The fabrication and characterization of heterojunctions composed with manganite perovskite

Abstract

Intensive research interests in condensed matter physics have been focused at heterojunctions composed of perovskite oxides. Complex oxides with a perovskite structure show many attractive characteristics for both basic research and applications. Among the perovskite oxides, the manganite displays high sensitivity to external disturbances such as magnetic fields, electric fields, currents, mechanical strain, and photo illumination. In this thesis, the field effects on the magnetic and electric properties have been explored. Modifications of the transport of manganite heterojunctions by applying various fields like electric/magnetic field, visible light, lattice strain, have been investigated systemically. To study the strain effect on the magnetic and electric properties, ferroelectric crystals of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) were used to induce tunable lattice strain into the grown films. Epitaxial thin films of La0.9Ca0.1MnO3 (LCMO) were grown on (100) oriented (PMN-PT) single-crystal substrates through pulsed laser deposition. Results from X-ray diffraction indicated that the ferroelectric poling significantly changed the in-plane tensile strain introduced in the LCMO film, leading to a tunable resistance and Curie temperature Tc. Furthermore, it is observed that the magnetoresistance of LCMO film was enhanced in ferromagnetic state (below Tc), but declined in the paramagnetic state (above Tc), due to the poling of PMN-PT. The resistance of the transition peak was increased, and the phase transition temperature was reduced at the same time after PMN-PT be poled. Residual strain by lattice mismatching was also studied. The strain effects on magnetoresistance can by interpreted by the Jahn-Teller distortion, and the polarization effect on the coexistence of multiple phases may cause the increased peak resistance after PMN-PT being poled. La0.9Hf0.1MnO3 has been known as a good electron doped manganite and has attracted lots of research interests. To reveal the effect of temperature on magnetic and electric properties in such a n-type manganite oxide, epitaxial thin films of La0.9Hf0.1MnO3 were grown on 0.05 wt% Nb-doped SrTiO3 substrates by pulsed laser deposition. X-ray diffraction measurements showed that the samples were of good epitaxy and single-crystal. The as-grown film of La0.9Hf0.1MnO3 exhibited a typical paramagnetic-ferromagnetic transition. Heterojunctions of La0.9Hf0.1MnO3/0.05 wt% Nb-doped SrTiO3 exhibited-asymmetric current–voltage characteristics and a remarkable temperature-dependent rectifying behavior in a wide temperature range. The most remarkable observation of this work is the existence of a crossover from positive to negative magnetoresistance in the heterojunction with the decrease of temperature. A temperature-dependent magnetoresistance was also studied. The rectifying characteristics and tunable magnetoresistance of these heterojunctions may be attributed to band structure of La0.9Hf0.1MnO3/0.05 wt% Nb-doped SrTiO3 heterojunction. Excellent photo detectors need to exhibit rapid response and demonstrate good repeatability for different applications. In this chapter, optoelectronic of heterostructures made with La0.9Hf0.1MnO3 and 0.05 wt% Nb-doped SrTiO3 were investigated. These thin films presented persistent and stable photovoltages upon light illumination. Rapid shift between small and large voltages was observed, demonstrating reliable photo detecting behavior. The injection of photoexcited charge carriers (electrons) could be responsible for the appearance of the observed opto-response. Such manipulative features by light irradiation exhibit great potential for light detectors for visible light.