Tunability Of The Magnetic And Electrical Transport Properties Of Co/Mn-based Perovskite Metal Oxides

The perovskite-type oxides ABO3 have become more and more popular since its discovery due to different combination of A-site and B-site metal elements.These perovskites possess rich electrical transport behaviors and magnetic properties,such as the giant magnetoresistance(GMM)effect and the metal insulators transition and so on,making them potential alternatives in spintronic devices,memory,solid oxide fuel cells,etc.These properties and potential applications have aroused worldwide attention and study.In this thesis,we mainly focused on the perovskite oxides whose combination of lanthanide rare earth elements at the A site and the 3d transition metal cobalt and manganese elements at the B site.The double exchange and super-exchange interactions between Mn ions with different valences can be explained for many physical phenomena.In addition to double exchange and super exchange interactions,Co with different valence states also exhibits spin state transitions.In addition to the effects of the combination of different elements at the A/B site on the magnetic and electrical conductivity of double perovskites,the factors such as crystal structure,valence state and spin state of magnetic metal also affect the them.This paper mainly includes the following parts.In Chapter 1,we introduced the crystal structure,electrical transport properties and magnetic related concepts of Co/Mn-based perovskite metal oxides in briefly,and introduced the current research status of these materials.In Chapter 2,we started from simple single perovskite,A-site doped Pr1-xBaXMnO3(x=0.25,0.30,0.33 and 0.36)polycrystalline ceramics were prepared and their magnetic and electrical transport properties were systematically studied.All samples show two metal-insulator transitions(MITs)corresponding to the high temperature TMI1 and low temperature TMI2,respectively,besides the non-Griffith phase above the ferromagnetic(FM)transition temperature Tc.Combining the results of the transport and magnetic properties,it is found that the FM transition temperature Tc coincides with the temperature TMI1,which is linearly related to the A-site ionic radius mismatch variance ?2,indicating the enhancement of FM interactions due to the increase of the degree of B-site ordering of Mn3+/Mn4+ions.The positive correlation between ferromagnetic insulators(FMI)and magnetic interactions,including the FM and short-range antiferromagnetic(AFM)interactions,is confirmed.It is suggested that the first MIT at TMI1 is attributed to the Mn3+/Mn4+double exchange interactions and the second MIT at TMI2 is closely related to the suppression of the AFM interactions under the internal FM field induced by the Mn3+/Mn4+ DE interactions.This work provides not only a theoretical understanding on the origin of MIT at low temperature,but also a new way for adjusting the FMI in perovskite manganese oxide Pri-xBaxMnO3 for application.In Chapter 3,In Chapter 3,we select a layered double perovskite LnBaCo2O5+?with a unique structure.This type of perovskite can be considered as half-doped(CoO2-BaO-Ln-O0.5-CoO2 layer stacking along the c direction,due to the lack of oxygen in the PrO0.5 layer,resulting in CoO5 pyramids and CoO6 octahedrons alternately arranged along the b axis)at A-site of the single perovskite oxides.PrBaCo2O5+?(PBCO)sample as prepared by a conventional solid-state reaction,and then annealed in O2 gas and Ar atmosphere(note as AP-PBCO,O2-PBCO and Ar-PBCO respectively).All samples show a magnetoresistance(MR)effect.As the temperature lowing,the MR effect changes from positive to negative for Ar-PBCO with low oxygen content,and is negative and increases in magnitude for AP-PBCO and O2-PBCO.A higher MR effect with?38%at around 70K and 9T for AP-PBCO is obtained.Besides,an abnormal decrease in magnitude for MR is observed for AP-PBCO and O2-PBCO.It is suggested that the spin disorder related polaron hopping energy WP is crucial for the MR effect in PBCO,besides the competition of AFM and FM interactions,which rests with the crystal structure dominated by the content and order of the oxygen vacancies.The observed abnormal MR can be attributed to the reduced polaron hopping energy WP related to the AFM order.In Chapter 4,we choose double perovskite Dy2CoMnO6(DCMO)whose A-site rare earth element with a larger magnetic moment,and use sol-gel method to prepare the double perovskite DCMO The structure and magnetic properties of them were systematically studied.Both X-ray diffraction and Raman spectrum indicate that double pervoskite DCMO has a monoclinic crystal structure with a space group P21/n.A Griffiths phase,a short-range ferromagnetic(FM)ordered state is proved to be existing in DCMO,which is significantly different from the recently reported non-Griffiths phase of Gd2CoMnO6(GCMO).It is suggested that the different 3d-4f exchange interactions in DCMO and GCMO can be attributed to the origin of the different short-range magnetic orders of them,besides the anti-site disorder,interrupting the long-range FM order of Co2+-O-Mn4+,resulting in the formation of short=range FM order.The effects of A-site rare-earth Re3+ions on the magnetism of double perovskite Re2Co(Ni)MnO6 are dominated by the radius and the moments of rare earth ions(or the 3d-4f exchange interactions),which are immanent connection and competition with each other.The FM/AFM coupling is not only related to the outer most 4f electrons of the rare earth ions,but also to the inner 5d/6s orbital electrons.In Chapter 5,we choose non-magnetic Al3+ ions to replace the magnetic ions Mn in the La2CoMnO6 to adjust the magnetic properties of the system.X-ray diffraction reveals a structural transition from monoclinic(space group P21/n)to rhombohedral phase(space group R-3c)with Al content increasing,and the unit cell parameters decreasing.The remanent magnetization Mr and ferromagnetic(FM)transition temperature Tc dramatically decrease with the increment of Al doping,which is due to the destruction of the long-range Co2+-O-Mn4+FM super-exchange interaction induced by Al ion doping.However,abnormally increase of coercive field HC and effective magnetic moment ?eff appear for x=0.25.The results of infrared spectrum,Raman spectrum,and X-ray photoelectron spectroscopy illustrate that this anomaly can be attributed to the spin-state transition of Co3+ions from intermediate spin(IS)to high spin(HS)state with x increasing to 0.25,rather than the valence changes of the magnetic ions.This spin-state transition is probably due to the increase of crystal symmetry with high disorder and the suppression of Jahn-Teller distortion.The suppressing Jahn-Teller distortion favors the stability of the HS state of Co3+ions.