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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.

The heat capacity of [Bi.sub.2]Cu[O.sub.4] has been measured from 5 to 350 K in an adiabatic calorimeter. A long range ordering peak was found at 42.7 K due to the establishment of three-dimensional antiferromagnetic order. The magnetic entropy calculated for the anomaly corresponds to the theoretical value for spin 1/2, [Delta] S = R ln 2, discarding the existence of any lower dimensional magnetic contribution at a higher temperature.

Adiabatic heat-capacity measurements of some of the intermetallic compounds [R.sub.2][Fe.sub.17][X.sub.y] (R = Er, Tm, Gd; X = N, D) have been performed from 5 K to 350 K. Magnetic ordering phase transitions have been observed at 291.9 K for [Er.sub.2][Fe.sub.17] and at 260 K for [Tm.sub.2][Fe.sub.17]. The anomalous enthalpy and entropy of these transitions are much smaller than expected in a pure localized model for the magnetic moments. In [Tm.sub.2][Fe.sub.17] a spin reorientation phase transition of second order character has been observed at 94 K. Theoretical calculations of the free energy and thermodynamic functions have been developed. In the interstitial compounds [Er.sub.2][Fe.sub.17][D.sub.3.4], [Tm.sub.2][Fe.sub.17][D.sub.2.13], [Gd.sub.2][Fe.sub.17][N.sub.2.18] and [Er.sub.2][Fe.sub.17][N.sub.2.4] no anomalies have been observed.