Preparation And Physical Properties Of Magnetic Double Perovskite Oxides

Since the Sr₂Fe Mo O₆ double perovskite oxide was reported in 1998,this material has always been much attention.The SFMO double perovskite is a half-metallic with the 100%spin-polarization at the Fermi level,it has a high Curie temperature（T_C）of～420 K which is higher than room temperature（RT）and it has a remarkable low-filed magnetoresistance（LFMR）behavior in RT.Due to these excellent physical properties,it has been paid a great deal of attention in views of its immense potential technological applications for spintronic and magnetoresistive devices.In this paper,we mainly investigate the effects of the Fe/Mo anti-site defect（ASD）contents and grain boundary（GB）strength on the MR behavior of SFMO.In addition,we try to optimize the MR behavior by modifying the GB environment of La_0.5Sr_1.5Fe Mo O₆（LSFMO）with organic materials.After comparison and analysis,some effective methods are supposed.The specific contents are as below:（1）The cooling rates of SFMO samples in the experiment were controlled with 5℃/min,10℃/min and 20℃/min respectively.The ASD content and GB property are adjusted simultaneously by this facile treatment.After the structural refinements,the ASD contents of the prepared samples are calculated to 9.6%,11.5%and 15.8%respectively.The resistivity of SFMO sample is enhanced about three orders of magnitude after the transport measurement.By systemically analyzing the structure and morphology,magnetic and the transport properties of samples,the combine effects of ASD and GB on MR behavior are detailedly discussed.Finally,we conclude that the MR behavior in this system is domaintly controlled by the positive GB effect and the negative effect by ASD is not shown evidently.（2）Two SFMO samples that have different ASD concentrations were prepared with different sintering temperature（T_S）,the ASD contents were assured to be 31%and 14%after the structural refinement.Then the GB strength of two samples was adjusted by the same glycerin soaking treatments.After a series of measurements,we find that the MR effects of SFMO samples which have the lower order degree but stronger GB strength is generally weaker than the one with higher order degree and lower GB strength.This phenomenon can be explained by that the suppression of MR behavior by ASD content is much intensive and this adverse effect can be hardly reduced by further enhancing the GB strength of materials.This manifests that in order to improve the MR behavior in SFMO material,it is necessary to make an extremely high order degree at first and then optimize the GB properties as far as possible.（3）The Curie temperature of electron-doped SFMO can be optimized significantly due to the band-filling effect,but accompanying an almost absent low-field magnetoresistance（LFMR）,which is unfavorable to applications in the magnetoresistive devices operated at room-temperature.In order to make the electron-doped double perovskite（LSFMO）material possess both high Curie temperature and significant MR effect,we try to modify the GB environment with organic molecule.We prepare the LSFMO/oleic acid composites at first,but the data show negligible enhancement of LFMR value.In order to explore this reason and find the effective method,the LSFMO/glycerin and SFMO/oleic acid composites are also synthesized.Based on the above experimental results and the related works,it is proposed that,maintaining high spin polarization of the carriers at the Fermi level and improving the tunneling process across the grain boundary by using the small molecule are decisive factors for optimizing the magnetoresistance behavior in the similar electron-doped double perovskites.