Unravelling the nature of ferromagnetic-paramagnetic phase transition and its bearing on colossal magnetoresistance in nanocrystalline La

The nature of the ferromagnetic (FM)-paramagnetic (PM) phase transition at $T = T_{C}$ changes from first order to second order when the average crystallite size, d, falls below 100 nm in optimally hole-doped nanocrystalline La1−xCaxMnO3. In zero field, the systems with d < 100 behave as a three-dimensional (3D) uniaxial dipolar ferromagnet in the asymptotic critical region but in finite fields, the asymptotic critical behavior changes over to that of a 3D Ising ferromagnet. How effectively polaron correlations couple magnetic degrees of freedom to the lattice decides the order of the FM-PM transition and the magnitude of colossal magnetoresistance (CMR). d alters the electron-lattice coupling and thereby enables the tuning of CMR over an unusually wide temperature range