Griffiths singularities and magnetoresistive manganites

The large, so-called colossal magnetoresistivity of doped LaMnO3 has attracted considerable attention, but is only one unusual feature of the ferromagnetic transition in these compounds. We examine in this paper the progression of magnetic and thermodynamic behavior as the transition temperature is made to vary from 360 K to 218 K by changing the divalent dopant. Single crystals of La0.7Sr0.3MnO3, as is well known, show modest magnetoresistivity and conventional critical behavior. La0.66Pb0.34MnO3, and to an even greater extent, La0.7Ca0.3MnO3, have unusual magnetic properties extending more than 100 K above the transition. We treat the properties of the latter samples in the context of a Griffiths phase in which the transition temperature is depressed from its maximum value T-G by random bond-angle bending.