Thermodynamic properties of ferromagnets with uniaxial and biaxial anisotropy

The thermodynamic properties of spin-one Heisenberg ferromagnets with uniaxial and biaxial anisotropy are investigated in the molecular field approximation (MFA) and random phase approximation (RPA). In MFA a full phase diagram comprising three lines of bicritical points, is obtained.Using the standard-basis operator method, the collective excitation spectrum is studied in detail; moreover the softening of the excitations at the phase boundaries is discussed. A self-consistent version of RPA with emphasis on the role of kinematic restrictions with regard to the standard-basis operators is analyzed.Moreover, the phase transitions are considered in the Ising model, where a line of tricritical points occurs, and the planar model, where a line of bicritical points, two lines of tricritical points and a line of triple points, occur.     

Crossover behaviour in random uniaxial ferromagnets with dipolar interactions

The crossover behaviour of random uniaxial ferromagnets with dipolar interactions is studied by a numerical solution of the renormalization group equations. It is found that as a function of temperature the crossover from pure behaviour to asymptotic random behaviour stretches over many decades on the temperature scale and is practically unobservable. However the effective critical exponent becomes nonuniversal for observable temperature distances from the critical point and depends on the impurity concentration. We also predict a similar nonuniversal behaviour for the measurable four spin coupling in these systems.     

The anisotropy of microeddy losses connected with rotation processes in uniaxial ferromagnets

A computation is produced of the anisotropy of microeddy losses, the defect of velocity of elastic waves and Young's modulus in uniaxial ferromagnets, based on the solution of a system of equations of rotational moments simultaneously with the wave equation. In the linear response approximation of the system, there are found the variance of the absorption coefficient, the velocity of the wave, Young's modulus, and the amplitude-independent components of the microeddy losses.Kursk Polytechnic Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 75–78, October, 1992.     

Macroscopic behaviour of ferromagnets with random and statistically vanishing anisotropy

A model is defined in which the anisotropy tensor is a random function of space point characterised by its mean square value σ and a correlation lenght a_c, and the exchange density A is uniform. If the magnetic moment density is M, it is shown that two dimensionless numbers can be defined a_c/a_p, where a_p is a typical Bloch wall width$\text{(}\text{A}\text{δ}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, and 4πM²/σ they define four types of macroscopic behaviour. For 4πM²/σ<1, the pole fields are a perturbation; if a_c<a_p, as in rare earth amorphous alloys, there is some frustration in the ground state and if a_p <a_c, as in inhomogeneous weak ferromagnets the magnetization direction is mainly fixed by the local anisotropy. For 4πM²/σ#62; 1, as in iron alloys, if a_p#62;a_c one has the case of soft alloys, the shape anisotropy is the dominant effect and if a_p<a_c the usual domain theory applies.     

Critical behaviour of the magnetic susceptibility of two nearly bidimensional Heisenberg ferromagnets

The parallel magnetic susceptibilities of ferromagnetic K₂ Cu F₄ and (CH₃ NH₃)₂ Cu Cl₄ have been measured near the critical temperature. In both ferromagnets, the thermal dependence cannot be described by the simple power law ?^-γ with $\text{? =}\text{T}\text{T}{}_{\mathrm{<mtext>C</mtext>}}\text{? 1}$ over a large range of temperatures. Due to crossover with dimensionality, the γ value increases from 1 for K₂ Cu F₄ and 1.23 for (CH₃ NH₃)₂ Cu Cl₄ in the immediate vicinity of T_c, to a maximum value of about 2.     

The magnetic susceptibility of spin-one Ising and Heisenberg ferromagnets with biquadratic exchange and uniaxial anisotropy

For spin-one Ising and Heisenberg ferromagnets with biquadratic exchange interaction J' and uniaxial anisotropy D, the magnetic susceptibility has been calculated by the use of the pair model approximation and its dependences on J' and D are discussed in comparative way.     

Critical behaviour of the compressible magnet with exchange anisotropy

Then-component magnet with exchange anisotropy on a compressible lattice, with isotropic elastic properties, is studied. The renormalization group method is applied ind =4 — dimensions. The fixed points and the stability regions are explored to the order ², and the analysis is concentrated upon the casen<4—2 +O( ²). Investigation of the fixed points reveals various crossover phenomena which are not present in the corresponding rigid model. Renormalization of the anisotropy crossover exponent is demonstrated. It is shown that macroscopic instabilities, leading to the first order phase transition, may appear.     

