Effects Of High Magnetic Fields On Tuning The Structure And Magnetic Properties Of Coevaporated Magnetic Nanoparticle Composite Films

Magnetic film is one of the most active fields in advanced materials.It is irreplaceable in many fields of such as high density storage,magneto-resistance sensor,micro electro mechanical system,micro-transformer,and microwave absorption.In order to improve properties,the particle size is required as small as possible.However,superparamagnetic effect will occur in films because small size nanoparticle is easy to be influenced by thermal disturbance.This restrains particle refinement and magnetic properties improvement.Insulating medium in nanoparticle composite films will affect intergranular exchange coupling interaction,magnetocrystalline anisotropy,demagnetization effect and so on.It offers an effective way to refine particle and to improve magnetic properties simultaneously.But it is hard to control particle size and distribution in the composite films.It needs to develop a new preparation or control method to realize nanoscale structural control and properties improvement of nanoparticle composite films.In this study,the magnetic nanoparticle composite films were prepared by molecular beam vapor deposition(physical vapor deposition)method.The nanoscale structure and magnetic properties of the nanoparticle composite films were tuned via applying magnetic fields and changing growth mode,growth temperature and so on.This dissertation studies the influence of structural evolution on magnetic properties in coevaporated Fe-SiO2,coevaporated FeNi-SiO2 and multilayer FeNi-SiO composite films under high magnetic field.It clarified the mechanism of microstructure evolution tuned by high magnetic field,and confirmed the relation between microstructures and magnetic properties of the films.Finally,a method was provided to tune the magnetic properties by varying particle size and distribution of nanoparticles.The main results are summarized as follows.(1)When high magnetic field was applied during coevaporation of Fe-SiO2 nanoparticle composite film,the SiO2 on the film surface accumulating around the nanoparticle boundary resulted in the decrease of Fe content in the top layer.Thus,the composite film was formed into two layers in direction perpendicular to substrate.Meanwhile,columnar crystal of Fe films changes to Fe-SiO2 equiaxial crystal and thus the nanoparticle composite films of Fe nanoparticles embedded in SiO2 matrix were formed.In addition,the SiO2 decreased magnetic phase content in composite films comparing with the Fe films in the same volume.This resulted in saturation magnetization Ms of the Fe-SiO2 composite film decreasing by 73.2%compared with that of the Fe film.The coercivity Hc increased by 350%than that of the Fe film deposited at a given SiO2 content.The results indicated that source temperature,substrate temperature,SiO2 content and high magnetic field can adjust particle size and improve magnetic properties of films(2)When high magnetic field was applied during coevaporation of FeNi-SiO2 nanoparticle composite film,the nanoparticles with diameter in the range of 3?5 nm distributed uniformly in the film whose thicknesses was 100 nm.With the increase of magnetic field intensity,Zeeman energy of magnetic field caused SiO2 accumulating around the FeNi particle and restraining particle coalescence and growth.Fe and Ni were different in each nanoparticle.Coexistence of bcc and fcc phases was found in the FeNi nanoparticles.The size and Ms of nanoparticles increased with the film thickness.The nanoparticle size decreased with applying high magnetic field which increased the Ms.However,the Hc had no significant decrease because the magnetic domain reversal was affected by disorder SiO2 among particles.This indicated outstanding performance of the?nanoparticle composite films.However,coalescence between nanoparticles was found during the process of the coevaporation method.(3)When high magnetic field was applied during multilayer FeNi-SiO nanoparticle composite film,the nanoparticle size was tuned by the thickness of magnetic layer.The coalescence of nanoparticles was suppressed by insulating layer thickness and insulating medium content in the magnetic layer.This leads to the uniform distribution of the nanoparticles.The contents and thicknesses of SiO had important effects on the magnetic properties.The Ms(397 emu/cm3)of the composite film with 6 nm magnetic layer thickness is 43.8%higher than that of the film with a 3 nm thickness magnetic layer because the magnetic phase content was lower in the thicker magnetic layer.Changing SiO volume fraction in the films can affect Hc when nanoparticle size in the film was small.