Is a Griffiths Phase a Prerequisite for Colossal Magnetoresistance?

Detailed measurements of the magnetic and transport behavior of the two La_1-xCa_xMnO₃ single crystals exhibiting colossal magnetoresistance are summarized. The x=0.21 sample exhibits unusual exponents (δ=20±1, γ=1.71±0.1, β=0.09±0.01, T_C=182±1  K) and, more importantly, a Griffiths phase characterized by an exponent λ=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 La_1-xCa_xMnO₃ near optimal doping, it does not appear to be a prerequisite for colossal magnetoresistance in this system.