Magnetoelastic excitations in ferromagnets with cubic single-ion anisotropy

Magnetic and elastic properties of three-axial (S=2) ferromagnets with single-ion anisotropy have been discussed. The phase diagram in the plane (single-ion anisotropy, temperature) has been constructed and two tricritical points driven by the magnetoelastic interactions have been found. The dispersion of the magnetoelastic excitations in the low temperature has been considered. The special attention has been given to the systems with single-ion anisotropy constantK–8 where the softening of a magnetoelastic mode can be observed.     

Surface anisotropy tensors for cubic and hcp ferromagnets

Point groups of the plane surfaces of cubic and h.c.p. ferromagnets are indicated together with the corresponding forms of the first and second rank surface anisotropy tensors.     

Temperature phase transitions in ferromagnets with oblique anisotropy in the presence of induced anisotropy

The formation and rearrangement of the stripe domain structure with a change in temperature and the effect of induced anisotropy on the temperature phase transitions have been considered. The obtained theoretical results are compared with the experimental data on the basis of numerical methods.     

Logarithmic corrections for dilute uniaxial ferromagnets at the critical dimension

The leading logarithmic corrections to the critical behavior of a dilute uniaxial (Ising) ferromagnet in the disordered phase are derived using renormalization group methods. The values of the exponents in the logarithmic terms differ from those given by previous authors.     

Critical behavior of Griffiths ferromagnets

From a heuristic calculation of the leading order essential singularity in the distribution of Yang-Lee zeroes, we obtain new scaling relations near the ferromagnetic-Griffiths transition, including the prediction of a discontinuity on the analogue of the critical isotherm. We show that experimental data for the magnetization and heat capacity of La(0.7)Ca(0.3)MnO(3) are consistent with these predictions, thus supporting its identification as a Griffiths ferromagnet.     

Critical dynamics of dipolar ferromagnets

The dynamical scaling functions for ferromagnets with dipolar interactions are computed by mode coupling theory above the critical temperature T_c. On the basis of this theory we explain apparently conflicting features of neutron scattering experiments on EuO, EuS and Fe. The position of the crossover from isotropic to dipolar critical dynamics is determined and further experiments are proposed.     

Critical temperature for two-dimensional ising ferromagnets with quenched bond disorder

The configuration-averaged free energy of a quenched, random bond Ising model on a square lattice which contains an equal mixture of two types of ferromagnetic bonds J₁ and J₂ is shown to obey the same duality relation as the ordered rectangular model with the same two bond strengths. If the random.system has a single, sharp critical point, the critical temperature T_c must be identical to that of the ordered system, i.e., sinh(2J ₁/kT _c) sinh(2J ₂/kT _c) = 1. Since _c ^(B) = 1/2, we can takeJ ₂ 0 and use Bergstresser-type inequalities to obtain(/dp) exp(–2J ₁/kT_c¦_p=p_c + = 1, in agreement with Bergstresser's rigorous result for the diluted ferromagnet near the percolation threshold.Work supported in part by National Science Foundation Grant No. DMR 76-21703, Office of Naval Research Grant No. N00014-76-C-0106, and National Science Foundation MRL program Grant No. DMR 76-00678.Paper presented at the 37th Yeshiva University Statistical Mechanics Meeting, May 10, 1977.     

Leaky modes of channel waveguides with uniaxial anisotropy

An exact numerical method for analyzing the propagation properties of leaky modes of inhomogeneous channel optical waveguides with a complex uniaxial diagonal permittivity tensor is developed. The method is based on solving the system of integro-differential equations formulated with respect to transversal components of the magnetic field and the longitudinal component of the electric field. Some results of investigation of leaky modes of diffused channel waveguides in LiNbO₃ and LiTaO₃ crystals are given.     

Features of magnetic reversal of highly anisotropic uniaxial ferromagnets

Specificity of magnetization and magnetization reversal processes of highly anisotropic uniaxial ferromagnets, caused by the possibility of microvolumes of a material to exist in multidomain and single-domain states, is discussed. The influence of the demagnetization method on the behavior of the magnetization curves, thermal magnetization, and ambiguity of the shape of Henkel plots is shown by the example of the SmCo₅ and Nd₂Fe₁₄B compounds.