Magnetic properties of M/sub 3/(AsO/sub 4/)/sub 2//spl middot/8H/sub 2/O (M=Co, Ni)

Magnetic susceptibility and heat-capacity measurements of Co/sub 3/(AsO/sub 4/)/sub 2//spl middot/8H/sub 2/O and Ni/sub 3/(AsO/sub 4/)/sub 2//spl middot/8H/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. >