  Spin-polarized density functional calculations, magnetization, and neutron
diffraction measurements are carried out to investigate the magnetic exchange
interactions and strong correlation effects in Yb substituted inverse spinel
nickel ferrite. In the pristine form, the compound is found to be a mixed
insulator under the Zaanen-Sawtazsky-Allen classification scheme as it features
both charge transfer and Mott insulator mechanism. Estimation of magnetic
exchange couplings reveals that both octahedral-octahedral and
octahedral-tetrahedral spin-spin interactions are antiferromagnetic which is
typical of a spin-frustrated triangular lattice. However, the latter is
dominant compared to the former leading to a forced parallel alignment of the
spins at the octahedral site which is in agreement with the results of neutron
diffraction measurements. The substituent Yb is found to be settled in +3
charge state, as confirmed from the XPS measurements, to behave like a
spin-half impurity carried by the localized $f_{z(x^2-y^2)}$ orbital. The
impurity $f$ spin significantly weakens the antiferromagnetic coupling with the
spins at the tetrahedral site, which explains the experimental observation of
fall in Curie temperature with Yb substitution.
