Magnetic phase diagram, static properties, and relaxation of the insulating spin glass Co1-xMnxCl2

Abstract

The magnetic behavior of ${\mathrm{Co}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$${\mathrm{Cl}}_{2}$\ensuremath{\cdot}2${\mathrm{H}}_{2}$O has been studied by dc magnetization and susceptibility measurements on a wide range of compositions. This system is a mixture of two isomorphous three-dimensional antiferromagnets of different magnetic periodicity, in which random competing short-range antiferromagnetic and ferromagnetic exchange interactions occur. Spin-glass behavior is observed and examined in several ways. In general, a particular composition exhibits a higher temperature transition to an essentially antiferromagnetic state and a second lower temperature transition to a spin-glass state, or perhaps a mixed spin-glass--antiferromagnetic state. The upper transition temperature is strongly composition dependent, while the lower, at 2.50\ifmmode\pm\else\textpm\fi{}0.10 K, is virtually independent of composition. An x=0.452 mixture, in which hysteretic and time-dependent effects are especially strong, is studied in detail. Substantial nonlinearity in its magnetization is observed, and the derived nonlinear susceptibility suggests that a phase transition occurs near 2.45 K. The thermoremanent magnetization (TRM) and isothermal remanent magnetization are also studied and exhibit features characteristic of spin glasses. The temperature dependence of the TRM is reminiscent of that in certain other insulating spin glasses, but does not seem to follow any simple functional form. The time dependence of the TRM is also studied in some detail. Of several theoretical decay expressions tested, the most satisfactory appears to be a stretched exponential with a power-law prefactor. The T-vs-x magnetic phase diagram appears to be of a qualitatively new type, though comparison with other systems can also be made.