MAGNETIC PHASE-DIAGRAM, STATIC PROPERTIES, AND RELAXATION OF THE INSULATING SPIN-GLASS COCL2-CENTER-DOT-H2O

The magnetic properties of CoCl2H2O are examined by dc magnetization measurements as a function of temperature and field. This insulating system has both ferromagnetic and antiferromagnetic interactions, the distribution of which may be randomized to some degree by removal of one water from the parent system CoCl22H2O. It behaves as a reentrant spin glass. A field-induced transition is observed in the 4-6 kG range, a much smaller field than the 31.3-kG metamagnetic transition in CoCl22H2O. The nonlinear susceptibility displays an unusual variation with temperature, but does not scale in the conventional way. The thermoremanent magnetization (TRM) is examined as a function of cooling field and temperature. Its time development approximates a stretched exponential decay and the parameters characterizing this are analyzed. It is also found that the TRM scales well according to a T log10(t/τ0) form, for temperatures not much above ∼0.7Tg, with τ0∼10-10-10-9 s. Irreversibility lines are derived from measurements of field-cooled and zero-field-cooled magnetizations. A weak irreversibility sets in at Tg(H=0)=9.0 K and displays no detectable field dependence up to 1 kG. A strong irreversibility sets in at Tg(H=0)=8.4 K and displays a strong field dependence. The results are analyzed using theories which consider anisotropy. The strong irreversibility line conforms much better to a (reduced) form τgh0.53, recently proposed for the 3D Ising short-range spin glass, than to the usual De Almeida-Thouless mean-field form τgh2/3. © 1995 The American Physical Society.