  We employ magnetic small-angle neutron scattering to investigate the magnetic
interactions in $(Fe_{0.7}Ni_{0.3})_{86}B_{14}$ alloy, a HiB-NANOPERM-type soft
magnetic nanocrystalline material, which exhibits an ultrafine microstructure
with an average grain size below 10 nm. The neutron data reveal a significant
spin-misalignment scattering, which is mainly related to the jump of the
longitudinal magnetization at internal particle-matrix interfaces. The field
dependence of the neutron data can be well described by the micromagnetic
small-angle neutron scattering theory. In particular, the theory explains the
'clover-leaf-type' angular anisotropy observed in the purely magnetic neutron
scattering cross section. The presented neutron-data analysis also provides
access to the magnetic interaction parameters, such as the exchange-stiffness
constant, which plays a crucial role towards the optimization of the magnetic
softness of Fe-based nanocrystalline materials.
