Electronic Transport Properties Of (La_1-xPr_x)_0.67Ca_0.33MnO₃ Thin Films And LaTiO₃/SrTiO₃ Heterostructures

Perovskite oxide materials possess rich physical phenomena,including colossal magnetoresistance,quantum oscillations,high-temperature superconductivity,magneto-thermal effects,ferroelectricity,and multiferroicity.These physical effects play important roles not only in fundamental science but also in practical applications.For example,the colossal magnetoresistance effect is used in magnetic storage,quantum oscillations are used in sensors and detectors,high-temperature superconductivity is used in efficient power transmission and quantum computing,magneto-thermal effects are applied in magnetic refrigeration,and ferroelectricity is used in integrated circuits and non-volatile memory.In this paper,(La_1-xPr_x)_0.67Ca_0.33Mn O₃（LPCMO）and La Ti O₃（LTO）perovskite oxide materials were chosen as research targets to investigate the effects of Pr doping concentration and substrate-induced strain on the electrical properties of LPCMO thin films,as well as the electromagnetic transport properties of the LTO/STO heterojunction.The main research contents and results of this paper are as follows:（1）This paper reports on the use of pulsed laser deposition to fabricate a series of(La_1-xPr_x)_0.67Ca_0.33Mn O₃（LPCMO）epitaxial films on Sr Ti O₃（STO）（001）single crystal substrates,and the investigation of their electrical properties.With increasing Pr doping concentration,the film resistivity increases,magnetic properties weaken,metal-insulator transition temperature reduces,and charge-ordered antiferromagnetic insulating phase becomes more stable.LPCMO films with x=0.2 grown on Sr Ti O₃（001）,（La Al O₃）_0.3（Sr₂Al Ta O₆）_0.7（LSAT）（001）,and La Al O₃（001）single crystal substrates were chosen for the systematic measurement of their electronic transport properties.It was found that electronic phase separation is sensitive to lattice strain induced by substrate,and films with little in-plane stress have the highest metal-insulator transition temperature.Under in-plane compressive or tensile stress,the metal-insulator transition temperature decreases,but LPCMO films with tensile in-plane stress have a more stable charge ordered phase and exhibit the largest negative magnetoresistance effect（MR=99.7%）.（2）High-quality LTO epitaxial films were prepared on STO（110）substrates by pulsed laser deposition.The LTO/STO heterojunction exhibits metallic conductivity in the temperature range of 1.8-300 K.At 1.8 K,the heterojunction has an electron mobility as high as 4045 cm²V^-1s^-1,and displays Sd H quantum oscillations and a non-trivial p Berry phase at high magnetic fields,indicating that the Fermi surface of the two-dimensional electron gas has quasi-two-dimensional characteristics.At low temperatures（1.8-3.2 K）and low fields（<250 Oe）,the heterojunction exhibits a large negative magnetoresistance（up to 61.8%）regardless of the magnetic field orientation perpendicular or parallel to the film plane.The negative magnetoresistance curve demonstrates hysteresis with a butterfly-shaped loop.Magnetic measurements show that the heterojunction has ferromagnetic properties at low temperatures,indicating the formation of a spin-polarized two-dimensional electron gas at the LTO/STO interface.Angle-dependent magnetoresistance measurement reveals isotropic negative magnetoresistance,suggesting the formation of island-like ferromagnetic clusters at the interface.