Is a Griffiths Phase a Prerequisite for Colossal Magnetoresistance?

Detailed measurements of the magnetic and transport behavior of the two ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ single crystals exhibiting colossal magnetoresistance are summarized. The $x=0.21$ sample exhibits unusual exponents ($\ensuremath{\delta}=20\ifmmode\pm\else\textpm\fi{}1$, $\ensuremath{\gamma}=1.71\ifmmode\pm\else\textpm\fi{}0.1$, $\ensuremath{\beta}=0.09\ifmmode\pm\else\textpm\fi{}0.01$, ${T}_{C}=182\ifmmode\pm\else\textpm\fi{}1\text{ }\text{ }\mathrm{K}$) and, more importantly, a Griffiths phase characterized by an exponent $\ensuremath{\lambda}=0.70\ifmmode\pm\else\textpm\fi{}0.2$. 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 ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ near optimal doping, it does not appear to be a prerequisite for colossal magnetoresistance in this system.