Investigation Of Magnetic Properties And Magnetoresistance In Co/ZnO Films

Granular films is one of the focuses of attention in recent years,because it plays an important role in the development of informationtechnology. The choice of non-magnetic materials in granular films isprimarily non-magnetic metal matrix or non-magnetic insulator. Theresearch about granular films composed of ferromagnetic metal andsemiconductor materials is a few, and there is a dispute about itstransport mechanism. Therefore, in this paper we propose asemiconductor with a small barrier height as well as ZnO as anon-magnetic substrate and a series of Co/ZnO multilayer granularfilms with different ZnO thickness and different sputtering pressurewere prepared on glass substrates by magnetron sputtering(JGP560CC model) at room temperature. The structure andmicrostructure of films were analyzed by x—ray diffraction (XRD) andtransmission electronic microscopy (TEM); the magnetic propertiesof films were measured by vibrating sample magnetometer (VSM) andsuperconducting quantum interference device (SQUID); we used a standard four-point-probe method to measure the electricalproperties of the films.Firstly, we prepared Co/ZnO films of the same thickness atdifferent sputtering pressure. We found that the films had a similarstructure that is the multilayer granular films of magneticnanoparticles wrapped in the semiconductor matrix particles. Themagnetic properties of the films had a transition from ferromagneticto superparamagnetic with the increase of sputtering pressure. Sucha large difference in magnetic properties has to do with carrierconcentration，the greater carrier concentration is，the stronger theeffect of coupling between the magnetic is. And the film is morelikely to exhibit ferromagnetic properties. From the dependence oflnρon T-1/2in zero magnetic field, we deduce that the MR is caused byspin-dependent tunneling.Secondly, on the basis of this study, we fixed the thickness of theCo layer and a series of Co/ZnO multilayer granular films withdifferent ZnO thickness were prepared at 0.8Pa sputtering pressure.The films became from amorphous into crystalline structure with theincrease of ZnO thickness. Formation of the films is a typical granularstructure from the TEM image. All films showed superparamagnetismat room temperature, saturation magnetization (Ms) decreased from249emu/cm3to 13emu/cm3with the increase of ZnO thickness. This shows that the size of metal particles in the films decreased. Themagnetoresistance value of the films as a whole showed adecreasing trend with the increase of ZnO thickness because the Coparticle size and particle spacing affect spin-dependent tunneling.And magnetoresistance was not easy to reach saturation withdecrease of the particle size. Based on the resistance value with thechange of magnetoresistance, it can be seen that one certainregularity may exist. There is an extreme value of about -11%.Lastly, we firstly study the effect of the total thickness of thefilms (the number of cycles) to the same thickness of Co/ZnO thinfilms. The results showed that，all of the films were thesuperparamagnetic. Magnetoresistance increased with the increaseof sputtering period number, this is mainly because the particletunneling in the films took place not only in the same layers but alsoin the adjacent layers.In conclusion, when magnetic metal particle size and particlespacing in the films achieves an appropriate value, themagnetoresistance of the films will be larger.