Magnetic Microstructure of a Nanocrystalline Ferromagnet - Micromagnetic Model and Small-Angle Neutron Scattering

Abstract

We report on a combined theoretical and experimental study of the magnetic microstructure of a single component, single phase, Pore-free nanocrystalline ferromagnetic material. From the equations of micro-magnetics we conclude that the magnetic microstructure is the convolution product of an anisotropy field microstructure and of a response function with a correlation length lH that depends on the applied field Ha. We derive equations for small angle neutron scattering by such structures, and present experimental scattering data for electrodeposited nanocrystalline Ni, the first where for a wide range of Ha the dominant scattering contribution is from the purely magnetic microstructure, not from nuclear or magnetic contrast at pores or second phases. The variation of the scattering cross section with Ha is in excellent agreement with the theory, indicating that the underlying changes in the magnetic microstructure with Ha are not displacements of domain walls, but changes in lH and hence in the magnetic response to an entirely stationary anisotropy field microstructure. At 20K the anisotropy fields are dominated by magnetocrystalline anisotropy, but at 300K the perturbation is from a much stronger interaction which maintains some moments aligned antiparallel to the field direction at Ha as high as 1.4MA/m (18kOe).

Small-angle neutron scattering by the magnetic microstructure of nanocrystalline ferromagnets near saturation

“…Studies of dislocation arrays in cold-worked ferromagnetic single crystals have demonstrated that a combination of SANS experiments with micro-magnetics theory can provide information on the nuclear microstructure [ 18 ]. Preliminary results of the present study [ 19 ] indicated that SANS experiments on single-phase bulk nanocrystalline materials with low porosity, hence low nuclear scattering cross-section, are in good agreement with predictions from micromagnetics over a wide range of applied magnetic fields and scattering vectors.…”

Electrodeposited Nanocrystalline Metals, Alloys, and Composites

“…Bakonyi et al [12] observed the same trend for nanocrystalline Ni, also prepared by electrodeposition. Weissmüller, et al [79] confirmed the earlier measurements by Aus et al, [73] reporting only small changes in M s for electrodeposited nanocrystalline Ni with 18 nm grain size. Kisker et al [80] presented further results for gas-condensed Ni which, in contrast to their earlier work, [70] now showed the saturation magnetization to be independent of grain size as long as the gas-condensed material was not exposed to air.…”

Magnetic Microstructure and Properties of the Nanocrystalline Hard Magnet Terbium

“…At present, the only known technique with a potential to resolve the magnetic microstructure in the bulk and on the length scale of nanometers is magnetic smallangle neutron scattering (SANS). SANS data have provided insight into the magnetic microstructure of soft magnetic nanocrystalline samples [2][3][4][5], yielding quantitative results for the magnetic microstructure, the exchange-stiffness constant, and the magnitude and microstructure of the magnetic anisotropy [5,6]. We have investigated nanocrystalline Tb as a model nanocrystalline hard magnet; the present paper complements a first report of the scattering data [7] with results of the magnetic characterization.…”