Magnetoelectronic phase separation in La1−xSrxCoO3single crystals: Evidence from critical behavior

Abstract

We have investigated the critical behavior of ferromagnetic La${}_{0.75}$Sr${}_{0.25}$CoO${}_{3}$ and La${}_{0.79}$Sr${}_{0.21}$CoO${}_{3}$ single crystals from the bulk magnetization measurements around their Curie temperature (${T}_{C}$). The detailed analysis of the dc magnetization data using different techniques such as the Kouvel-Fisher, the Arrott-Noaks, and critical isotherm plots yield the critical exponents of $\ensuremath{\beta}=0.362\ifmmode\pm\else\textpm\fi{}0.002$, $\ensuremath{\gamma}=1.304\ifmmode\pm\else\textpm\fi{}0.006$, and $\ensuremath{\delta}=4.75\ifmmode\pm\else\textpm\fi{}0.01$ with ${T}_{C}=213.93\ifmmode\pm\else\textpm\fi{}0.02$ K for La${}_{0.75}$Sr${}_{0.25}$CoO${}_{3}$ and $\ensuremath{\beta}=0.491\ifmmode\pm\else\textpm\fi{}0.004$, $\ensuremath{\gamma}=1.217\ifmmode\pm\else\textpm\fi{}0.003$, and $\ensuremath{\delta}=3.51\ifmmode\pm\else\textpm\fi{}0.01$ with ${T}_{C}=187.67\ifmmode\pm\else\textpm\fi{}0.01$ K for La${}_{0.79}$Sr${}_{0.21}$CoO${}_{3}$, characterizing these second-order phase transitions. For both the crystals, the scaling of the magnetization data above and below ${T}_{C}$ obtained using the respective critical exponents and the consistency in the values of the critical exponents determined by different methods confirm that the calculated exponents are unambiguous and intrinsic. The obtained values of exponents suggest that for La${}_{0.75}$Sr${}_{0.25}$CoO${}_{3}$ the transition falls into the three-dimensional Heisenberg universality class of the near-neighbor interaction as proposed for double-exchange systems, whereas in the case of La${}_{0.79}$Sr${}_{0.21}$CoO${}_{3}$ the transition is characterized by mean-field-like values of the critical exponents. We have also estimated the reduced critical amplitudes and observed that for La${}_{0.75}$Sr${}_{0.25}$CoO${}_{3}$ they fall well within the range of the Heisenberg model prediction for spin $Sg1/2$, whereas for La${}_{0.79}$Sr${}_{0.21}$CoO${}_{3}$ they are found to be shifted toward the mean-field values. The deviation of the critical exponents from 3D Heisenberg values toward mean-field ones is attributed to the presence of magnetoelectronic phase inhomogeneity in the $x=0.21$ single crystal. The detailed analysis of the specific-heat data in the vicinity of ${T}_{C}$ for the $x=0.33$, 0.25, and 0.21 samples also supports the phase separation scenario at around $x=0.21$.