Study On The Microstructure And Physical Properties Of Perovskite La_2/3Sr_1/3MnO₃ Thin Films

Perovskite manganite films have received extensive attention due to their unique physical properties such as colossal magnetoresistance,ferroelectrics and half-metallicity.In this thesis,La_2/3Sr_1/3MnO₃?LSMO?films with different thicknesses are epitaxially grown on different substrates using pulsed laser deposition?PLD?technique.The angular-dependent magnetoresistance?MR?and the effect of the lattice strain on the physical properties of these films are systematically investigated.In chapter 1,the microstructure,physical properties and research progress of perovskite manganite films are generally given.From the point of the crystal,magnetic and electronic structures of perovskite oxides,their unique physical properties and recent research progress are introduced.In chapter 2,the preparation,microstructure characterization,experimental equipment and density functional theory calculations of perovskite manganite films are introduced.In chapter 3,the effect of thickness on angular-dependent MR of LSMO thin films grown on Sr TiO₃?STO?is systematically investigated.When the magnetic field is parallel to the film surface,an additional weak peak appears in the MR curves of the films thinner than 8 nm.As revealed by density functional theory calculations,this peak signals the formation of two-dimensional electron gas?2DEG?at the interface which arises from a magnetic reconstruction.However,the 2DEG gradually disappears with increasing film thickness.When the film is thicker than 10 nm,the 2DEG completely disappears and an anomalous MR valley simultaneously appears when the magnetic field is perpendicular to the film surface.Combining high-resolution transmission electron microscopy observations and the critical thickness calculations,it demonstrates that the appearance of this valley is associated with the formation of pure edge dislocations at the interface between the film and the substrate.In chapter 4,the effect of the lattice strain on the transport properties of LSMO film is systematically investigated.The temperature-dependent resistivities of LSMO films grown on STO,?La,Sr??Al,Ta?O₃?LSAT?and LaAlO₃?LAO?are measured.The LSMO/STO and LSMO/LSAT films show metallicity,whereas an unexpected resistivity is found when the film is thinner than 10 nm.This is attributed to the formation of 2DEG at the interface.However,for the films grown on LAO,the resistivity rising disappears,and the films with a thickness below 20 nm are under insulating state.It shows that thecompressive stress can lead to the mobility reduction of eg electrons,resulting in the resistivity increase.The compressive stress can hinder the formation of 2DEG.Furthermore,the applied magnetic field can lead to the decrease of resistivity.The substrate orientation has no significant influence on the transport properties of LSMO films.