Critical temperature of an anisotropic ferromagnet in the presence of biquadratic exchange

A formulation, using the double time Green's function method, is presented for the anisotropic spin-one ferromagnetic system in the presence of biquadratic exchange and an expression is given for the critical temperature T_c in terms of parameters associated with the uniaxial anisotropy (treated exactly) and the biquadratic interaction (only the longitudinal part, treated in RPA). The prediction that T_c increases with the biquadratic exchange is found to be consistent with the recent calculation.     

On the curie temperature of an anisotropic Heisenberg-Ising ferromagnet with crystal-field anisotropy

The effect of crystal-field anisotropy on the Curie temperature is studied in an S = 1 anisotropic Heisenberg-Ising ferromagnet. The Green function technique with a Callen-type approximation is used. The results show that, when the correlations between spins of nearest neighbouring sites are taken into account, the change in the Curie temperature with crystal-field anisotropy is not so great as that calculated in the random phase approximation.     

On the Curie temperature of a spin one XY ferromagnet with uniaxial anisotropy

A spin one XY ferromagnet with uniaxial anisotropy has been investigated, using Green's function technique in random phase approximation (RPA). The Green functions associated with the anisotropy energy are treated without decoupling. A set of coupled equations have been obtained to find the critical temperature T_c and (S^Z)² at T_c as function of the uniaxial anisotropy parameter D. T_c and (S^Z)² at T_c are found to increase with D. The results are compared with the earlier results obtained in the Narath type of RPA.     

Critical behavior in the amorphous Ising ferromagnet with a random field

Within the differential operator method and the effective-field approximation, the critical behaviour of the amorphous Ising ferromagnet with a random field is studied. Tricritical points and reentrant phenomena are discussed. The influence of the random field and the amorphization on the transition temperature is also investigated.     

Replica-symmetric solutions of a dilute Ising ferromagnet in a random field

We use the replica method in order to obtain an expression for the variational free energy of an Ising ferromagnet on a Viana-Bray lattice in the presence of random external fields. Introducing a global order parameter, in the replica-symmetric context, the problem is reduced to the analysis of the solutions of a nonlinear integral equation. At zero temperature, and under some restrictions on the form of the random fields, we are able to perform a detailed analysis of stability of the replica-symmetric solutions. In contrast to the behaviour of the Sherrington-Kirkpatrick model for a spin glass in a uniform field, the paramagnetic solution is fully stable in a sufficiently large random field.     

The Curie temperature of a spin-one Heisenberg ferromagnet with biquadratic exchange and uniaxial anisotropy

With the pair model approximation, the Curie temperature of a spin-one Heisenberg ferromagnet with biquadratic exchange interaction J' and uniaxial anisotropy term D is calculated for both positive as well as negative values of J' and D.     

Effect of an AC magnetic field on the steady-state magnetization distribution in a weak ferromagnet

Possible steady-state magnetization distributions in a domain wall are found in a weak ferromagnet subjected to an ac magnetic field. The character of the rotation of the magnetization vector in the domain wall is determined. It is predicted that domain structures can be rearranged and reoriented under an ac magnetic field.     

Magnetization of a random ferromagnet

The relative magnetization of a random fcc ferromagnetic alloy is calculated as a function of temperature and concentration. Results are in good agreement with published measurements on Ni-Cu alloys.     

Magnetic field effect on sound velocity below the curie temperature of an itinerant electron ferromagnet

Starting from a general consideration on the exchange interaction effect on the screening of the ion-ion interaction we present a new theory concerning the magnetic field effect on the longitudinal acoustic sound velocity in the ferromagnetic state of a metal.     

Transition temperature of an Ising ferromagnet

By calculating the partition function over all one-break configurations the transition temperature of a one-dimensional Ising ferromagnet, with inverse-square interaction, is determined.     

Effect of electric field and temperature on the binding energy of bound polaron in an anisotropic quantum dot

The ratio between the confinement lengths in the xy-plane and the z direction plays an important role in determining the properties of anisotropic quantum dot. Within a variational approach of Pekar type, we investigated theoretically the effects of electric field and temperature on the ground-state binding energies of hydrogenic impurity polarons in KBr anisotropic quantum dot. The obtained results illustrate that the binding energies increase with the electric field strength and temperature but decrease with the position of the impurity when considering different confinement lengths in the xy-plane and the z direction and present the properties of the anisotropic quantum dot.     

Thermodynamic properties of the site-bond diluted Ising ferromagnet on an anisotropic square lattice

Within the framework of an effective-field theory, we calculate the magnetization, specific heat and susceptibility of a quenched site-bond diluted Ising ferromagnet on a square lattice with both anisotropic coupling constants and anisotropic bond occupancy probabilities. It is shown that these thermodynamic quantities exhibit some interesting behavior as a function of temperature for selected concentrations of magnetic atoms and anisotropic bonds with anisotropic coupling constants.     

Effect of mechanical boundary conditions on the dynamic and static properties of a strongly anisotropic ferromagnet

The spectra of coupled magnetoelastic waves in a semi-infinite strongly anisotropic easy-plane ferromagnet with a rigidly fixed face are analyzed for two variants of fixation (in the basal plane and perpendicularly to it). The phase states of the system are determined. Differences in the phase diagrams and elementary excitation spectra depending on the choice of the sample fixation plane are considered. When rotational invariance is taken into account, the nonreciprocity effect for the velocities of sound in a crystal appears. It is shown that the velocity of sound in the sample considerably depends on the symmetry of the imposed mechanical boundary conditions. The phase diagrams of the system under investigation are presented.     

Effect of an external electric field on liquid crystalline polymers

The effect of an external electric field on the order parameter and on the isotropic-anisotropic phase transition temperature for semi-flexible liquid crystalline polymers is studied by a mean-field approximation. For the polymers whose electric dipole moments are parallel to the chain backbone, the critical transition temperature T_c is extensively changed by gDT ∼ ∥E∥², where E is the external electric field.     

The magnetic properties of the two-dimensional square lattice mixed-spin anisotropic Heisenberg ferromagnet with a transverse magnetic field

The two-dimensional square lattice mixed-spin anisotropic Heisenberg ferromagnet with a transverse magnetic field is studied by means of the double-time Green's function. The analytic expressions of the critical temperature, the high-temperature zero-field susceptibilities, the spin-wave velocity, spin-wave stiffness and spin-wave gap are obtained. The phase diagrams in which the critical temperature, the reorientation temperature and the reorientation magnetic field are shown as a function of single-ion anisotropic parameter are discussed.     

Diffraction of random light emission in a monochromatic sound wave field in an anisotropic disperising medium

Institute of Automated Control Systems and Radioelectronics, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 32, No. 1, pp. 94–105, January, 1989.