Study Of Preparation And Room Temperature Ferromagnetic Properties Of CeO₂ Nanoparticles

Spintronics is a new inter-discipline relating to magnetism,electronics and informatics.Diluted magnetic semiconductors(DMS),as main promising materials for the future spintroncis devices,have attracted a great deal of attention nowadays. Generally speaking,DMS can be realized by replacing the no-magnetic cations of the semiconductors with magnetic transition ions or rare earth ions.Then the semiconductors would have the magnetism after dopping.Since 1960,people were searching for the room temperature DMS(RTDMS) all along.A major boost to these efforts was realized by a theoretical work by Dietl et al.in which they predicted the RTDMS in Mn doped ZnO and GaN.Only within a few years,RTDMS has been observed in several DMS systems,for instance,3d transition metal doped ZnO,TiO₂, SnO₂,In₂O₃ and CeO₂.However,most unexpectedly,there are several research groups reported the observations of room temperature ferromagnetism in the un-doped nanostructured metal oxides,which was called as "d0 ferromagnetism".The origin of such ferromagnetism is unclear now,which has challenged our current magnetism theory greatly.Therefore,great deals of work need to do to clarify this phenomenon.In this thesis,various kinds of chemical methods,including sol-gel,sonochemical co-precipitation,hydrothermal,microemulsion methods,are used to prepare the Co or Fe doped CeO₂ nanoparticles.X-ray photoelectron spectrometer(XPS),X-ray diffraction(XRD),vibrating sample magnetometer(VSM),nuclear magnetic resonance(NMR),high-resolution transmission electron microscope(HRTEM),and Raman spectroscopy(Raman) are performed on samples to investigate microstructure, magnetic properties,and chemical state of samples.The main conclusions are as following:1) All as-prepared undoped CeO₂ nanoparticles using various chemical methods mentioned above are ferromagnetic in room temperature,and their magnetization saturation are in the same order of magnitude.2) Co or Fe doped CeO₂ nanoparticles prepared by sol-gel and co-precipitation methods are ferromagnetic in room temperature too.However there was not an obvious enhancement of magnetization saturation,comparing to the undoped samples.3) After being annealed in H₂/Ar,the ferromagnetism of doped and pure CeO₂ nanopartilces prepared by sol-gel method enhanced.By using Raman spectroscopy it is proved that there is a larger amount of oxygen vacancies in the H-annealed samples. However,it can not be confirm to prove that the ferromagnetism arose from oxygen vacancies.4) After being annealed in different reduction temperatures,duration and partial pressure of hydrogen,it was observed that ferromagnetism of samples respectively prepared by sonochemical co-precipitation,hydrothermal,microemulsion,and homogeneous precipitation method increased at first and then decrease with increasing the reduction degree.5) After deconvoluting the XPS spectra of samples prepared by co-precipitation method we obtained the concentration of Ce³⁺.And then we got the concentration of Ce³⁺ in the surface monolayer though semiquantitative calculation.What was observed that the higher the number of surface Ce³⁺/Ce⁴⁺ pairs,the more robust the ferromagnetism appears.Thus,we suggest that the double exchange interaction between the mixed valent Ce³⁺/Ce⁴⁺ pair via O^2- ion on the surfaces causes the ferromagnetism in CeO₂ nanoparticles.