  We have investigated the magnetic microstructure of two-phase Fe-Nb-B~based
Nanoperm alloys using unpolarized small-angle neutron scattering (SANS) and
small-angle X-ray scattering (SAXS). Our SANS analysis reveals a significantly
large magnetic scattering contribution due to spin misalignment, primarily
originating from the substantial jump in the longitudinal magnetization at the
interfaces between the particles and the matrix. The magnetic scattering
exhibits an angular anisotropy that resembles a clover-leaf-type pattern,
consistent with the predictions of micromagnetic SANS theory. Analysis of the
one-dimensional SANS data yields values for the exchange-stiffness constant and
the average anisotropy and magnetostatic fields. The micromagnetic correlation
lengths for all three samples exhibit a similar field variation with sizes
ranging between about 10-35 nm. We also find that the nuclear and magnetic
residual scattering component of the SANS cross section exhibits a similar $q$
dependency as the SAXS data. These findings further validate the applicability
of the micromagnetic SANS theory, and the mesoscopic information obtained is
important for the advancement of the soft magnetic properties of this class of
material.
