EXPERIMENTAL-STUDY OF SIMPLE CUBIC, S=1/2, XY MAGNET .2. SPECIFIC-HEAT OF CO(C5H5NO)6(NO3)2 AND CO(C5H5NO)6I2 BELOW 1 K

Specific-heat data below 1 K are presented on CoL6(NO3)2 and CoL6I2, where L C5H5NO = pyridine-N-oxide. The magnetic ordering temperatures are found as Tc = 0.458(5) K and 0.500(5) K, respectively. The materials are approximations of the simple cubic XY model, the anisotropy in the magnetic exchange being ∝ = J/J⊥≅ 0.31. The data are compared with theoretical predictions for both the pure XY model (∝ = 0) and for intermediate anisotropy (∝ = 0.31). The predictions have been obtained from Padé-approximants to the high-temperature series expansions and from spin-wave theory including higher-order corrections. The variation in the exchange constants obtained through the series CoL6X2, with X = BF4, ClO4, NO3, and I, is discussed in terms of the change in interatomic and in anion size. © 1978.