  Magnetic double perovskite compounds provide a fertile playground to explore
interesting electronic and magnetic properties. By complementary macroscopic
characterizations, neutron powder diffraction measurements and first-principles
calculations, we have performed comprehensive studies on the magnetic ordering
in the double perovskite compound Sr$_2$NiWO$_6$. It is found by neutron
diffraction to order magnetically in a collinear type-II antiferromagnetic
structure in a tetragonal lattice with $k$ = (0.5, 0, 0.5) below $T\rm_N$ = 56
K. In the ground state, the ordered moment of the spin-1 Ni$^{2+}$ ions is
determined to be 1.9(2) $\mu\rm_{B}$, indicating a significant quenching of the
orbital moment. The Ni$^{2+}$ moments in Sr$_2$NiWO$_6$ are revealed to cant
off the $c$ axis by 29.2$^{\circ}$, which is well supported by the
first-principles magnetic anisotropy energy calculations. Furthermore, the
in-plane and out-of-plane next-nearest-neighbor superexchange couplings
($J\rm_2$ and $J\rm_{2c}$) are found to play a dominant role in the spin
Hamiltonian of Sr$_2$NiWO$_6$, which accounts for the stabilization of the
type-II AFM structure as its magnetic ground state.

Strongly Correlated Electrons