Range of magnetic correlations in nanocrystalline soft magnets.

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.