Magnetic properties of M3(AsO4)2 center dot 8H2O (M=Co, Ni)

Magnetic susceptibility and heat-capacity measurements of [Co.sub.3][(As[O.sub.4]).sub.2] [center dot] 8[H.sub.2]O and [Ni.sub.3][(As[O.sub.4]).sub.2] [center dot] 8[H.sub.2]O have shown the existence of long range antiferromagnetic ordering at 7.1 and 8.3 K, respectively. The analysis of the measurements together with the structural features have shown a stronger ferromagnetic dimers interaction. The ordering of the Co compound has been characterized as a planar one with S=1/2 and having a three-dimensional crossover below 2 K. For the Ni compound the magnetic ordering at [T.sub.N] is three-dimensional Heisenberg with S=1.