Structure, Magnetism And Magnetoresistance, Of Nanostructured Co-ZnO Films

Since the discovery of giant magnetoresistance effect in Fe/Crmultilayers, the magnetoresistance in magnetic multilayers,granular films and magnetic tunnel junctions composed of magneticmetals and non-magnetic materials have been studied extensivelyfor potential application in spintronic devices,such as magneticsensors, hard disk reading heads and magnetic random accessmemories. There is little research about the MR of semiconductorbarrier. Semiconductor can provide a much lower barrier heightcompared to insulator with a decrease of resistivity. Moreover, theincreased thickness of semiconductor reduced the probability ofpinhole. According to the reports in the literature, roomtemperature MR has been observed in magnetic metal/semiconductor films. However, there is ample room for furtherinvestigation of exploring the origin of the MR in magnetic metal/semiconductor films and improving the MR ratio.In this paper, three series of Co-ZnO based granular films (GF) were deposited by magnetron sputtering. The GF was grown bysequentially depositing ultrathin layers and ZnO layers. The GFswere deposited at various substrate temperatures. On the base ofthis, GF(400℃)/ZnO/GF(RT) samples with a sandwitch structurewere designed. Moreover, the influence of Al-doped ZnO onGF(400℃)/ZnO/[Co-ZnAlO] GF(RT) samples was studied. Thestructure, magnetic properties, and magnetoresistance effect wereinvestigated. The results are summarized as follows:(1) GF samples were deposited at various substrate temperatureof100℃、200℃、300℃、400℃, With the increasing substratetemperature of the granular films, the Co grain size was growing,but the MR was drastically decreased. By comparing magnetic andmagnetoresistance of four granular film deposited at differenttemperature. We found that the small size of magnetic particles isadvantageous to the magnetoresistance effect, and the large sizeof the magnetic particles is advantageous to the magnetic.(2) In order to obtain one material include both largemagnetoresistance effect and excellent magnetic We havedesigned [Co(0.6nm)/ZnO(0.7nm)]10(400℃)/ZnO(xnm)(RT)/[Co(0.6nm)samples with a sandwich structure include both “hot” and “cold”layer. Because of the different substrate temperature of top andbottom GFs. There are two dimensions of Co particles exist in the trilamellar membrane.The resistance of the samplesGF(400℃)/ZnO/GF(RT) with sandwich structure can be regarded asthe result of the top, bottom and the middle ZnO layer threeresistance in parallel, The MR value was rapidly decreasing, whichis closer to the MR value of "hot" GF layer. Most of the samplecurrent flowing through the "hot" bottom GF because of the lowresistance of the hot GF layer. Due to the coupling effect betweenthe top and bottom films,the coerccivity was decreased. However,with increasing of ZnO layer thicknesses, the coupling effectbetween the top and bottom become weakened, the MR value andmagnetic of the sample have be improved.(3) GF (400℃)/ZnO/Co/[ZnAlO)10samples was prepared by Aldoped ZnO.The MR effect is enhanced with Al doping, because Aldoping is an effective modulation at the top "cold"[Co/ZnAlO]10layerresistance, which is close to bottom "hot" GF resistance.And thenimproving the magnetoresistance obviously.In conclusion, we designed and optimized for both the "hot","cold" GF thin samples with sandwich structure. Which obtain alarger coercivity and considerable magnetoresistance value atroom temperature. So that the film include both excellent magneticand magnetoresistive properties. The study will lay a goodfoundation for the application of Spintronics devices devices.