Is a griffiths phase a prerequisite for colossal magnetoresistance?

Detailed measurements of the magnetic and transport behavior of the two La-1 - xCaxMnO3 single crystals exhibiting colossal magnetoresistance are summarized. The x = 0: 21 sample exhibits unusual exponents (delta = 20 +/- 1, gamma = 1.71 +/- 0.1, beta = 0.09 +/- 0.01, T-C = 182 +/- 1 K) and, more importantly, a Griffiths phase characterized by an exponent lambda = 0.70 +/- 0.2. By contrast, the x = 0.20 specimen displays Heisenberg model behavior with no evidence of such a phase. Thus while a Griffiths phase accounts for the behavior of La1-xCaxMnO3 near optimal doping, it does not appear to be a prerequisite for colossal magnetoresistance in this system.