Spontaneous magnetization of randomly dilute ferromagnets

We consider Ising ferromagnets on random subgraphs of the square lattice. These are obtained by independent random selections either of sites or of bonds. We assume that for each site (or, respectively, bond) the probability of being selected exceeds the critical percolation probability. Then, at sufficiently low temperatures and zero external field, spontaneous magnetization occurs. Some further related results are obtained.     

Low-temperature heat capacity and magnetization of ferromagnets

The change in the magnetization of a ferromagnet due to disordering in the system of atomic magnetic moments with increasing temperature is put in correspondence with the change in the entropy of the magnetic subsystem. A relationship is obtained for calculating the magnetic component of the heat capacity of the ferromagnet, which satisfactorily describes the temperature dependence of the heat capacity for Fe, Co, TbB₂, and DyB₂ at low temperatures.     

Nonconcavity of the magnetization in Ising ferromagnets

Some Ising ferromagnets having nonconcave magnetization are presented as counterexamples to the often assumed case of concave magnetization.     

High-magnetic-field magnetization and magnetoresistance of some amorphous ferromagnets

Measurements of isothermal magnetization and magnetoresistance at T = 4.0-4.2 K and 15 ? H ? 132 kG on the amorphous ferromagnets Fe₈₀B₂₀, Fe₇₈Mo₂B₂₀, Fe₄₀Ni₄₀P₁₄B₆, and Fe₃₂Ni₃₆Cr₁₄P₁₂B₆ indicate a relatively large H-induced increase (3.6%) in the high-magnetic-field magnetization of the latter alloy, and marked differences in the magnitude and sign of the high-field magnetoresistance of the four alloys. The results are qualitatively interpreted in terms of internal effective field distributions which include a small fraction of atomic spins in negative field sites.     

The spatial fluctuations of the magnetization in amorphous ferromagnets

The spatial variation of the magnetization of amorphous ferromagnets containing transition metal and metalloid atoms is shown to depend explicitly on that of the alloy concentration and of the nearest neighbour distance. This variation will thus be in general on an atomic scale for these materials.     

Reorientation of magnetization with temperature in 2D ferromagnets

We investigated 2D Heisenberg ferromagnet (monolayer) with the account of dipolar forces and uniaxial anisotropy and found a reorientation phase transition in temperature from out-of-plane to in-plane phase. This phase transition is of the first order with hysteresis. We estimated the temperatures of switching both analytically and numerically.     

Remanence studies of interparticle interactions in Ba-ferrite

Interparticle interactions in barium ferrite dispersions have been studied as a function of orienting field strenght, milling time and angle with respect to the orientation direction through the use of isothermal and dc remanence measurements. The measurements have been analyzed through the use of Henkel plots, σM plots and σH plots.     

Domain-like magnetization fluctuations in the paramagnetic phase of uniaxial ferromagnets

It is shown that for a model ferromagnet described by the usual Landau-Ginzburg functional with both uniaxial anisotropy and demagnetizing energies included, the magnetization fluctuations in the paramagnetic phase contain all necessary information on the domain structure which develops below some critical temperature. Expressions for the period of the domain structure, critical temperature and correlation functions are obtained.     

Simulation of magnetization reversal processes in finite ferromagnets with defects

The magnetization and magnetization reversal processes that occur through the mechanism of incoherent rotation of magnetic moments in cubic ferromagnets with limited sizes are investigated theoretically. It is established that the appropriate model representation of magnetic inhomogeneities arising in the region of defects is provided by 0° domain walls. The influence of the external magnetic field on the structure and the stability region of the 0° domain walls is determined. This makes it possible to reveal the characteristic features of the magnetization reversal of real crystals as a function of the material and defect parameters, in particular, in the vicinity of the spin-reorientation phase transition.     

Stabilization of the long-range magnetic ordering by dipolar and magnetoelastic interactions in two-dimensional ferromagnets

The possibility of the long-range magnetic order stabilization in two-dimensional ferromagnets with the account of dipolar and magnetoelastic interactions is investigated. The mechanisms of the magnetic order stabilization by both types of interactions are studied. The Curie temperature is estimated. The comparisons with experimental data are made. Received 22 June 2001 and Received in final form 17 January 2002     

The influence of interparticle interaction on magnetization curves of nano-and microcrystal ensembles

Reconstruction of magnetization curves for real close-packed systems of highly anisotropic ferrimagnetic BaFe₁₂O₁₉ single-domain nano-and microcrystals is carried out on the basis of the results of a study of interparticle magnetic interaction. These curves allowed us to distinguish general laws of magnetization of ensembles of randomly oriented single-domain particles predicted by the Stoner-Wohlfarth theory and to discuss special features found for microcrystals.     

Long-time relaxation of the magnetization and tunneling magnetoresistance of granular ferromagnets

Magnetic anisotropy and orientational variance as well as shape diversity of granules largely determine the magnetic properties of granular ferromagnetic metals. The model of magnetically anisotropic ellipsoidal granules explains the glassy nature of the magnetic state of such systems. The relaxation of the magnetization and the magnetoresistance of granular ferromagnetic metals is examined on the basis of this model.     

Spectroscopic diagnostics of plasma with strong interparticle interactions

Taking as examples the emission spectra of low-temperature dense plasma of organic composition obtained in jets of capillary and slot discharges with an evaporating wall, it is shown that some lines of singly ionized carbon that are broadened under the conditions of significant screening of plasma microfields can be used as reliable “standards” for determination of the electron concentration.     

Effect of interparticle magnetostatic interaction on the magnetization reversal in neodymium-iron-boron alloys

A program for modeling the hysteretic behavior of permanent magnets based on Nd₂Fe₁₄B alloys and visualizing their domain structure was developed on the basis of a phenomenological approach to the elementary event of magnetization reversal of highly anisotropic uniaxial ferromagnets. It is shown that the magnetostatic interaction between microvolumes of the material leads to cooperative magnetization reversal of microvolumes with the formation of reverse-magnetization channels and interaction domains.     

On the continuity of the magnetization and energy in Ising ferromagnets

We investigate the phase diagram of ferromagnetic Ising spin systems satisfying the G.H.S. inequality. We show that they cannot have a normal first-order phase transition as the temperatureT is varied, i.e., one where the magnetization is discontinuous and there are three coexisting phases. Furthermore, for n.n. interactions, discontinuity in the magnetization at 0 <T ₀ T _c implies an uncountable infinity of pure phases atT ₀.     

Roles of nonequilibrium conduction electrons on the magnetization dynamics of ferromagnets

The mutual dependence of spin-dependent conduction and magnetization dynamics of ferromagnets provides the key mechanisms in various spin-dependent phenomena. We compute the response of the conduction electron spins in a spatial and time varying magnetization M(r,t) in the time-dependent semiclassical transport theory. We show that the induced nonequilibrium conduction spin density in turn generates four spin torques acting on the magnetization-with each torque playing a different role in magnetization dynamics. By comparing with recent theoretical models, we find that one of these torques which has not been previously identified is crucial to consistently interpreting experimental data on domain wall motion.