Study Of Stain Effect And Abnormal Transport Behavior At Low Temperature In Strongly Correlated Electronic Systems

Materials with perovskite structure are potential candidates for spintronics and avariety of complex electronic ground state, induced by the coupling between spin,charge, orbital and lattice degrees of freedom have already been studied. A very highspin polarization and colossal magneto-resistance in perovskite oxide occur nearCurie temperature and arouse extensive attentions. Manganite oxides belong to oneof the complex systems. In particular, lattice elastic stress can strongly modulate theintrinsic properties of the electronic phase separation. On the other hand, theelectronic transport properties at low temperatures are extraordinarily complex andcan reflect the characteristics of low-energy excited states and electron scatteringmechanisms. Especially the main influence of phonons on conduction electrons atlow temperatures is weak or even disappears, which shows the intrinsic physicalmechanism. Based on these properties, the thesis focus on the study onnarrowband-based single crystal Pr_5/8Ca_3/8MnO₃film and ZrO₂dopedLa_2/3Sr_1/3MnO₃film. Detailed studies are conducted on the change of magneticproperties and electrical transport properties under the stress regulation, as well as onthe abnormal low temperature transport behavior in perovskite oxidesLa_2/3Sr_1/3MnO₃film with different concentration ZrO₂particles andY_1-xPr_xBa₂Cu₃O_7-δ（x=0.44-0.48）. In addition, about the training effect, severaltheoretical models were proposed based on theoretical and experimental research tostudy its great damage to the magnetic spin valve design. Moreover, the trainingeffect in a lot of exchange bias material is a great disservice to the design ofelectronic devices. The related mechanism is far more from clear, though theemergence of giant magneto-resistance effect is for the decreasing of training effectin Co/CoO bilayer samples. This thesis includes six chapters:In chapter one, we review the history of the development of spintronics. Theemergence of the giant magnetoresistance effect is a milestone in spintronics, which is often related to exchange bias phenomenon. There are a lot of designeddeficiencies for itself, such as the limitations of the material itself and the low spinpolarization （less than50%）, etc. Perovskite manganite oxides aroused widespreadattention from the researchers due to a very high spin polarization in the vicinity ofthe Curie temperature. In particular, there is very strong modulation of the intrinsicproperties of the electronic phase separation through lattice elastic stress in thissystem. On the other hand, the electronic transport properties at low temperatures areextraordinarily complex and show interesting low temperature resistance minimumphenomenon. The mechanism of this low-temperature resistance minimumphenomenon is very complex, and is far from understanding.In chapter two, we focus on the main methods for sample synthesis in thecourse of the experiment and the principle of thin-film sample growth. High-qualitysample synthesis is the cornerstone for experimental study. In addition, two precisioninstruments---Superconducting Quantum Interference Device （SQUID） and thePhysical Properties Measurement System （PPMS） are reviewed, which ensure thecorrectness of the experimental results.In chapter three, we focus on the impact of stress on the magnetic propertiesand transport properties of manganites. By growing a high-quality film,Pr_5/8Ca_3/8MnO₃[100] and [001] directions are locked in LaAlO₃[010] and [021]respectively. In-plane lattice mismatch induce tensile and compressive strains in thefilm field along two different crystal orientations. The magnetization measured alongtensile orientation is much bigger than counterpart compressive orientation,indicating the magnetization easy axis of polarization and hard polarization axis.Under the tensile strain along z-axis, the long Mn–O bonds of the elongatedoctahedral form a zigzag pattern in the x–y plane at low temperatures, giving rise tothe preferential magnetization axis below the transition temperature Tc. We alsonotice that the exchange bias value in compressive orientation is bigger than that intensile orientation. The reason is the coupling interaction between AFM and FM influenced by the structure distortion. On the other hand, La_2/3Sr_1/3MnO₃film withdifferent concentration ZrO₂particles as a secondary phase provides an opportunityto tune the strain state, which is induced by mismatch between film and substrate.The epitaxial strain in La_2/3Sr_1/3MnO₃film is tuned by introducing appropriateconcentrations of nano-scaled non-magnetic ZrO₂particles. The results show agradual modification of the average lattice parameters and in turn tune the strainstate.In chapter four, the magnetic and transport properties of La_2/3Sr_1/3MnO₃filmwith different concentrations of ZrO₂particles and Y_1-xPr_xBa₂Cu₃O_7‐δ（x=0.44-0.48）systems were systematically studied. Nonmagnetic ZrO₂can induce the disordereffect on the matrix without changing the spin condition in La_2/3Sr_1/3MnO₃filmswith different concentrations of ZrO₂particles （0、3%、6%and20%）. Thetemperature dependence of resistivity generally shows a minimum at lowtemperatures and a resistivity upturn that appears to be increasingly stronger withincreasing ZrO₂precipitate concentration. We found that Kondo effect contributionto resistivity minimum can be excluded. Nonmagnetic ZrO₂particles in the metallicLa_2/3Sr_1/3MnO₃thin films modify the diffusion constant and increase the weaklocalization at low temperatures. On the other hand, for the Y_1-xPr_xBa₂Cu₃O_7‐δ（x=0.44-0.48） samples, the unconventional Kondo-like resistivity minimum is foundbelow Tc, where Tminincreases with increasing magnetic field below0.05T, and thendecreases for H>2T. Furthermore, Tcremains unchanged from0to0.05T, butdecreases linearly with increasing H from2T. The results reveal that theunconventional Kondo-like effect is related to magnetic scattering of the coexistingstates of the AFM and SC phases and FM clusters.In chapter five, a detailed study of the anisotropic magneto-resistance ofCo/CoO double-film samples grown on single-crystal Si（100） substrate is carried out.By using perturbation field to break the magnetic training effect after the formationof magnetic domains, induces magnetic training effect recovery. Through analysis, we know that the reason for training effects has two parts. The first one is from theweakening of anti-ferromagnetic spin, where the total intensity in the direction of thecooling field is partially dispersed into other easily polarization axis direction. Thesecond reason is from the formation of magnetic domains. The measurement resultshows that when the perturbation field deviation angle is less than30°, magneticexercise effect is not fully recovered, mainly because the ferromagnetic layer formedinside the magnetic domains is not fully destroyed. This indicates the maximumdeviation between the spin of the magnetic domain internal angle of30°.In chapter six, a summary of the present work is given. Meanwhile, thediscussion of the expectations about the regulation of stress on the nature of theperovskite oxide and on the anomalous resistance minimum phenomenon at lowtemperatures is also given in this chapter..