The study of magnetic and polaronic microstructure in Pr1-xCaxMnO3 manganite series

Abstract

Manganites have been studied intensively due to their extraordinary properties: colossal magnetoresistance and ferroelectricity. The inhomogeneities present in certain regions of the phase diagram are believed to be responsible for these novel properties. The nature of these inhomogeneities that critically depends on the interaction between charge, atomic and spin degrees of freedom are yet poorly understood. Furthermore, there still remains a significant scope to theoretically investigate the characteristics of the long- living metastable states experimentally found in manganites. We propose a microscopic model of manganites to study the overall electronic, atomic and magnetic microstructure of the three-dimensional Pr1−xCaxMnO3 manganite series. Firstly, we investigate the phase diagram of a hypothetical one-dimensional system to find out the relevant length scales. The study of relaxation dynamics in the one-dimensional system under external light pulse give us the relevant timescales of the response from the charge, lattice and spin subsystem. The length and time scales extracted from our one-dimensional study assist us to investigate the three-dimensional real systems. Secondly, we carried out a comprehensive study of the low-temperature phase diagram of the three-dimensional Pr1−xCaxMnO3 manganite series with the proposed model. The model predicts several distinct phases with characteristic charge and orbital ordering together with an associated magnetic structure. The structures at x=0.125, 0.25, 0.66 and 0.25 are different from the one obtained by earlier theoretical studies. With the large system size, our study reveals the presence of inhomogeneous phases in certain doping regions. The nature of the inhomogeneous states are capable of demonstrating several experimental findings in manganites. The model also allowed us to study the non-collinear spin structures. The charge and orbital order at x=0.125 and x=0.66 are associated with canted and a spiral spin-structure. The defect-induced local disorders in the CE-type phase leads to formation of the metastable structures close to x=0.5. Finally, we studied the dynamics of the three-dimensional manganites under external light pulse to investigate the role of spin and lattice degrees of freedom in their relaxation mechanisms. The system subjected to the high-and low-intensity light pulse undergo different relaxation pathways. In the low-intensity case at x=0.5, a long-lived carrier state characterised by no charge and orbital ordering is observed due to emergence of ferromagnetism in the system. On the other hand, the electronic subsystem relaxes through conical interactions and form single site JT polarons in the high-intensity case. The conical intersection is lattice and spin assisted. The longer timescale relaxation process is dominated by thermally activated polaron hopping.