Critical behavior and transport properties of single crystalPr1−xCaxMnO3(x=0.27, and 0.29)

Abstract

A summary of analyses of critical behavior in both the ac susceptibility and dc magnetization in single crystal ${\text{Pr}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$ ($x=0.27$, and 0.29) is presented supplemented by magnetotransport data. The magnetic data indicate that both single crystals exhibit a continuous/second-order paramagnetic to ferromagnetic phase transition but with marginally different critical exponents. Estimates of the latter for the $x=0.27$ sample yield $\ensuremath{\delta}=4.81\ifmmode\pm\else\textpm\fi{}0.02$, $\ensuremath{\gamma}=1.36\ifmmode\pm\else\textpm\fi{}0.02$, and $\ensuremath{\beta}=0.36\ifmmode\pm\else\textpm\fi{}0.02$ (consistent with both the predictions for the nearest-neighbor three-dimensional-Heisenberg model and with those reported for its double exchange-dominated metallic counterparts) with ${T}_{C}=127\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{K}$; whereas at $x=0.29$, $\ensuremath{\delta}=4.62\ifmmode\pm\else\textpm\fi{}0.05$, $\ensuremath{\gamma}=1.38\ifmmode\pm\else\textpm\fi{}0.01$, and $\ensuremath{\beta}=0.37\ifmmode\pm\else\textpm\fi{}0.02$ with ${T}_{C}=114\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{K}$. The absence of metallicity\char22{}and by inference, the suppression of charge fluctuations\char22{}does not, therefore, appear to influence the universality class of the transition. No evidence for a Griffiths-type phase was found in either specimen. These single crystals also display comparable values of the acoustic spin-wave stiffness $D\ensuremath{\sim}70\text{ }\text{meV}\text{ }{\text{\AA{}}}^{2}$ well below that found in the charge-ordered field-induced metallic regime of the same system. These samples remain insulators below ferromagnetic ordering temperature ${T}_{C}$ in fields up to 90 kOe; they display nonabiabatic small polaron mediated transport behavior in the paramagnetic regime in zero field characterized by activation energies comparable to those reported previously for a range of doped perovskites.