in the specific heat curve just above T N was characteristic to the low dimensional spin system. This tendency was remarkable in the system with isotropic coupling such as Mn 2+ ions and not so distinct in the case of anisotropic Co 2+ ions. The present cases of Fe 2+ and Ni 2+ ions gave the examples of those with intermediate anisotropy.

Stationary self-localized magnons in Heisenberg antiferromagnets

Magnon-magnon interactions in a d -dimensional version of the simple cubic antiferromagnet with anisotropic nearest-neighbor exchange interaction and uniaxial anisotropy energy are studied to show the existence of a stationary self-localized magnon mode appearing below the magnon frequency band. This is an intrinsic nonlinear mode corresponding to a local large-angle, low-frequency precessional motion of spins, quantum states of which are characterized by the indefiniteness of the number of relevant magnons. The formulation of the problem is made by employing the Dyson-Maleev transformation and a coherent-state ansatz. Approximate analytical expressions for the eigenfrequency and profile functions of the stationary self-localized mode are obtained in the strong-localization limit for arbitrary d .

Moving and stationary self-localized magnons as spin-wave solitons in Heisenberg ferromagnets

A theory of elementary excitations in Heisenberg ferromagnets with anisotropic exchange interactions is formulated without assuming the smallness of spin deviations from the ground state. This is done by using a path integral formalism in the SU(2) coherent state representation combined with the use of the stationary-phase approximation. Equations for spin-deviation field variables are solved in an approximate analytical form to show the existence by the intrinsic nonlinearity of mobile and immobile self-localized magnons in one- and d -dimensional cases, respectively. Discussions are given on their characteristic properties such as the energy eigenvalues lying below the linear spin-wave energy band, dispersion curves and localized-mode profiles. These intrinsic nonlinear modes can be considered as a natural extension of conventional (linear) spin waves to the nonlinear, soliton regime.

SU(2) coherent state path integral formulation of nonlinear collective modes of magnons in Heisenberg antiferromagnets : self-localized magnons and vortices

A path integral theory in the SU(2) coherent state representation of elementary excitations in Heisenberg antiferromagnets is developed, in which no assumption is made on the smallness of spin deviations from the Neel state. Use of the stationary phase approximation leads to the Lagrangian equations in which the nonlinearity intrinsic to the magnon system is fully included at the cost of neglecting quantum fluctuations in the coherent state basis. Full nonlinear equations so obtained are of a simple renormalized form in the boson coherent state path integral formulation using the Dyson-Maleev transformation. The formal theory is illustrated to show the existence under certain conditions of self-localized magnons below the linear magnon energy band and of vortex-like modes in two-dimensional cases.

Exact and approximate analytical solutions of stationary vortexlike modes in the d-dimensional anisotropic classical O(2) (XY) spin model

Analytical solutions to nonlinear difference equations describing stationary states of the d -dimensional classical O(2) (XY) spin model are studied. The Hirota bilinear operator formalism is employed to obtain exact and approximate stationary vortexlike-mode solutions having a form somewhat similar to a discrete version of vortex solutions to the two-dimensional sine-Gordon equation. The former exists for a specific case, while the existence condition for the latter is much less restrictive. The obtained analytical result is demonstrated in numerical calculations of vortex profiles for a two-dimensional model O(2) spin system, in which a single vortex and a periodic array of vortex-antivortex pairs are shown.

Statics and dynamics of antiferromagnetic Ising system on a triangular lattice. I

The Ising model on a triangular lattice with antiferromagnetic nearest neighbor and ferromagnetic next nearest neighbor interactions is investigated by treating the master equation by molecular field approximation(MFA) and the cluster decoupling approximation(CDA). For the fully frustrated case, CDA gives no phase transition in contrast with the MFA result which gives the finite Neel temperature. The change of states (i.e., phase transition) is explained by words of bifurcation theory and the relaxation process is investigated by studying the flow in the space of sublattice magnetizations in each approximation.

Dynamical Behaviors of Spin-Peierls Transition: 1-D S=1/2 XY Antiferromagnets

The observation of spin-Peierls transition (at the temperature T SP ≃14 K) in the inorganic compound CuGeO 3 has attracted considerable attention. We investigate characteristic dynamical behaviors of the system accompanied by temperature-dependent lattice displacements from the theoretical point of view. In the special case of 1-D XY antiferromagnets with which the present paper mainly concerned, the fermion representation allows rigorous calculations, so that intrinsic dynamical behaviors of spin-Peierls transition are elucidated without any approximation. The generalized susceptibility is calculated by means of the two-time Green functions. We find interesting dynamical properties that will be a clue to the extensive study on the corresponding Heisenberg antiferromagnet.

Theory of quantum nonlinear localized modes in quantum lattices

Abstract A theory of quantum nonlinear localized modes is developed for three kinds of spatially discrete quantum systems, Bose lattices, Heisenberg spin systems and Frenkel excitons, by employing the coherent state path integral formulation. Applying the stationary-phase approximation leads to nonlinear lattice equations in c-number forms, in which the intrinsic nonlinearity of the quantum systems is incorporated to all orders. The general theory is illustrated by solving the obtained nonlinear equations both analytically and numerically to show the existence of stationary self-localized magnons in one-dimensional Heisenberg antiferromagnets and moving nonlinear localized modes in two-dimensional Heisenberg ferromagnets containing a hole.

Specific Heat of Insulating EupSr1-pS

The specific heat is studied by high-temperature series expansion up to β 6 for an fcc Heisenberg spin system with nearest-neighbor ferromagnetic exchange interaction J 1 and next-nearest-neighbor antiferromagnetic exchange interaction J 2 , between randomly distributed magnetic moments of arbitrary concentration p . The obtained results are compared with recent experimental data on Eu p Sr 1- p S, where due to the frustration effect, various magnetic phases occur. Reasonable agreement is found near the onset of the ferromagnetic or spin-glass phase from the paramagnetic phase.

Effects of dimensionality and randomness on magnetism of Heisenberg paramagnets

Abstract Both the statics and the dynamics of a randomly diluted quasi-low dimensional Heisenberg paramagnet are analyzed in terms of high temperature series expansions. Despite the relative shortness of the series, some crossover effects arising from the interplay between changes in dimensionality and randomness are noticed.