Range of Magnetic Correlations in Nanocrystalline Soft Magnets

Abstract

We have obtained the magnetic field dependence of static ferromagnetic correlations in nanocrystalline electrodeposited Co and Ni by means of the correlation function of the spin misalignment, determined from small-angle neutron scattering data. The approach yields a correlation length l(C), which is a measure for the spatial extent of inhomogeneities in the magnetization distribution. The correlation length depends strongly on the applied magnetic field with values ranging from 94 nm in nanocrystalline Co at low fields to about 15 nm at saturation. The results for l(C) indicate that in Co the main source of nonuniformity in the spin system is the anisotropy field of each individual crystallite, whereas in nanocrystalline Ni the main sources of spin disorder originate from twin faults or from the defect cores of grain boundaries.

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

“…Indeed, the latter equation has been found to describe excellently the strongly field-dependent spin-misalignment correlations in nanocrystalline Co and Ni [25].…”

Depth-resolved investigation of the lateral magnetic correlations in a gradient nanocrystalline multilayer

“…Note that random fluctuations of the spin misalignment were already introduced in order to describe small angle scattering data from soft nanocrystalline bulk ferromagnets. [56][57][58] The average magnetic scattering length density in layer l, leading to non-spin-flip specular reflectivity is given by l M = l Msat cos͑⌽ l ͒ where l Msat is the magnetic scattering length density in layer l at magnetic saturation. Two types of scattering processes are expected.…”

Small angle neutron scattering investigations of spin disorder in nanocomposite soft magnets

“…SANS studies on the nc elemental soft magnets Fe, Co, and Ni as well as on the two-phase Fe-based compound Vitroperm (Fe 73 Si 16 B 7 Nb 3 Cu 1 ) suggest the existence of long-range magnetization fluctuations which are correlated over several neighboring crystallites [3,5]. By contrast, magnetically hard nc Tb reveals a relatively small correlation length of only a few nanometers, smaller than the experimental grain size of 9 nm [6].…”

“…On the microscopic scale, the nc microstructure is reflected in a nonuniformity of the orientation of the magnetic moments, with characteristic length scales which depend on the grain size and on the applied magnetic field [1][2][3]. The random anisotropy model (RAM) [1], where the interplay between the exchange and anisotropy energies over the magnetically correlated local magnetic moments is considered, has successfully related this nanoscaled microstructure to macroscopic magnetic properties.…”

“…Microscopic techniques, such as magnetic-force or Lorentz microscopy are only just approaching the required spatial resolution, and they are unfortunately restricted to imaging the domain structure at the surface or in thin cross sections, where the magnetic nanostructure may be quite different from the bulk. Small angle neutron scattering (SANS) is the only known method which can unambiguously resolve spin disorder at the nanoscale inside the bulk of an nc ferromagnet, since static variations in the magnitude and orientation of the magnetization on the scale of a few nanometers to hundreds of nanometers give rise to intense elastic magnetic scattering [3,4].…”