The Magnetoresistance Of Co-ZnO Granular Films

The magnetoresistance （MR） has attracted a great deal of interestowing to the potential application in information technology. Up to thepresent time, MR has been observed in layered structures andgranular films. In these films, the non-magnetic layer （or matrix）includes mainly metals and insulators. Anyway, the reports about MRof the systems, in which a semiconductor is as a non-magnetic matrix,are very rare. In this work, ZnO semiconductor was as a non-magneticmatrix; Co-ZnO granular films were designed by multilayer forms anddeposited by magnetron sputtering. Based on studying the influenceof several kinds of buffer layers and substrate temperature onmicrostructures, magnetic properties and MR of the films, themechanism of the MR was discussed.Experimental parameters including the concentration of Co andZnO in Co-ZnO systems were often optimised in order to obtain largeMR values as previously reported. In this work, the influence of ZnObuffer layer on the MR of Co-ZnO granular films is firstly investigated.From the results of the XRD, TEM, and AES of ZnO（0¹50nm）/[Co（0.6 nm）/ZnO（0.7nm）]₁₀films, the [Co（0.6nm）/ZnO（0.7nm）]₁₀film behavesexactly like a granular film, although it was grown in multilayer form.And the Co particle sizes are different due to the influence of thedifferent thickness of ZnO buffer layer. Therefore, the films consist ofCo particles of different sizes dispersed in a semiconductor matrix.The MR ratios of Co-ZnO granular films are enhanced if they aregrown on a ZnO buffer-layer. The room temperature MR of a Co-ZnOfilm with a100nm ZnO buffer-layer is reached to-13.5%.Secondly, both the ZnO/α-C/[Co（0.6nm）/ZnO（0.7nm）]₁₀films withα-C as inserting layer and ZnAlO/[Co（0.6nm）/ZnO（0.7nm）]₁₀film weredesigned. It is found that the MR of the films is drastically decreasedwhen the structure includes more conductive materials such as a α-Cinserted layer, and a ZnAlO buffer-layer. Furthermore, the films thatincluded thicker α-C layers exhibit smaller MR ratios. In view of theMR, resistance, and Co particle size, it is found that both small Coparticle size and high resistance of buffer-layers are beneficial toenhance the MR of Co-ZnO granular films.Thirdly,[Co（0.6nm）/ZnO（0.7nm）]₁₀/ZnO/[Co（0.6nm）/ZnO（0.7nm）]₁₀films with different ZnO layer thicknesses were deposited.And the top and bottom [Co（0.6nm）/ZnO（0.7nm）]₁₀granular layerswere deposited at different substrate temperatures. The MR of[Co（0.6nm）/ZnO（0.7nm）]₁₀/ZnO/[Co（0.6nm）/ZnO（0.7nm）]₁₀films deposited at room temperature exhibits clear oscillatory behaviouras a function of ZnO layer thickness. For the [Co（0.6nm）/ZnO（0.7nm）]₁₀film deposited at high substrate temperature, their MR andresistance are drastically decreased. And the MR versus field curveshows two well-separated peaks at the coercive fields due to theincrease of Co grain size in the films deposited at high temperature.In summary, the influence of ZnO, C, and ZnAlO buffer layer andsubstrate temperature on MR of Co-ZnO granular films wereinvestigated. And it is found that both small Co particle size and highresistance of buffer-layers play an important role in inducing MR ofCo-ZnO granular films. These results may give a clue to enhance andanalyse MR in various inhomogeneous films, especially ferromagneticmetal-semiconductor granular films.