Preparation,Magnetism,and Magnetoelectric Transport Properties Of Nanostructured Ferromagnetic/Nonmagnetic Composite Thin Films

Magnetoresistance(MR)materials have wide applications in the field of the magnetic sensor and magnetic recording.The film composed of ferromagnetic(FM)metal and non-magnetic(NM)substrate has a significant effect on the electron transport properties because of its rich interface effect.Although the room temperature magnetoresistance effect of FM/NM matrix films has been extensively studied,an universal explanation is missed,and the magnetoresistance mechanism and the approaches to further improve the magnetoresistance value are still needed to explore.Previous studies have shown that the nano composite films of the carbon and silicon nitrogen non-magnetic matrix doping with magnetic Fe,Co,or Ni metals not only exhibit a large magnetoresistance effect but also have a conductive regime transition when the doping amount of the metals changes.Therefore,figuring out the mechanism of magnetoresistance effect and the effect of magnetic metals on the structure and mobility of the carriers of non magnetic matrix films is crucial.It has great scientific and practical significance,and is beneficial to promote the application of the high performance,low cost carbon materials and silicon materials in the field of electronic information technology.In this thesis work,by doping transition elements(mainly high spin polarization FeCo)into various non-magnetic matrix,including narrow band gap semiconductor SiN,wide band gap semiconductor oxide ZnO,amorphous C,and an insulator MgO,the effects of composition and structure on the magnetic and transport properties were studied.The granular and multilayer FM/NM nanocomposite films were prepared by sputtering deposition.The X-ray diffraction(XRD),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),atomic force and magnetic force microscope(AFM,MFM)were used to observe and analyze the structure and crystallization phases.The magnetic,electric and magnetoelectric transport properties were measured by using the PPMS(Physical Property Measurement System).The results are summarized as follows:Firstly,the FeCo-Si-N composite films were prepared by reactive magnetron sputtering deposition.The structure of FeCo particles distributed in the Si-N matrix were successfully obtained in the as-deposited films.The structural and magnetic characteristics were clearly characterized by electron microscopies and MFM.The magnetic transport properties of FeCo-Si-N were studied systematically,and a large room temperature positive magnetoresistance was obtained.The MR varies linearly with the magnetic field in a wide range of magnetic field.Based on the analysis of the relationship between the magnetic properties and the change of the magnetoresistance with the composition,the domain structure of the granules were determined indirectly,and the structural characteristics of getting the maximum MR were concluded.The study of the influence of temperature on the MR found that the electron transport transport mechanism at low temperature is tunneling conduction,and the magnetoresistance is tunneling magnetoresistance(TMR).The sign of the magnetoresistance effect is changed near the blocking temperature,indicating the effect of magnetic moment alignment of the MR.Both the orbit effect and spin effect play important roles in the origin of the positive MR.Secondly,the FeCo-Zn O thin films were prepared,and their magnetoelectric properties were studied.It is found that the sputtering power has significant impact on the structure and properties.For the films prepared with low sputtering power,the Fe and Co are not in the zero valent metal state,and no FeCo cluster was found.The MR is positive(PMR)and not sensitive to the composition and thickness,indicating the low relativity between magnetism and magnetoresistance.The results mean the PMR comes from the orbit effect rather than spin effect.This PMR increases up to 350% at 220 K,and the dramatic change indicated the phonon scattering influence.For the films prepared with higher sputtering power,Fe and Co atoms in metal state were found,and the film is superparamagnetic.Negative magnetoresistance effect occurs in thicker samples,and it may be derived from the tunneling magnetoresistance generated by the FeCo clusters.Thirdly,the FeCo and amorphous carbon composited [C/FeCo]_n multilayer films with various periods were also prepared,and the structure and magnetoelectric properties were studied.It is found that the MR changes from positive to negative as the period number n increases.The PMR varies linearly with the magnetic field,and is not saturated in the high field.The conducting regime was studied by analyze the relationship of the resistivity and the temperature.The fitting curves of ln R--1/2and ln R--1/4 of different n shows that,at low temperature,they are both in variable range hopping(VRH)regime,and the MR are both negative,but at higher temperature,the films with less period is no longer in VRH regime because of the thermal energy,and its MR changes to positive.For the films with more periods,because of its stronger localization caused by the interfaces and defects,it remains VRH and negative MR even the temperature increases.The significant different MR effects at various conduction regimes indicate that the type of transport regime plays an important role in MR origin.In the last part of this work,the potential of the FePt-C and FePt-MgO FM-NM composite films in the perpendicular magnetic recording was explored.The magnetron sputtering co-deposition was used to prepared FePt-C and MgO/[FePt-MgO] films.The immiscibility between FePt and C and MgO was used to form the C or MgO separator between the FePt granules.The influence of the composition and annealing temperature on the microstructure,magnetism was studied.And it is found out that proper separator amount is needed to get high coercivity.Increasing the annealing temperature is beneficial to the formation of FePt phase in the films,but too high temperature destroys the film quality,and reduces the coercivity.The magnetoresistance effects of the FePt-C and FePt-MgO were also studied.In conclusion,the FM/NM nanocomposite films were prepared by the magnetron sputtering deposition,the high magnetoresistance was obtained in the FeCo-Si-N,FeCo-ZnO and FeCo/C films.By combining the analysis of the structure,magnetism,transition of the sign of magnetoresistance and the resistance temperature dependence,the electric transport and magnetic transport mechanisms of the FM-NM composited films were studied.A further understanding of the spin-dependent transport of FM-semiconductor composited films was obtained.For the FePt-C and FePt-MgO granular films,the effect of composition and annealing on the structure and magnetism was studied,and the magnetoresistance effect was also studied.These work are of significance for the development of the spintronic theories and devices and new magnetic recording medium materials.