Manipulating physical properties of complex materials by processing

Abstract

Transition metal oxides (TMOs) are a class of materials in which charge, orbital, spin and lattice strongly interact with each other. In the present thesis, a special attention is paid to the physical properties of TMOs as a function of processing. First, TMOs are studied as substrates, where the surface is of prime importance. Second, the thin films are fabricated and their physical properties are presented. The SrTiO3 (001) and (111) surfaces are weakly and strongly polar and the free energy of intact surfaces should diverge. It is shown that in TiO2 terminated SrTiO3(001) the top most layer shows rumpling of O and Ti atoms. Rearrangement of atoms at the surface is one way of minimizing the surface free energy and avoiding polar catastrophe. In the case of SrTiO3 (111), we have systematically investigated the annealing effect on the structure and composition of the polar surface of SrTiO3(111). Heteroepitaxial growth of transition-metal oxide films on the open (111) surface of SrTiO3results in significant restructuring due to the polar mismatch. Using processing condition that we found for SrTiO3 (111), we have solved the polar catastrophe issue that occurs between materials with polar mismatch. we have studied La0.66Sr0.33MnO3 (001) under specific processing conditions and found a new interfacial magnetic interaction which is primarily driven by broken symmetry in oxide heterostructures. We show direct evidence that the symmetry breaking between conventional ferromagnetic metal and diamagnetic band insulator leads to dramatic modification of structure and chemistry at the interface. As a result, antiferromagnetic coupling emerges at the interface region which leads to spontaneous magnetic reversal and inverted hysteresis persisting well above room temperature. By further processing the La1-xSrxMnO3-y (001) thin films, it was shown that fine control of growth parameters can drastically change the physical properties of transition metal oxides. Magnetic properties of La1-xSrxMnO3-y was shown to differ from the target material La0.66Sr0.33MnO3 Appearance of exchange bias, inverted hysteresis and spontaneous magnetic reversal once more show that intimate competition in electronic, spin and lattice degrees of freedom in transition metal oxides can lead to new functionality, even in a monolithic thin film.