HEAT-CAPACITY AND MAGNETIC-PROPERTIES OF COCL2-CENTER-DOT-H2O

Recent magnetic studies of polycrystalline CoCl2H2O showed an antiferr omagnetic ordering transition at T(N) approximately 14 K and an appare nt spin glass transition at approximately 7 K. We now have extended th e magnetic susceptibility measurements up to 300 K. The results above T(N) can be well described if the lowest electronic states of Co2+ in this salt are two Kramers doublets separated by DELTAE/k = 230 K, and if a predominantly ferromagnetic mean field interaction is introduced. The heat capacity of CoCl2.H2O over the range 0.6-80 K has also been measured. Below approximately 10 K, C(p) almost-equal-to aT-2 + bT3. T he first term is the hyperfine contribution of the Co-59(2+) nuclei. T he second is the sum of lattice vibrational and electronic spin parts. C(p) exhibits a lambda-type peak at T(N1) = 15.0 +/- 0.05 K, and a se cond lambda anomaly of comparable size at T(N2) = 13.9 +/- 0.05 K, whi ch appears to be associated with a spin reorientation transition. Corr ecting the observed C(p) data for the nuclear spin contribution, one m ay calculate the sum of lattice vibration and electron spin entropies, and thus obtain at T(N1) an upper limit to the critical magnetic entr opy. This amounts to 0.38 R In 2, suggesting that a substantial part o f the spin ordering in this system is of short-range and lower dimensi onality. C(p) also exhibits an inflection point near 4.5 K, where an u nusual feature in the temperature dependence of the spin glass thermor emanent magnetization was found.