Condensed Matter Theory

Abstract:

Neutron scattering experiments with full polarization analysis have been performed with a single crystal of chromium to study the low-energy spin fluctuations in the transverse spin density wave (TSDW) state. A number of remarkable results have been found. Inelastic scattering observed close to the TSDW satellite positions at (1+/-delta,0,0) does not behave as expected for magnon scattering. In particular, the scattering corresponds to almost equally strong magnetization fluctuations both parallel and perpendicular to the ordered moments of the TSDW phase. As the Neel temperature is approached from below, scattering at the commensurate wavevector(1,0,0) increases in intensity as a result of critical scattering at "silent" satellites (1,0,+/-delta) being included within the spectrometer resolution function. This effect, first observed by Sternlieb et al, does not account for all of the inelastic scattering around the (1,0,0) position, however. Rather, there are further collective excitations, apparently emanating from the TSDW satellites, which correspond to magnetic fluctuations parallel to the ordered TSDW moments. These branches have a group velocity that is close to that of (1,0,0) longitudinal acoustic (LA) phonons, but assigning their origin to magneto-elastic scattering raises other unanswered questions.

Abstract:

The aim of this review is to describe some aspects of the dynamic properties of magnetic multiparticle systems in the micrometer range. These particles include monodisperse magnetic microspheres, and magnetic holes, i.e. nonmagnetic particles dispersed in ferrofluids.The complementary use of analogue simulations and computer simulations to explore the dynamic properties is demonstrated.The ability to describe the complex dynamics of magnetic holes in quantitative symbolic dynamic terms using the notion of knot- and braid theory will be discussed. In particular, statistical analysis of braid words, diffusion, memory effects and correlations for the complex dynamics is illustrated.

Abstract:

A nucleation picture of magnetization switching in single-domain ferromagnetic nanoparticles with high local anisotropy is discussed. Relevant aspects of nucleation theory are presented, stressing the effects of the particle size on the switching dynamics. The theory is illustrated by Monte Carlo simulations and compared with experiments on single particles.