  We combine magnetometry and magnetic small-angle neutron scattering to study
the influence of the microstructure on the macroscopic magnetic properties of a
nanocrystalline Ni bulk sample, which was prepared by straining via
high-pressure torsion. As seen by magnetometry, the mechanical deformation
leads to a significant increase of the coercivity compared to nondeformed
polycrystalline Ni. The neutron data reveal a significant spin-misalignment
scattering caused by the high density of crystal defects inside the sample,
which were created by the severe plastic deformation during the sample
preparation. The corresponding magnetic correlation length, which characterizes
the spatial magnetization fluctuations in real space, indicates an average
defect size of 11 nm, which is smaller than the average crystallite size of 60
nm. In the remanent state, the strain fields around the defects cause spin
disorder in the surrounding ferromagnetic bulk, with a penetration depth of
around 22 nm. The range and amplitude of the disorder is systematically
suppressed by an increasing external magnetic field. Our findings are supported
and illustrated by micromagnetic simulations, which, for the particular case of
nonmagnetic defects (holes) embedded in a ferromagnetic Ni phase, further
highlight the role of localized spin perturbations for the magnetic
microstructure of defect-rich magnets such as high-pressure torsion materials.
