O. F. de Alcantara Bonfim

Renormalization group treatment of a spin-one ising model with biquadratic exchange interaction

Abstract The critical behavior of a spin-one Ising model with biquadratic exchange interactions is studied using the mean field renormalization group method. The phase diagram for various lattices is obtained. The nature of the phase transition with biquadratic interaction is analysed and comparison with other methods is made.

Mean field renormalization group analysis of the Blume-Capel model

The critical properties of the d-dimensional Blume-Capel model is studied by using the mean field renormalization group method. The phase diagram and tricritical behaviour of the square lattice and various three-dimensional lattices have been analysed. Results are compared with those of high-temperature series expansion and variational methods.

Order and chaos in ferromagnetic spin wave instabilities

Abstract Spin wave instabilities driven by microwave fields display chaotic phenomena similar to other nonlinear dynamical systems. We present observations of period-doubling and odd-period bifurcation routes to chaos in microwave parallel pumping experiments with Yttrium Iron Garnet (YIG). The results are interpreted on the basis of numerical solutions of a symmetric nonlinear two-mode model. An asymmetric two-mode model is also proposed to explain recent similar experiments with the perpendicular second-order Suhl instability in YIG.

Bond and site diluted Ising-model systems: a unified treatment

Abstract A unified effective-field approximation scheme for both quenched bond- and site-diluted Ising magnets is presented. The method, which can systematically include correlation effects, is illustrated by applying to a honeycomb lattice. We also comment on the relation between the present method and that recently proposed by Boccara.

A new pulse-like soliton solution for the uniaxial 1-D Heisenberg ferromagnet

Abstract Nonlinear excitations in a Heisenberg chain with uniaxial anisotropy in the presence of an external field are studied within a semi-classical approach. In addition to the solitary-wave solution obtained by Long and Bishop, a new soliton solution, energetically more favorable than magnons, is also obtained.

Phase transitions in the two-dimensional superantiferromagnetic Ising model with next-nearest-neighbor interactions

We use Monte Carlo and transfer matrix methods in combination with extrapolation schemes to determine the phase diagram of the two-dimensional superantiferromagnetic (SAF) Ising model with next-nearest-neighbor (NNN) interactions in a magnetic field. The interactions between nearest-neighbor (NN) spins are ferromagnetic along x, and antiferromagnetic along Y. We find that for sufficiently low temperatures and fields, there exists a region limited by a critical line of second-order transitions separating a SAF phase from a magnetically induced paramagnetic phase. We did not find any region with either first-order transition or with reentrant behavior. The NNN couplings produce either an expansion or a contraction of the SAF phase. Expansion occurs when the interactions are antiferromagnetic, and contraction when they are ferromagnetic. There is a critical ratio R(c)=1/2 between NNN and NN couplings, beyond which the SAF phase no longer exists.

Quantum fidelity approach to the ground-state properties of the one-dimensional axial next-nearest-neighbor Ising model in a transverse field

In this work we analyze the ground-state properties of the s=1/2 one-dimensional axial next-nearest-neighbor Ising model in a transverse field using the quantum fidelity approach. We numerically determined the fidelity susceptibility as a function of the transverse field B_{x} and the strength of the next-nearest-neighbor interaction J_{2}, for systems of up to 24 spins. We also examine the ground-state vector with respect to the spatial ordering of the spins. The ground-state phase diagram shows ferromagnetic, floating, and 〈2,2〉 phases, and we predict an infinite number of modulated phases in the thermodynamic limit (L→∞). Paramagnetism only occurs for larger magnetic fields. The transition lines separating the modulated phases seem to be of second order, whereas the line between the floating and the 〈2,2〉 phases is possibly of first order.

Two-step renormalisation group for Ising systems

A decimation transformation preceded by a mean-field renormalisation group has been used to obtain the critical properties of various d-dimensional lattices. The method improves substantially both exponents and critical couplings even for small size clusters when compared with decimation and mean-field renormalisation group methods.

Temperature dependence of magnon pair-mode energies in MnF2:Fe++

Abstract The temperature dependence of the energies of the A lg (D) + E g pair-mode and of the two-magnon peak of the pure host were measured in MNF 2 :Fe ++ using Raman scattering. The data is explained within the framework of molecular field theory using NMR and Mossbauer measurements of the magnetization around the impurity.