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).