Preparation, Structures And Physical Properties Of Magnetic Double Perovskite Ceramics

Owing to their unique structure, physical properties, and potential applications in magnetic storage devices and spintronics devices, the magnetic double perovskites oxides are extremely attractive for condensed matter physics and materials science researchers. As a representative of the magnetic double perovskites, ferrimagnetic material Sr2FeMoO6(SFMO) has a half-metal property with100%spin polarization, remarkable low-field magnetoresistance (LFMR) at room temperature and a considerable high Curie temperature of～420K. Therefore, since1998, the SFMO have been widely investigated in view of its immense values in both fundamental physical research and potential technological applications. In this dissertation, we select SFMO as the main object of investigation. We paid a great of efforts by adjusting Fe/Mo ordering degree at B-site, to tune the properties of transport and magnetization. Furthermore, we have investigated the intrinsic relation between structure and properties. We have comparatively investigated the competitive and combining effects of grain boundary (GB) and Fe/Mo anti-site defects (ASD) on the LFMR in SFMO. Based on these conclusions, we intend to optimize the LFMR in SFMO without reducing its magnetization. In addition, we have also studied the Sr2MnWO6-based double perovskites. The details are following:(1) Based on systematically investigating the effects of experimental parameters (H2content in H2/Ar mixed gas, flowing rate of the gas, sintering temperature, and sintering time) on phase purity and ASD content, a facile method only by adjusting the sintering time (16-4h) was developed to quantitatively control the ASD concentration in SFMO ceramics in a wide range of2%-21%. We systematically investigated the relationship of ASD and magnetization, transport, magnetoresistance and Tc. The dependences of ASD content on the statured magnetic moment (Ms) and Tcare investigated by the Monte Carlo simulations too (Chapter3).(2) Based on the results in (1), SFMO ceramics with a wide range of2%-38%ASD content were also prepared by controlling sintering time and flowing rate of the gas flexibly. The competitive and combining effects of GB and ASD on the LFMR behavior in double perovskite SFMO ceramics have been investigated systematically. With these results, we get a conclusion that to enhance the LFMR of this material, the efficient strategy is to synthesize SFMO ceramics with the ASD content less than26%, and then improve the strength of the GB as much as possible (Chapter3).(3) Based on the conclusions in (2), in order to optimize LFMR performance, the SFMO ceramics with10%ASD concentration was processed by subsequently soaking in the mixed solution of glycerol and water at room temperature. The volume ratio of glycerin and water were VGiy/VH2o=1/19and VGly/VH2o=1/1respectively. The former ratio can induce the formation of insulating SrMoO4in GB, the latter allow for the insulating glycerin to penetrate in GB. Both of them can enhance the strength of GB and thus improve the LFMR response remarkably. Moreover, magnetization of the compound keep unchanged obviously (Chapter4).(4) In5d-substituted SFMO, i.e. Sr2(FeMo)1-xIrxO6ceramics, the effects of Ir-substituted on the structure, magnetization, transport and magnetoresistance were investigated. The limit concentration of Ir-substitution is only0.06, however, this small amount affect corresponding physical properties remarkably. This might stimulate investigation of other element substitution at A/B sites in SFMO ceramics (Chapter4). (5) Series of off-stoichiometic Sr2Mn(1+x)W(1-x)O6(0<x<3/5) ceramics were synthesized successfully. With the increasing of Mn-content (x), the structural phase transformation from monocline to tetragonal take place gradually, accompanying with the sensitive variance of the pure or mixed Mn chemical valence (+2、+2/+3、+3、+3/+4). It is confirmed that the predominant magnetic interaction changes from antiferromagnetic (AFM) to weak ferromagnetic (FM). The resistivities of all the samples are very large and even can not be measured. We also analyzed underlying physical reasons of the above experimental phenomena.