Preparation And Magnetic Properties Study Of Ca_2-xNa_xFeMoO₆ Double Perovskites

double perovskites(Dp-s) are one of key spintronic materials with 100%spin polarization, and are widely used in material realm such as Room-Temperature Low Field Magneto-Resistance effect(RT-LFMR), spin-transport etc. At first, we review the essential properties such as crystal structure of FeMo based Dp-s, as well as the relation among the electronic structure, the influence of hole-doping effect, structure defects and the magnetism in the Dp-s. To enhance the Curie temperature (Tc), the study of physical nature about the mechanism of the ferromagnetic interaction is significant. Thus, a serial of Ca2-xNaxFeMoO6 Dp-s oxides were prepared in order to study how the A-site hole-doping influences the magnetism of Dp-s oxide.Since the Sol-gel method can prepare the polycrystalline Dp-s in low sintered temperature, which will have the features of small grains. To meet the need of our research, in this dissertation we adopt Sol-gel method to make the precursor of Dp-s. And the presintering condition is obtained through DSC-TG (Differential Scanning Calorimetry, Thermal Gravity) analyzing.One can find that sintering temperature, duration of annealing and the H2/Ar ratio (H2/Ar%) of reduction gas atmosphere are key factors to form the pure phase of Dp-s during the final preparation procedure. The parameters of making the pure crystalline of Ca2-xNaxFeMoO6 (0=x=0.4) were obtained by lots of experiments. Finally the trend of decreasing sintering temperature as x increasing was gained.XRD was employed to investigate the hole-doping effect on the crystalline structure and lattice parameters of the samples. The magnetic properties of Ca2-xNaxFeMoO6 at room temperature and low temperature were investigated by the VSM (vibrating sample magnetometer) and SQUID (superconducting quantum interference device), respectively. The Curie temperature (Tc) is also obtained. While the permittivity e and permeabilityμspectra are measured using the Vector-network-analysis, and the microwave absorption coefficient is finally calculated. And then we primary explore how hole-doping and the Structure defect affect the magnetism of Dp-s. The anisotropy constant K1 and the saturation magnetization Ms in polycrystalline Ca2-xNaxFeMoO6 at 5K have been calculated by the law of approach to saturation (LATS) method. The correctness and practicability of the results have been analyzed. The calculation result shows that the LATS method is suitable for the calculation of K1 and Ms in Dp-s polycrystalline.