Magnetic phase diagram, static properties and relaxation of the insulatingspin glass Co1-xMnx(SCN)(2)(CH3OH)(2)

The magnetic behavior of Co1-xMnx(SCN)(2)(CH3OH)(2) has been studied by DC magnetization and susceptibility measurements on mixtures spanning the comp lete composition range. The pure components are a quasi-two-dimensional Hei senberg antiferromagnet (Mn system) and a three-dimensional Ising antiferro magnet (Co system). The crystal structure of the cobalt constituent is dete rmined, and is closely related to that of the manganese constituent. Compet ing orthogonal spin anisotropies should occur in mixtures, and frustration effects arising from competing ferromagnetic and antiferromagnetic interact ions may also arise. The Curie and Weiss constants, in chi(M) = C/(T - thet a), vary regularly with composition. C versus x is essentially linear while theta versus x shows a definite curvature, analysis of which reveals that the unlike-ion exchange interaction is antiferromagnetic and stronger than the like-ion interactions. The magnetic susceptibility is field dependent, more markedly so with increasing x. Plots of M/H versus T exhibit maxima at low temperatures only for mixtures substantially richer in cobalt than man ganese. Magnetic transition temperatures are estimated from these data. Mag netization versus held isotherms evolve with composition and with temperatu re; those for x = 0.24(5) and 0.16(9) exhibit S-shapes for temperatures at or below the identified transitions. The nonlinear susceptibility versus te mperature for x = 0.24(5) displays structure but does not diverse. The temp erature dependence of the thermoremanent magnetization (TRM) for x = 0.24(5 ) shows characteristic features but does not follow any simple form. The ti me-dependence of the TRM is fitted at a series of temperatures employing a stretched exponential decay form. The thermal variation of the fit paramete rs is systematic and suggests that temperatures just below 3 K and slightly above 6 K have special significance. Over a limited temperature range the TRM is found to scale approximately as T log(10)(t/tau(0)), with tau(0) app roximate to 10(-12) s. Strong and weak irreversibility lines are determined for x = 0.24(5); both vary as tau(g) proportional to h(0.56), With zero-fi eld\ temperatures of T-s(0) = 5.5(5) K and T-w(0) = 9.8(5) K, respectively. The exponent is closer to that recently predicted (0.53) for a short-range three-dimensional Ising spin glass than to the value 2/3 of the DeAlmeida- Thouless line in the infinite range mean-field Ising model. The existence o f strong random anisotropy may account for the presence of a weak irreversi bility line with the observed exponent. The T-x magnetic phase diagram exhi bits a crossing of paramagnetic-ordered state phase boundaries and an assoc iated tetracritical point at x approximate to 0.20(5) and T approximate to 2.6(0) K. Spin glass properties are apparent for compositions close to the tetracritical point. (C) 1999 Elsevier Science B.V. All rights reserved.