Magnetotransport Properties Of Co-based Heusler Half-metallic Granular Composite Materials

Spintronics materials are the basic materials for fabrication of morden magnetoelectric devices with miniaturization, rapid reading/writing, high density and sensitivity. Recently, Magnetic Random Access Memory (MRAM) has been significantly focused on its experimenatal research. The MRAM requires a large magnetoresistance (MR) efect at room temperature and lower magnetic field. However, the large MR efect is strongly dependent upon electrode materials with high spin polarization. On the other hand, half-metallic materials are theoretically predicted to be100%spin polarization. The experiment research also demonstrates the high spin polarization of half-metals. Heusler alloys are promising high spin polarized half-metallic materials, which show some desired magnetic properties, such as high spin polarization, large magnetic moment, and high Curie temperature, which are considered as potential application in spintronics, in comparison with other half-metallic materials. Recently, a lot of researchers pay much attention to studying on composites, which exhibit not only the properties of single component, but also some additional properties coming from the interaction between the components of the composites. Therefore, ferromagnetic/antiferromagnetic (FM/AFM) coupled Co2MnSi nanocomposite structure and Co2FeSi nanocomposite structure were designed to study the magnetic, transport and exchange bias properties of the nanocomposites. The investigation details were summarized as follows:(1) The polycrystalline Co2MnSi alloy powder was prepared by high vacuum arc melting and the ball milling method, which has a complete Heusler cubic L21structure and show fine metallic property. The pure NiO and pure CuO were obtained by precipitation method. There was no observable chemical interaction between Co2MnSi and NiO during the mixing and ball milling process.(Co2MnSi)0.8-(NiO)0.2composite existed the exchange bias of10.7Oe at magnetic field cooling hysteresis loop, which increased scattering effect at the interface between Co2MnSi nanoparticles and NiO nanoparticles. The values of MR were very approach among (Co2MnSi)1-x-(NiO)x composites at50K and300K. The MR of (Co2MnSi)1-x-(NiO)x composites reaches the maximum value at x=0.2, the MR values are-0.323%at50K and-0.296%at300K, comparing with the NiO-free Co2MnSi, the addition of NiO dramatically enhances the MR values, corresponding to the increment of19.8and26.4times. The (Co2MnSi)1-x-(CuO)x (x=0.2,0.4,0.6,0.8) composites reaches the maximum value of MR at x=0.2at50K and300K, comparing with the NiO-free Co2MnSi, the addition of NiO dramatically enhances the MR values, which are-0.312%at51K and-0.219%300K, corresponding to the increment of19.1and19.6times.(2) The polycrystalline Co2FeSi alloy powder was prepared by high vacuum arc melting and the ball milling method, which has a small amount of defects and show fine metallic property. There was no observable chemical interaction between Co2FeSi and NiO during the mixing and ball milling process. With the increase of antiferromagnetic NiO nanoparticles,(Co2FeSi)1-x-(NiO)x composites existed FM/AFM coupled effect at the interface between Co2FeSi nanoparticles and NiO nanoparticles, which greatly enhance the value of MR. The MR of (Co2FeSi)1-x-(NiO)x composites reaches the maximum value at x=0.2, the MR values are-0.336%at51K and-0.280%at300K. Both (Co2FeSi)o.9-(NiO)0.1and (Co2FeSi)0.8-(NiO)0.2composites existed metal-insulator transition temperature (T=107.4K) and insulator-metal transition temperature (T=156.5K).(Co2FeSi)0.7-(NiO)0.3composite shows insulating property from50K to300K.