A Study Of Electric, Magnetic Properties And TMR Effect Of Co(Fe)-Cr₂O₃ Nano-scaled Granular Films

Recently, insulating granular films coomposed of nanostructured ferromagnetic(FM) metallic granules dispersed in an oxide insulating matrix have receivedconsiderable interests due to the discovery of pronounced Tunnelingmagnetoresistance (TMR) effect in them and to superior high frequency softmagnetic properties, giant Hall effect and mechanical properties, etc., they hold.This is a new development after the discovery of Giant magnetoresistance (GMR) inmetallic superlattice, metallic granular films and in magnetic tunneling junctions,and raises interesting fundamental problems,all of which make them very promisingfor applications. It is known that the tunnel-type GMR effect in insulating granularfilms is closely related to their microstructure, electrical transport and magneticproperties, however, up to now no profound systematical experimental andtheoretical studies on the origins and physical properties in these systems have beenproceeded. In this dissertation, Co-Cr2O3 and Fe-Cr2O3 granular films with superiorproperties have been successfully fabricated by RF co-sputtering technique underroom and 77K substrate temperatures for the first time. The microstructure,electrical transport and magnetic properties as well as their relation to TMR effectare systematically investigated by various techniques such as X-Ray diffraction(XRD), transmission electron microscopy (TEM), electrical diffraction (ED), X-Rayphotoelectron spectroscopy (XPS) and VSM, etc., the change of electrical transportand magnetic properties in these films with their microstructure is also shown. Someinteresting and valuable results have been obtained after the theoretical fitting ofexperimental data with the existed theory. The temperature dependence of TMReffects in these films and the way to get larger TMR effect is also studied. Our mainprincipal results obtained are as follows:(1) Microstructure and Magnetism:I. Series Co-Cr2O3 and Fe-Cr2O3 granular films have been successfully fabricated by RF co-sputtering technical under room and 77K substrate temperatures.II. We have found that, in the Cr2O3-type insulating granular films, whether is Co-based or Fe-based, insulating media is polycrystal, and the size of FM granules are large, which made all the films exhibit ferromagnetic properties at RT, even to the films made under 77K substrate temperature. This implies that this phenomena is closely related to the properties of this insulating matrix.III. By using XPS technique, it is confirmed that, in the whole metallic volumefraction fv range we studied, Co-Cr2O3 and Fe-Cr2O3 granular films are all consisted of metallic Co(Fe) granules and Cr2O3 insulating media. This implies that the samples we have made is the same as we expected. IV. We have found that, for the Co-Cr2O3 granular films fabricated under room temperature (RT), as the contents of Co in the film increases, the fcc-Co granules in the film gradually grow and their size increases from 20 nm for fv=0.15 to 33nm for fv=0.35, then decrease as fv further increase, to 18nm for fv=0.55, that is, there is a maximum size for fcc-Co granules. At the meantime, some Co in the films exist in the form of hcp-Co. As fv increases, the ratio of fcc-Co to the total Co-phase in the films also exhibits a maximum, and its content is closely related to the size of fcc-Co granules. When the proportion of fcc-Co in the total Co-phase is large, the size of fcc-Co granules is also large, correspondingly, Hc of the film is small. V. We have found that in the Fe-Cr2O3 granular films fabricated under RT that, as fv increases, Fe granules in the films become larger, their average size increases from 15nm for fv=0.15 to 19nm for fv=0.39, and further increases to 24nm for fv=0.55, that is, the size of Fe granules increases monotonously with fv; while Hc of the film decreases with fv increase, and reaches a minimum at fv=0.39, with fv further increase, Hc also increases, despite the fact that Fe granules are very large at that case, which needs to be further studied.(2) Transport Properties: I. Measurements of temperature coefficient of resistivity (TCR) for Co-Cr2O3 granular films fabricated under RT indicates that their TCR change sign from negative to positive as fv increases. For fv≤0.5, TCR<0, these films are in the dielectric state; for fv>0.5, TCR>0, there is a normal metallic conductance. When fv=0.55, TCR changes its sign at about 65K: when T>65K, TCR>0, and metallic conductance exists in the films; but when T<65K, TCR<0, thermally activated tunneling becomes the main conductance mechanism. This implies the co-existing and competition of metallic and tunnel conductances. II. TCR measurements for Fe-Cr2O3 granular films fabricated under RT indicates that their TCR change sign from negative to positive as fv increases. For fv≤0.55, TCR<0, these films are in the dielectric state; for fv=0.28, TCR also changes its sign at low temperature: when T>105K, TCR>0, but when T<105K, TCR<0, implying the competition of conductance mechanisms. Increase in resistivity at low temperature may be related to the Coulomb Block effect.