What is the Mechanism Underlying 3-D Heisenberg-like Ferromagnetism Across the Compositional Metal-Insulator Transition in La1-xCaxMnO3 (0.18 < xc < 0.22)?

Detailed measurements of the magnetic and transport properties of the two La1-xCaxMnO3 (x = 0.18, x = 0.20) single crystals straddling the compositional metal-insulator transition boundary (0.18 < xc < 0.22) are summarized. The analysis of magnetization/susceptibility data reveals the occurrence of a second order/continuous ferromagnetic-paramagnetic phase transition described not only by nearest neighbour, 3-D Heisenberg model exponents (gamma = 1.387, beta = 0.365, delta = 4.783), but also with comparable values of the critical amplitudes in both the insulating and the metallic samples. These results support the assertion that double exchange cannot be the underlying mechanism supporting ferromagnetism in this composition range, and arguments are presented that the relevant interaction is ferromagnetic super exchange modulated by proximity to the orbitally ordered to disordered transition.