Static magnetic properties and relaxation of the insulating spin glassCo1−xMnxCl2⋅H2O

Abstract

The magnetic properties of ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{Cl}}_{2}{\mathrm{\ensuremath{\cdot}}\mathrm{H}}_{2}\mathrm{O}$ are examined by dc magnetization and susceptibility measurements, for $x=0.05,$ 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, and 0.95 between 1.8 and 300 K. The pure components are a quasi-one-dimensional Heisenberg antiferromagnet (Mn) and an antiferromagnetic reentrant spin glass (Co) with some low-dimensional character. The Curie and Weiss constants, in ${\ensuremath{\chi}}_{M}=C/(T\ensuremath{-}\ensuremath{\theta}),$ show regular composition dependence, with $\ensuremath{\theta}(x)$ varying nonlinearly from positive to negative values as x increases. Antiferromagnetic maxima often occur, and transition temperatures are estimated for most mixtures. The $T\ensuremath{-}x$ diagram shows two descending boundaries from either composition extreme; any transition temperatures for $x=0.5--0.8$ are lower than we can measure. Magnetization isotherms evolve with composition, and associated hysteretic effects weaken with increasing x. The nonlinear susceptibility for $x=0.30$ shows structure, but does not diverge. The thermoremanent magnetization (TRM) is examined in detail for $x=0.30,$ 0.40, and 0.50. Its temperature dependence shows characteristic features, but does not follow any simple form. Systematic variation in the TRM with cooling field and composition is apparent. The time dependence of the TRM is fit using a stretched exponential decay form. Systematic variations in the fit parameters with temperature, cooling field, and composition emerge. For low to moderate temperatures, the TRM is found to scale according to $T{\mathrm{log}}_{10}(t/{\ensuremath{\tau}}_{0}),$ with ${\ensuremath{\tau}}_{0}\ensuremath{\approx}{10}^{\ensuremath{-}12}--{10}^{\ensuremath{-}13}\mathrm{s}.$ For $x=0.30$ and 0.50, strong and weak irreversibility lines are determined. The former conform better to a recent prediction for the short-range three-dimensional Ising spin glass, ${\ensuremath{\tau}}_{g}\ensuremath{\propto}{h}^{0.53},$ than to the DeAlmeida-Thouless mean-field form ${\ensuremath{\tau}}_{g}\ensuremath{\propto}{h}^{2/3};$ best-fit exponents are slightly less than 0.53. For the weak irreversibility lines, the dependence of ${\ensuremath{\tau}}_{g}$ on field is much weaker than the Gabay-Toulouse form ${\ensuremath{\tau}}_{g}\ensuremath{\propto}{h}^{2}.$ The presence of strong random anisotropy is a possible explanation. A monotonic decrease of ${T}_{s}$ with increasing x is found. A reentrant spin glass state still occurs for $x=0.30;$ for $x=0.40$ and 0.50, the spin glass transition is not reentrant. Irreversible effects are maximized in the $x=0.3--0.4$ region; they are absent for mixtures richer in manganese than cobalt.