  Ferromagnetic insulators (FM-Is) are the materials of interest for new
generation quantum electronic applications. Here, we have investigated the
physical observables depicting FM-I ground states in epitaxial Sm$ _{2} $NiMnO$
_{6} $ (SNMO) double perovskite thin films fabricated under different
conditions to realize different level of Ni/Mn anti-site disorders (ASDs). The
presence of ASDs immensely influence the characteristic magnetic and anisotropy
behaviors in SNMO system by introducing short scale antiferromagnetic
interactions in predominant long range FM ordered host matrix. Charge
disproportion between cation sites in form of $ Ni^{2+}+Mn^{4+} \longrightarrow
Ni^{3+}+Mn^{3+} $, causes mixed valency in both Ni and Mn species, which is
found insensitive to ASD concentrations. Temperature dependent photo emission,
photo absorption measurements duly combined with cluster model configuration
interaction simulations, suggest that the eigenstates of Ni and Mn cations can
be satisfactorily described as a linear combination of the unscreened $ d^{n} $
and screened $ d^{n+1} \underline{L} $ ($ \underline{L} $: O 2\textit{p} hole)
states. The electronic structure across the Fermi level (E$ _{F} $) exhibits
closely spaced Ni $ 3d $, Mn $ 3d $ and O $ 2p $ states. From occupied and
unoccupied bands, estimated values of the Coulomb repulsion energy ($ U $) and
ligand to metal charge transfer energy ($ \Delta $), indicate charge transfer
insulating nature, where remarkable modification in Ni/Mn $ 3d $ - O $ 2p $
hybridization takes place across the FM transition temperature. Existence of
ASD broadens the Ni, Mn $ 3d $ spectral features, whereas spectral positions
are found to be unaltered. Hereby, present work demonstrates SNMO thin film as
a FM-I system, where FM state can be tuned by manipulating ASD in the crystal
structure, while I state remains intact.

Materials Science