Theory of magnetic small-angle neutron scattering of two-phase ferromagnets

Abstract

Based on micromagnetic theory we have derived analytical expressions for the magnetic smallangle neutron scattering (SANS) cross section of a two-phase particle-matrix-type ferromagnet. The approach-valid close to magnetic saturation-provides access to several features of the spin structure such as perturbing magnetic anisotropy and magnetostatic fields. Depending on the applied magnetic field and on the magnitude Hp of the magnetic anisotropy field relative to the magnitude ∆M of the jump in the longitudinal magnetization at the particle-matrix interface, we observe a variety of angular anisotropies in the magnetic SANS cross section. In particular, the model explains the "clover-leaf"-shaped angular anisotropy which was previously observed for several nanostructured magnetic materials, and it provides access to the magnetic interaction parameters such as the average exchange-stiffness constant. It is also shown that the ratio Hp/∆M decisively determines the asymptotic power-law exponent and the range of spin-misalignment correlations.

Magnetic SANS study of a sintered Nd–Fe–B magnet: Estimation of defect size

“…The "correlation length" L of the magnetic anisotropy field appears to be the average size over which the direction and/or magnitude of H K changes. For instance, for a statistically isotropic polycrystalline material, where each crystallite is a single crystal with magnetocrystalline anisotropy only, the parameter L is sensibly related to the average crystallite size [26]. For the present sample, which consists of micron-sized Nd 2 Fe 14 B crystallites 1 that are separated by a Nd-rich interface with a thickness ("size") of the order of a few tens of nanometers (compare Fig.…”

Analysis of magnetic neutron-scattering data of two-phase ferromagnets

“…2 and 3 in Ref. 14), and by the results of full-scale three-dimensional micromagnetic simulations for d M /d (lower row). For further details on the micromagnetic simulation methodology, we refer the reader to Refs.…”

Third-order effect in magnetic small-angle neutron scattering by a spatially inhomogeneous medium

“…The motivation for the present study is to probe the limits of the second-order magnetic SANS theory [4] by looking for prominent third-order effects, specifically, those which are not masked by the second-order ones. One such effect-a central result of this work-is pinpointed at the end of the paper (Sec.…”