Critical Dynamics of Magnets

We review our current understanding of the critical dynamics of magnets above and below the transition temperature with focus on the effects due to the dipole--dipole interaction present in all real magnets. Significant progress in our understanding of real ferromagnets in the vicinity of the critical point has been made in the last decade through improved experimental techniques and theoretical advances in taking into account realistic spin-spin interactions. We start our review with a discussion of the theoretical results for the critical dynamics based on recent renormalization group, mode coupling and spin wave theories. A detailed comparison is made of the theory with experimental results obtained by different measuring techniques, such as neutron scattering, hyperfine interaction, muon--spin--resonance, electron--spin--resonance, and magnetic relaxation, in various materials. Furthermore we discuss the effects of dipolar interaction on the critical dynamics of three--dimensional isotropic antiferromagnets and uniaxial ferromagnets. Special attention is also paid to a discussion of the consequences of dipolar anisotropies on the existence of magnetic order and the spin--wave spectrum in two--dimensional ferromagnets and antiferromagnets. We close our review with a formulation of critical dynamics in terms of nonlinear Langevin equations.Comment: Review article (154 pages, figures included

EXTRAORDINARILY LOW MÖSSBAUER-FRACTION IN AEROSOLED Fe FINE PARTICLES

Le spectre Mössbauer de particules de fer préparées par une technique aérosol a été étudié, et en particulier la fraction sans recul. Les résultats suggèrent un "adoucissement" des vibrations de réseau.Mössbauer spectra of Fe small particles prepared by an aerosol technique have been studied with special reference to the recoilless fraction. It is suggested that a softening of the lattice vibrations occurs in these mist-like particles

MAGNETIC STRUCTURE OF SOME AMORPHOUS ALLOYS

Analysis of Mössbauer spectra of the amorphous alloys Fe78B12Si10, Fe75.4B14.2Si10.4, and Fe40Ni40P14B6 (METGLAS® 2826) indicates that the magnetic structure is either collinear or almost collinear

NON-COLLINEARITY AS A CRYSTALLITE-SIZE EFFECT OF γ-Fe2O3 SMALL PARTICLES

The surface magnetic structure of γ-Fe2O3 particles, known to be non-collinear, is explored in terms of the size of the crystallites that make up the particle. Mössbauer spectroscopy is employed

Origin of Elevated Ordering Temperature in MnFe2O4 Nanometer Particles

Mössbauer spectra of MnFe2O4 with various particle sizes in a longitudinal 50 kOe magnetic field show that the Fe ion occupancy of the A and B crystallographic sites depends on the particle size. The elevated Néel temperatures with decreasing particle sizes are attributed to an increased iron occupancy on the A sites ; this in turn leads to an increase in the total superexchange interaction