Investigation On Spin/orbital-ordering And Phase Separation Of Perovskite Manganite

In the perovskite manganites，the doped manganites R1_xAE_xMnO₃（R=La, Pr.AE=Sr, Ca） is a strongly electron-correlated system with strong coupling among spin,charge, and degrees of orbital freedom, possessing abundant physical phenomena: thecolossal magnetoresistance （CMR） effect, the metal-insulator transition and theelectrical-magnetic phase separation. In the manganite systems, the egsingle-electronband, Mn-O-Mn bond angle and Mn-O bond length could be modulated by A-sitedoping. Thus, the amount of the ferromagnetic metallic and the charge-orderedinsulating phases could be modified. The magnetic and electrical transport propertiesare modified by controlling the competition between the double-e_xchange andsuper-e_xchange coupling. The main results of this dissertation are as follows:1) The magnetic and the electrical properties of polycrystallineLa5/8（Pr_xCa1-_x）3/8MnO₃prepared by solid-state reaction were investigated. X-raydiffraction （XRD） indicated that all La5/8（Pr_xCa1-_x）3/8MnO3samples are pureorthorhombic perovskite phase with pnma space group at room temperature. Thetemperature dependence of magnetization showed that the Curie temperature （TC）decreased with increasing the Pr concentration, and the TCfor La5/8（Pr_xCa1-_x）3/8MnO₃（_x=0.25，0.5，0.75） was around160K,150K, and100K, respectively. Furthermore,upon increasing the Pr concentration the degree of the lattice distortion increases,which weakens the ferromagnetic interaction and results in the spin glass state. Thespin-frozen temperatures of La5/8（Pr_xCa1-_x）3/8MnO₃（_x=0.25，0.5，0.75）were around150K,75K, and70K, respectively. The electrical resistance as a function oftemperature showed a double-peak phenomenon for_x=0.25sample, which wasmainly due to the the different phases in this sample. Magnetoresistancemeasurements confirmed that the CMR could be modulated by the A-site doping inthese compounds.2) La5/8（Pr1-_xCa_x）3/8MnO3/La1-_xSr_xMnO3（_x=0.25,0.5,0.75） films were depositedon STO by RF magnetron sputtering. With the help of XRD and physical propertymeasure system （PPMS）, the structure and properties of La1-_xSr_xMnO3/La5/8（Pr1-_xCa_x）3/8MnO3（_x=0.25,0.5,0.75） films were characterized. It’s indicated thatthe films present the favorable orientated growth when deposited on （100） STOsubstrate; samples are small-polaron transition mode. Stress can inhibit the formationof charge ordering state. As the increasing of Pr3+, lattice constant of samplesincreased, the stress between LPCMO and LSMO decreased. This is the direct reasonfor the formation of charge order （CO） state could be easily. When the samples in ane_xternal magnetic field, the charge order （CO） state could be melted, CMR would beappeared.3) The Ba0.5Sr_0.5Co_0.8R0.2O₃（R=Ni、Fe、Mn） samples prepared by solid-state reaction were investigated. Discuss the impact B-doping of perovskite. It could beshown from the Ba_0.5Sr_0.5Co0.8R_0.2O₃（R=Ni、Fe、Mn） samples that there was theCo⁴⁺-O^2--Mn4+super-exchange only with Mn doping, with the TCwas near50K.Pr0.5Ca0.5Mn1-xRuxO3（x=0.09）, Ru⁴⁺plasma enhanced double exchange interaction.There were Mn³⁺-O^2--Mn⁴⁺and Mn³⁺-O^2--Ru⁵⁺in the Pr_0.5Ca_0.5Mn1-xRu_xO₃（x=0.09）sample. It’s indicated that Ru doping on Mn-site induced the TChigher than TIM.