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The description of the collective magnetorheological effect induced by magnetic field in magnetoactive elastomers is proposed. The condition of consistency is used between magnetic and mechanic momenta of forces exerted on magnetically uniaxial ferromagnetic particles in elastomer at their magnetization. The study shows that even in the case of small concentration of particles, the value of magnetically-induced shear can be anomalously large, reaching up to tens of percent. The deformation of magnetoactive elastomer can evolve critically, as a second-order phase transition, if magnetic field is aligned along the easy axis of particles. [ABSTRACT FROM AUTHOR]

Quantum phase transitions induced by an external magnetic field in a Van Vleck paramagnet with ion spin S = 1, competition of an easy-plane single-particle anisotropy and Ising spin-spin interactions have been investigated. Phase transformations have been described using the Lagrange function, minimization of which is performed by the coefficients of linear combination of the spin wave functions that determine the magnet ground state. It was shown that such an approach is consistent with the Landau theory of phase transitions of the 1st kind. The result obtained showed that the transition from the Van Vleck paramagnetic (singlet) phase to the ferromagnetic phase occurs by forming an intermediate state with only one magnetized spin sublattice. [ABSTRACT FROM AUTHOR]

A preface to the October 2008 issue of "Low Temperature Physics" is presented.

Comments on the use of phenomenological model in quantum subsystem analysis of phase transitions in antiferromagnetic cobalt fluoride in longitudinal magnetic field. Criterion in determining the order of collinear-noncollinear phase transition; Multiparameter phenomenological theory.

Shape variation under the action of small external fields is a peculiar feature of soft matter. In the present paper we demonstrate a possibility of the analogous shape variation in the solids that combine the properties of antiferro- and ferromagnetic materials and show strong magnetoelastic coupling. The antiferromagnetic subsystem provides a macroscopic deformation of a sample in the external magnetic field while the ferromagnetic component ensures high susceptibility of the domain structure. [ABSTRACT FROM AUTHOR]