Static magnetic properties and relaxation of the insulating spin glass Co1-xMnxCl2 center dot H2O
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The magnetic properties of (Formula presented) are examined by dc magnetization and susceptibility measurements, for (Formula presented) 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 (Formula presented) show regular composition dependence, with (Formula presented) varying nonlinearly from positive to negative values as x increases. Antiferromagnetic maxima often occur, and transition temperatures are estimated for most mixtures. The (Formula presented) diagram shows two descending boundaries from either composition extreme; any transition temperatures for (Formula presented) are lower than we can measure. Magnetization isotherms evolve with composition, and associated hysteretic effects weaken with increasing x. The nonlinear susceptibility for (Formula presented) shows structure, but does not diverge. The thermoremanent magnetization (TRM) is examined in detail for (Formula presented) 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 (Formula presented) with (Formula presented) For (Formula presented) 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, (Formula presented) than to the DeAlmeida-Thouless mean-field form (Formula presented) best-fit exponents are slightly less than 0.53. For the weak irreversibility lines, the dependence of (Formula presented) on field is much weaker than the Gabay-Toulouse form (Formula presented) The presence of strong random anisotropy is a possible explanation. A monotonic decrease of (Formula presented) with increasing x is found. A reentrant spin glass state still occurs for (Formula presented) for (Formula presented) and 0.50, the spin glass transition is not reentrant. Irreversible effects are maximized in the (Formula presented) region; they are absent for mixtures richer in manganese than cobalt. 1998 The American Physical Society.
