Magnetic phase diagram, static properties, and relaxation of the insulating spin glass CoCl2

Abstract

The magnetic properties of ${\mathrm{CoCl}}_{2}$\ensuremath{\cdot}${\mathrm{H}}_{2}$O 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 ${\mathrm{CoCl}}_{2}$\ensuremath{\cdot}${2\mathrm{H}}_{2}$O. 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 ${\mathrm{CoCl}}_{2}$\ensuremath{\cdot}${2\mathrm{H}}_{2}$O. 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 ${\mathrm{log}}_{10}$(t/${\mathrm{\ensuremath{\tau}}}_{0}$) form, for temperatures not much above \ensuremath{\sim}0.7${\mathit{T}}_{\mathit{g}}$, with ${\mathrm{\ensuremath{\tau}}}_{0}$\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}10--}$${10}^{\mathrm{\ensuremath{-}}9}$ s. Irreversibility lines are derived from measurements of field-cooled and zero-field-cooled magnetizations. A weak irreversibility sets in at ${\mathit{T}}_{\mathit{g}}$(H=0)=9.0 K and displays no detectable field dependence up to 1 kG. A strong irreversibility sets in at ${\mathit{T}}_{\mathit{g}}$(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 ${\mathrm{\ensuremath{\tau}}}_{\mathit{g}}$\ensuremath{\propto}${\mathit{h}}^{0.53}$, recently proposed for the 3D Ising short-range spin glass, than to the usual De Almeida--Thouless mean-field form ${\mathrm{\ensuremath{\tau}}}_{\mathit{g}}$\ensuremath{\propto}${\mathit{h}}^{2/3}$.