  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.
