Abstract
The colossal magnetoresistance effect (CMR), the drop of the electric resistance by orders of magnitude in a strong magnetic field, is a fascinating property of strongly correlated electrons in doped manganites. Here, we present a detailed analysis of the magnetotransport properties of small polarons in thin films of the low bandwidth manganite Pr0.68Ca0.32MnO3 with different degrees of preparation-induced octahedral disorder. The crystal and defect structure is investigated by means of high-resolution transmission electron microscopy. We apply the small polaron theory developed by Firsov and Lang in order to study the hopping mobility in the paramagnetic phase and its changes due to the formation of the antiferromagnetic charge ordered (CO) and the ferromagnetic metallic phases. Although it represents a single particle theory, reasonable estimates of small polaron properties such as formation energy, activation energy and transfer integral are possible, if the effects of interactions and disorder are taken into account. Beyond the well-known effect of the magnetic double exchange on the transfer integral, we show that the emergence of band transport of small polarons in the CMR transition sensibly depends on the degree of octahedral disorder, the polaron–polaron interactions and the resulting long range order leading to a structural phase transition in the CO phase.
Highlights
Almost 20 years ago, a seminal publication of Millis et al [1] stated that magnetic double exchange alone cannot explain the magnetoelectric transport behavior of manganites and the interplay of double exchange and polaronic effects had to be addressed
Beyond the well-known effect of the magnetic double exchange on the transfer integral, we show that the emergence of band transport of small polarons in the colossal magnetoresistance effect (CMR) transition sensibly depends on the degree of octahedral disorder, the polaron–polaron interactions and the resulting long range order leading to a structural phase transition in the charge ordered (CO) phase
D is controlled by the octahedral tilt, which determines whether a ferromagnetic metallic (FMM) or an antiferromagnetic charge ordered (CO) ground state is formed at low temperature and intermediate doping [12]
Summary
Almost 20 years ago, a seminal publication of Millis et al [1] stated that magnetic double exchange alone cannot explain the magnetoelectric transport behavior of manganites and the interplay of double exchange and polaronic effects had to be addressed. The magnetic field dependent electronic overlap, the strong influence of crystallographic defects, and inter-site Coulomb interactions due to the high polaron carrier density have to be taken into account The latter are expected to increase significantly the activation energy for hopping transport [16] and are involved in the formation of polaron long-range order, which induces a structural phase transition between the charge disordered and the CO phases [17]. In order to present the effect of disorder on the phase formation and the small polaron properties in the different phases, the manuscript is organized as follows: after presenting some details on thin film preparation (section 2), we give a brief summary of the studies of the crystallographic structure and defects and show that nano-twins and octahedral disorder are the most common defects in as-prepared PCMO thin films They result in an inhomogeneous stressstrain state of the films (section 3).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
