We report on the discovery of a novel triangular phase regime in the system La1-xSrxMnO3 by means of electron spin resonance and magnetic susceptibility measurements. This phase is characterized by the coexistence of ferromagnetic entities within the globally paramagnetic phase far above the magnetic ordering temperature. The nature of this phase can be understood in terms of Griffiths singularities arising due to the presence of correlated quenched disorder in the orthorhombic phase.

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The critical behavior of two-dimensional (2D) anisotropic systems with weak quenched disorder described by the so-called generalized Ashkin-Teller model (GATM) is studied. In the critical region this model is shown to be described by a multifermion field theory similar to the Gross-Neveu model with a few independent quartic coupling constants. Renormalization group calculations are used to obtain the temperature dependence near the critical point of some thermodynamic quantities and the large distance behavior of the two-spin correlation function. The equation of state at criticality is also obtained in this framework. We find that random models described by the GATM belong to the same universality class as that of the two-dimensional Ising model. The critical exponent ν of the correlation length for the 3-and 4-state randombond Potts models is also calculated in a 3-loop approximation. We show that this exponent is given by an apparently convergent series in ǫ = c − 1 2 (with c the central charge of the Potts model) and that the numerical values of ν are very close to that of the 2D Ising model. This work therefore supports the conjecture (valid only approximately for the 3-and 4-state Potts models) of a superuniversality for the 2D disordered models with discrete symmetries.