The Investigations On The Electrochemical Fabrication And Related Properties Of Nanocrystalline Magnetic Thin Films

Magnetic materials with low high-frequency loss, high saturationmagnetization intensity, excellent corrosion resistance and resistance arepotential candidates in MEMS applications. Among various methodsemployed to prepare magnetic films, the electrodeposition method is one ofthe most promising technologies. That is because it is easy to control thecomposition, structure and property of the film through the adjustment oftechnical parameters and electrodeposition bath.This dissertation consists of three parts. In the first part (chapter2andchapter3), nanocrystalline CoNiFe soft magnetic film and CoNiFe-Si3N4composite films have been successfully prepared through cyclic voltammetry(CV) method. In addition, the effects of technical parameters such asterminate potential, Si3N4concentration, pH and agitation speed on the filmstructure have been studied. Then the surface morphologies, magneticproperties, constituent phases and hardness of the CoNiFe and CoNiFe-Si3N4films have been characterized by x-ray diffraction (XRD), scanning electronmicroscopy (SEM) and vibrating sample magnetometer (VSM). The optimalcondition has therefore been determined. The conclusions are follows:(1) The CoNiFe soft magnetic thin film with nano-structure has beenelectrodeposited through CV method. It possesses a high magnetization Bsof 2.03T and a low coercivity Hcof851.2A/m.(2) The electrodeposited CoNiFe-Si3N4composite film possesses highermicro-hardness and smaller nanocrystalline particles than the CoNiFe film.Meanwhile, its magnetic properties (Bs=1.82T, Hc=716.2A/m) is comparableto that of the CoNiFe film.In the second part(chapter4-5), the electrodeposition mechanism andkinetics process of nanocrystalline CoNiFe and CoNiFe-Si3N4thin filmshave been studied by using CV, electrochemical impedance spectroscopy(EIS) and chronoamperometry (CHR). In addition, the corrosion evolutionand corrosion resistance of the CoNiFe and CoNiFe-Si3N4thin films in3.5%NaCl solution have been investigated by using Tafel, EIS, EN combinedwith XRD and SEM. The conclusions are as follows:(1) In the electrodeposition bath of CoNiFe and CoNiFe-Si3N4thin films,both EISs consist of a capacitive arc at high frequency and an inductive arcat low frequency, respectively. As negative bias is applied and increased, theinductive component at low frequency is replaced by another capacitive arc.Meanwhile, the charge transfer resistance Rtand CPE of the theCoNiFe-Si3N4sysytem is higher than those of the CoNiFe system.(2) The addition of nano-sized Si3N4particles into the CoNiFeelectrodeposition bath makes the electrodeposition potential shift in apositive direction. Furthermore, the adsorption of the nano-sized Si3N4particles to the cathode leads to an increase in charge transfer resistanceduring the cathodic reduction process of CoNiFe-Si3N4codeposition, andtherefore increases the cathodic polarization. Finally, the synergetic functionof higher cathodic polarization and grain growth obstruction caused by sterichindrance of absorbed nano-sized Si3N4particles results in the grain refinement and surface densification of the CoNiFe-Si3N4film.(3) The electrodeposition of CoNiFe and CoNiFe-Si3N4thin films belongto anomalous co-deposition. Meanwhile, their nucleation-growth procedurefollows the style of3D transient nucleation/growth mechanism.(4) The CoNiFe-Si3N4film possesses higher corrosion resistance than theCoNiFe film. During the corrosion process of the CoNiFe-Si3N4thin films,two corrosion parameters SEand SGobtained by dimensional analysis methodthrough EN parameters can well describe the fast reaction (such as pittingunder electro-chemical control) and slow reaction (such as corrosion productformation under diffusion control).In the third part(chapter6), the electrodeposition of NdFeB rare earthpermanent magnetic film through CV method has been preliminarily explored.The complexing agent (C2H5NO2and NH4Cl) as additive during NdFeBelectrodeposition has been discussed. Then the effects of NdCl3concentrationand terminate potential on the morphologies of NdFeB thin films have beeninvestigated. It has been shown that the rare permanent magnetic NdFeB filmcould be obtained by electrodepositionin aqueous solution as the codepositionof Nd3+could be induced by Fe2+. Meanwhile, high Nd3+content acceleratesthe growth rate of NdFeB and results in grain coarsening. In addition, due tohigh activity of Nd3+in aqueous solution, the electrodeposition reaction of theNdFeB film is radical. This factor lead to crack in the NdFeB filmelectrodeposited in the bath with higher Nd3+concentration.