Mean-field transfer-matrix study of the magnetic phase diagram of CsNiF3_{\bf 3}

A method for treating ferromagnetic chains coupled with antiferromagnetic interactions on an hexagonal lattice is presented in this paper. The solution of the $1D$ part of the problem is obtained by classical transfer-matrix while the coupling between the chains is processed by mean-field theory. This method is applied with success to the phase diagram and angular dependence of the critical field of CsNiF$_3$. Results concerning the general influence of single-ion anisotropy on the magnetic ordering of such systems are also presented.Comment: J1K 2R1 14 pages (RevTex 3.0 preprint), 7 Postscript figures uuencoded at the end, Report# CRPS-94-2

Thermal phase diagram of a model Hamiltonian for columnar phases of liquid crystals

We present the phase diagram and critical properties of a coupled $XY$-Ising model on a triangular lattice using the mean-field approximation, the Migdal-Kadanoff scheme of renormalization group and Monte-Carlo simulations. The topology of the phase diagram is similar for the three techniques, with the appearance of a phase with $XY$ order and Ising disorder. The results suggest a line of transitions belonging to the 2D-Ising universality class in contrast with previous data indicating a new universality class. This model is relevant to the columnar phases of discotic liquid crystals [such as hexa(hexylthio)triphenylene (HHTT)] in the limit of weak intercolumn coupling.Comment: 11 figures available upon reques

Atomic level micromagnetic model of recording media switching at elevated temperatures

An atomic level micromagnetic model of granular recording media is developed and applied to examine external field-induced grain switching at elevated temperatures which captures non-uniform reversal modes. The results are compared with traditional methods which employ the Landau-Lifshitz-Gilbert equations based on uniformly magnetized grains with assigned intrinsic temperature profiles for $M(T)$ and $K(T)$. Using nominal parameters corresponding to high-anisotropy FePt-type media envisioned for Energy Assisted Magnetic Recording, our results demonstrate that atomic-level reversal slightly reduces the field required to switch grains at elevated temperatures, but results in larger fluctuations, when compared to a uniformly magnetized grain model.Comment: 4 pages, 5 figure

A study of aircraft fire hazards related to natural electrical phenomena Final report, Jun. 1966 - Jul. 1967

Natural electrical phenomena as aircraft fire hazards, with study of flame propagation in fuel vent system

Magnetic Phase Diagram of the Ferromagnetically Stacked Triangular XY Antiferromagnet: A Finite-Size Scaling Study

Histogram Monte-Carlo simulation results are presented for the magnetic-field -- temperature phase diagram of the XY model on a stacked triangular lattice with antiferromagnetic intraplane and ferromagnetic interplane interactions. Finite-size scaling results at the various transition boundaries are consistent with expectations based on symmetry arguments. Although a molecular-field treatment of the Hamiltonian fails to reproduce the correct structure for the phase diagram, it is demonstrated that a phenomenological Landau-type free-energy model contains all the esstential features. These results serve to complement and extend our earlier work [Phys. Rev. B {\bf 48}, 3840 (1993)].Comment: 5 pages (RevTex 3.0), 6 figures available upon request, CRPS 93-

A model of magnetic order in hexagonal HoMnO3

Symmetry arguments are used to develop a spin Hamiltonian for the description of the complex magnetic ordering in HoMnO$_3$. Using a novel application of the Landau Lifshitz Gilbert dynamic torque equations to this model of the frustrated Mn ions on an $AB$ stacked triangular antiferromagnetic, it is shown that the four principal spin configurations observed in this compound are stabilized. Ho-Mn coupling is found to be a consequence of an unusual trigonal anisotropy term which is responsible for simultaneous Mn spin reorientation and onset of Ho magnetic order. Based on these microscopic considerations, a mean-field Landau-type free energy is derived which reproduces the succession of observed temperature driven magnetic phase transitions at zero field, including re-entrant behavior. In addition, our analysis suggests that the basal-plane magnetic order should be slightly incommensurate with the lattice.Comment: 9 pages, 3 figure

Biquadratic antisymmetric exchange and the magnetic phase diagram of magnetoelectric CuFeO2_2

Biquadratic {\it antisymmetric} exchange terms of the form $- [C_{ij} e^{\alpha}_{ij}({\bf s}_i\times{\bf s}_j)_z]^2$, where ${\bf e}_{ij}$ is the unit vector connecting sites $i$ and $j$ and $\alpha = x,y$, due partially to magnetoelectric coupling effects, are shown to be responsible for the spin-flop helical phase in CuFeO$_2$ at low magnetic field and temperature. Usual biquadratic {\it symmetric} exchange, likely due to magnetoelastic coupling, is found to support the stability of axial magnetic states at higher fields in this nearly-Heisenberg like stacked triangular antiferromagnet. A model Hamiltonian which also includes substantial interplane and higher-neighbor intraplane exchange interactions, reproduces the unique series of observed commensurate and incommensurate periodicity phases with increasing applied magnetic field in this highly frustrated system. The magnetic field-temperature phase diagram is discussed in terms of a Landau-type free energy.Comment: 7 pages, 9 figure