Magnetic signatures of ferromagnetic polarons in La_0.7Ca_0.3MnO₃: Colossal magnetoresistance is not a Griffiths singularity

Magnetic measurements of La_0.7Ca_0.3MnO₃ reveal a departure from Curie-Weiss behavior below the orthorhombic (O) to rhombohedral (R) structural transition at T_O−R=705 K. Between T_O−R and T^∗=273 K, the magnetic response is clearly paramagnetic but with an enhanced magnetic moment attributed to the existence of small ferromagnetic polarons. These polarons grow in density and begin to interact below T^∗ as evidenced through low-field magnetic-susceptibility and field-dependent magnetization measurements, which show hysteresis and saturation, respectively. At the ferromagnetic transition temperature T_c=247.1 K, the polarons coalesce and the system exhibits long-range ferromagnetism. It is asserted that the transition at T_c should be viewed as a polaronic to ferromagnetic phase transition. The relevance of the Griffiths phase is addressed.