[Ni(phen)(fum)]: an S = 1 two-dimensional quantum magnet

[Ni(phen)(fum)] comprises of weakly bound covalent-layers containing Ni(II) ions forming a spatially anisotropic square lattice with pronounced intradimer exchange coupling. Susceptibility studies performed from 1.8 to 300 K revealed significant differences between field and zero-field cooling below 10 K suggesting the presence of easy-axis anisotropy. Specific heat, investigated in zero magnetic field from 0.3 to 20 K, did not reveal any lambda-like anomaly only a tiny hump was observed at about 1.2 K. The amount of magnetic entropy removed down to 1.2 K supports the notion of magnetic low dimensionality. The specific-heat data are dominated by a round anomaly at 4.2 K which was analyzed within limiting models of a paramagnet with E/D = 0.27 and D/k_B = -13.4 K and antiferromagnetic S = 1 dimers with D/J = -0.8 and D/k_B = -5.3 K. The analysis indicates the importance of introducing an interdimer coupling responsible for the formation of magnetic order on the square lattice.