  The functional characteristics of double perovskites with unique
ferromagnetic-insulator ground state have been controversial due to the
unavoidable presence of anti-site disorders (ASDs). Here, we aim to investigate
the origin of magnetic ordering on local and global scales in
Sm$_{2}$NiMnO$_{6}$ (SNMO) double perovskite system. Different calcination
routes are exploited to generate different cation arrangements in SNMO and the
corresponding magnetic configurations are examined using the high energy (E $
\sim $0.3 eV) `hot neutrons', which has helped to overcome Sm absorption as
well as to record total (Bragg's+diffuse) scattering profiles with high
momentum transfer (Q$ _{max} \sim $24 angstrom$ ^{-1} $). We have observed that
the Ni-Mn sublattice adopts long range collinear ferromagnetic $ F_{x}F_{z} $
structure with commensurate $k$=(0, 0, 0) propagation vector, below ordering
temperature T $ \lesssim $ 160 K, irrespective of variable ASD concentrations.
In addition, the signatures indicating the antiparallel polarization of Sm
paramagnetic moments with respect to Ni-Mn network, are noticed in the vicinity
of anomalous magnetic transitions at T $ \lesssim $ 35 K. The real space pair
distribution function calculations have provided a direct visualization of ASDs
by means of broadening in Ni/Mn-Mn/Ni linkage. Employing the Reverse Monte
Carlo approach on diffuse magnetic scattering profiles, we have observed the
negative spin-spin correlation function which suggests the Ni-Ni
antiferromagnetic exchange interactions ranging up to first nearest neighbor
distance. These results confirm that the existence of ASDs in cation ordered
host matrix leads to competing ferromagnetic-antiferromagnetic phases in a
broad temperature range, which quantitatively governs the temperature dependent
bulk magnetic observables of SNMO system.
