Structural And Physical Properties Of Double Perovskite Ln₂Ni/CoMnO₆

Double perovskite oxides with general formula A2BB’O6, where A is rare earth oralkaline earth metals and B/B’are transition metals, have been widely investigateddue to their rich physics and the prospect of novel applications. In this dissertation, wefocused on the typical double perovskite compound Ln2Ni/CoMnO6(Ln=rare earth,Y), and carried out a series of investigations on their structure and physical properties,especially on the magnetic properties, some important conclusions have been obtained.The research content can help us further understanding the basic propertis andphenomenon in this stongly correlated electron system. On the other hand, the resultsobtained in this dissertation can provide some usefull information for the practicalapplication in future. Details are as follows: In chapter one:Firstly, we introduce some basic concepts in perovskitemanganite which pocess interesting colossal magnetoresistivity effect. Secondly, wesummarize the basic structural, magnetic and electrical preoperties of doubleperovskite Ln2Ni/CoMnO6systerm, where Ln represents the rare elements or Y. Atlast, we summerize the research progress of perovskite manganite with smallerparticle size, especially the corresponding nanoparticls. In chapter two:Lai.6Sro.4NiMn06nanoparticles of different sizes (18～150nm)have been prepared by a sol-gel method, and the size effects on their magneticproperties are investigated. It is found that there is a ferromagnetic (FM) transitionaround245K for the sample with particle size of D～150nm. As the particle sizedecrease, a spin-glass (SG) transition appears around60K and becomes moreobvious,meanwhile, the FM transition shifts to a lower temperature and becomesindistinct. At low temperature, the exchange bias (EB) effect is clearly observed forlarge particles and becomes indistinct as particle size D≤32nm. On the other hand,the particle size dependent saturation magnetization (Ms) and coercivity (He) shownon-monotonic variation, which indicates the magnetization sate at low temperature isvery complicated. We propose a modified core-shell model to understand the particlesize dependent magnetic property in this systerm. In this modified model, the SGphase mainly resides on the surface (shell) of each particle, and both AFM APBs andFM domains coexist in the core, the FM domains arrange antiparallel across APB. Wededuce that the EB effect in the large particles originate from the exchange couplingbetween FM domains and the APBs. As particle size decrease, the size effect and the surface effect become more and more obvious, resulting in the apparent spin-glass transition in the small particles. The size effects on the FM phase, AFM APBs and surface SG phase in grain are different, even are opposite, which result in the complicated size-dependent magnetic properties.In chapter three:Bulk and nanosized Pr2NiMnO6samples were prepared by standard solid-state reaction method and sol-gel method, respectively, and the magnetic properties are investigated. It is found that both the two samples show FM behavior at low temperature. As particle size decrease to nanoscale, the ferromagnetism become weak obviously. At low measuring magnetic field, the inverse susceptibility (χ-1) of Pr2NiMnO6bulk sample and nanosized sample exhibits an upword and downword derivation from the C-W behavior well above Tc, respectively. The derivation of χ-1above Tc confirms the presence of the short-range magnetic ordered state before the long-range magnetic ordered state formed. The short-range ordered state in the nanosized sample can be well described as the Griffiths phase. These results suggest that both APBs and FM domains coexist in the bulk sample, the FM domains arrange antiparallel across APB, correspongding to the AFM exchange coupling, which results in the χ-1upword derivation. As particle size decrease to nanoscale, there is no APB in the particle due to the small particle size, while the FM short-range ordered domains still exist, which result in the χ-1downword derivation, paving the way for the Griffiths phase.In chapter four:The ceramic Pr2CoMnO6of double perovskite structure was prepared by a solid-state reaction method and the magnetic properties, phonon behaviors are studied in detail. Two FM transitions at174K and140K are observed, the high temperature transition can be attributed to the FM superexchange interaction of the ordered Co2+-O-Mn4+pair, while the second transition at140K is related to the introduction of a small number of Co3+and Mn3+in the disordered phase. Compared with La2CoMnO6, both the FM transition temperatures are lowered for Pr2CoMnO6, which can be understood mainly based on the decrease in the Co-O-Mn bond angle due to the smaller ionic radius of Pr3+. Temperature-dependent Raman scattering experiment reveals obvious softening of the phonon mode involving stretching vibrations of the (Co/Mn)O6octahedra around FM transitions, indicating a close correlation between magnetism and lattice in Pr2CoMnO6. On the other hand, the field-cooling magnetic hysteresis loop reveals that EB effect is present, which is supposed to origin from the exchange coupling between Co/Mn ordered FM domains with APB caused by the partially Co/Mn antisite disorders.In chapter five:Pr2CoMnO6nanoparticles of different sizes (45nm～1μm) have been synthesized by a sol-gel method. The particle size increases with the annealing temperature increasing from700to1200℃. Both magnetic and Raman studies indicate that the rates of disordered phases decrease as the annealing temperature decreases, this variation is deviate from the thermodynamic and the size effect rules. Through a contrast test, it is suggested that the annealing temperature dependent variation of the oxygen vacancies plays a key role on the magnetic properties and Raman features in these Pr2CoMnO6nanoparticles. The samples annealed at relatively high temperature pocess more oxygen vacancies, correspngding to the high disorder degree, resulting in the weak ferromagntism and low Tc, the relatively lower annealing temperature (700～1000℃) in air in favor of decreasing the concentration of oxygen vacancies in this system, correspngding to the lower disorder degree, which result in the high Tc for the small particles. This reaseach provide a basis for understanding the peculiar annealing temperature dependent (size-dependent) magnetic properties for Ln2CoMnO6systerm.