Investigation Of Magnetoresistance Effect And Spin Injection In The Films Composed Of Co And ZnO Semiconductor Etc.

Since the discovery of giant magnetoresistance effect in Fe/Crmultilayers, the magnetoresistance (MR) in magnetic multilayers,granular films and magnetic tunnel junctions composed of magneticmetals and non-magnetic materials have been studied extensively forpotential application in spintronic devices, such as magnetic sensors,hard disk reading heads and magnetic random access memories.There is little research about the MR of semiconductor barrier.Semiconductor can provide a much lower barrier height compared toinsulator with a decrease of resistivity. Moreover, the increasedthickness of semiconductor reduces the probability of pinholes.According to the reports in the literature, room temperature MR hasbeen observed in magnetic metal/semiconductor films. However,there is ample room for further investigation of exploring the origin ofthe MR in magnetic metal/semiconductor films and improving the MRratio. In this paper, the films composed of Co and ZnO semiconductoretc. were deposited on glass substrates by sequential magnetronsputtering at room temperature. The structure, magnetic properties,magnetoresistance and spin injection at the interface between metaland semiconductor were studied.â‘ the magnetization and roomtemperature MR in Co/ZnO films was enhanced by Al doping;â‘¡theorigin of the MR in Co/ZnO films was studied compared to that inCo/Al₂O₃, Co/C, and Co/Cu films;â‘¢the room temperature MR ofCo/ZnO films depends on the resistance of Co/ZnO films. The resultsare summarized as follows:(1) Co/ZnO films were deposited by sequential deposition ofultrathin Co layer fixed 0.6 nm and ZnO layer varied from 0.4 to 3.0 nm.The granular films consist of Co particles dispersed in ZnO matrix anda few of +2 valence cobalt ions substitute for Zn2+ions in ZnO lattices.The large negative MR of 11.9% and 26% at room temperature and lowtemperature, respectively, are observed in the Co/ZnO film. Theobserved linear relation between lnρand T^.1/2indicates aninter-particle tunneling conduction mechanism in the films. At highertemperatures, lnρstarts to slightly deviate from linear relation,suggesting the presence of spin-independent high-order hopping athigher temperatures due to defects within the ZnO matrix. Theenhanced MR at low temperature is ascribed to spin-dependent high-order tunneling.(2) The structure and MR effect for Co/ZnO, Co/Al₂O₃, Co/C andCo/Cu granular films were studied comparatively. Four types of filmsexhibit similarities in structure, comprising Co particles embedded innonmagnetic matrix. The average size of Co particles decreases withan increase of the difference of surface energy between metallic Coand the nonmagnetic matrix, which are smaller than that of cobalt.The absence of MR effect in Co/C films at room temperature isprobably due to the conductivity mismatch between the metallic Coand the semimetal carbon. The large room temperature MR in Co/ZnOgranular films is related to the partially polarized electrons at theinterface between Co particles and ZnO matrix.(3) The Co/ZnAlO films were prepared using sequential magnetronsputtering at room temperature by Al doped ZnO (Al: 2at.%). The filmshave a similar structure consisting of Co nanoparticles dispersed insemiconductor matrix. The magnetization of the films comes frommetallic Co particles and the graded magnetic semiconductor at theinterface between Co particles and semiconductor matrix. The MR ofCo/ZnAlO granular film reaches -12.3% at room temperature, which isthe largest value in magnetic metal/semiconductor films by far. Boththe magnetization and the MR effect are higher for films containingCo/ZnAlO than for Co/ZnO. The enhanced magnetization of the films with Al doping is probably due to the enhanced carrier concentrationenhancing the magnetization of the graded magnetic semiconductor.The larger MR in the film with ZnAlO occurs because of the larger spinpolarization of the semiconductor which acts as the spin filter, whichis very promising to improve the spin injection efficiency intosemiconductors.(4) Many influential factors of room temperature MR in Co/ZnOfilms was studied by a lot of experimental data by changing theconditions of the sputtering pressure and ZnO normal thickness. Thestudy indicates that the room temperature MR is related to theresistance of Co/ZnO films instead of the sputtering pressure and ZnOthickness. The large room temperature MR (more than 8%) of thefilms is observed on the scale of 1300Ω<R<6000Ω. And the MR valuedecreases if the resistance is too small or too large. The structure,magnetic properties and electrical transport properties of Co/ZnOfilms were investigated. We found that the MR dependence on theresistance is ascribed to the electron transport mechanism in the film.The large room temperature MR is related to the tunnelingconductance between the magnetic nanoparticles. Whereas, thedecreased probability of tunneling induced by too large resistancebetween magnetic particles results in a small MR value.In conclusion, large room temperature negative MR and spin injection efficiency were observed in Co/ZnO and Co/ZnAlO films,which is caused by spin-dependent tunneling throughsemiconducting barriers. We found that the enhanced MR comesfrom the spin filtering of the magnetic semiconductor at theinterface between the metallic particles and semiconductor matrix.This provides a new way for the effective electron spin injection atroom temperature between the metal and semiconductor, which isexpected for potential application in spintronics devices.