  Magnetic insulators are often antiferromagnetic (AFM) and layered AFM
compounds usually show low ordering temperature. On the other hand, layered
ferromagnetic (FM) insulators with high-TC are very rare although they could be
quite useful for spintronic applications. Here, using crystal field level
analysis in combination with density functional theory calculations as well as
Monte Carlo simulations, we predict that the layered insulator Sr2NiRuO4 would
have a strong FM coupling with TC as high as 240 K. The tetragonal crystal
field in the Ni-O-Ru square plane stabilizes the S=1/2 Ni+ and S=3/2 Ru3+
states. The unique level ordering and occupation optimize the FM Ni-Ru
superexchange interactions in the checkerboard arrangement, thus suggesting
Sr2NiRuO4 as an unusual high-TC layered FM insulator. This work highlights the
potential of charge-spin-orbital degrees of freedom for stabilizing strong FM
coupling in layered oxides.
