Magnetic Ordering in La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ Films Driven by Nanostructural Disorder

Completely amorphous, partly disordered (a perfect crystalline matrix with randomly-oriented nanocrystalline inclusions), and lattice-strained La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ films were prepared by rf magnetron sputtering. The amorphous film demonstrates a temperature-dependent magnetization, typical for the paramagnet with freely-moving individual Mn spins. The partly disordered film represents a superposition of the ferromagnetic (FM) phase, which belongs to the crystalline matrix, and the superparamagnetic (SPM) one, corresponding to the nanocrystalline inclusions. Analysis of the magnetic hysteresis loops reveals that about 70\% of the film volume belongs to the FM phase. It was shown that the average magnetic moment of SPM particles is controlled by an applied magnetic field, which is explained by a partial SPM to FM transition due to an enhancement of the ferromagnetic coupling between adjacent nanocrystalline clusters. The lattice-strained film turns out to be in an inhomogeneous FM state.