Magnetic field effects on the local tunneling conductivity of La0.75 Ca0.25 MnO3 /MgO thin films

Abstract

Manganites are known for their exceptional properties, such as the metal-insulator transition (MIT) and the colossal magnetoresistance (CMR) effect. These materials show a very rich phase diagram in which the properties of the different phases do not only depend on the doping and temperature but also on electric and magnetic fields. Manganites are interesting candidates for studying the physics of correlated electrons and are also possible candidates for technological applications. It is known that a strong interplay between lattice, charge, spin and orbital degrees of freedom plays a very important role, which has been described in different theoretical models. Until now the basic mechanisms responsible for the special electronic and magnetic properties and the CMR effect are far from being understood completely. Hence more experimental work is crucial for understanding manganites. Scanning tunneling microscopy (STM) and spectroscopy (STS) are very local and surface sensitive techniques for probing the topography and the local electronic properties. They were used in this work to examine manganites with a metallic and ferromagnetic low temperature ground state. Overall strain-free thin films of La3/4 Ca1/4 MnO3 were deposited on MgO(100) substrates and showed different microscopic growth modes and crystal symmetries depending on the deposition parameters. The films were examined by STM/STS with respect to their temperature behavior and especially their behavior in external magnetic fields in the vicinity of the MIT. Furthermore, the films were compared with respect to their growth modes. The experiments reveal that the local tunneling conductivity changes continuously with temperature and magnetic field. The tunneling conductivity also varies locally, but neither distinct phases nor a domain-like growth of some regions with a magnetic field were found. This is inconsistent with the most popular theory of a percolation scenario. In a nutshell, a percolation of insulating and metallic phases does not seem to be a necessary prerequisite for the CMR effect.