Study On The Electrochemical Fabrication And The Related Structure/Properties Of Nanocrystalline CoNiFe Soft Magnetic Thin Films

Soft magnetic materials have been widely used in the areas of magnetic data storage devices (such as hard disk drives (HDD)), magnetic-MEMS actuators, and other magnetic sensing devices. Up to date, various methods have been employed to prepare soft magnetic thin films. Among them, the electroplating method is one of the most promising technologies in industrial level because of its ability to "tailor" deposit structure and properties and to form a high-aspect-ratio film. A strong candidate of soft magnetic materials in many MEMS applications is the CoNiFe ternary alloy with its capable of providing an extremely high B_s combined with reasonably low H_c.In this thesis, we have tried to prepare nanocrystalline CoNiFe soft magnetic films with better magnetic properties (higher saturation magnetization B_s and lower coercivity H_c) and higher corrosion resistance by using cyclic voltammetry (CV) and probe into the corresponding deposition mechanisms, especially its nucleation/growth process. It is believed that the research has not only a significant theoretical value but also broad application prospects.Firstly, the effects of principle technological parameters (such as the concentration of three metal ions, potential range of CV, pH value and so on) on the microstructure of electrodeposited CoNiFe films were studied in this thesis, and finally we improved a set of operating conditions. Then the surface morphology, composition, crystal phases and magnetic properties of the optimized CoNiFe alloy deposits were determined using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), X-ray diffractometer (XRD) and a vibrating sample magnetometer (VSM). Based on the above results, the optimized CoNiFe film is composed of very spherical and uniform nanograins, and it also possesses good magnetic properties (with a high B_s value of 2.03T and a low H_c of 10.7Oe) and high corrosion resistance.Secondly, the kinetics and mechanism for electrocrystallization of nanocrystalline CoNiFe thin film formed on brass substrate was studied using CV, chronoamperometry (CHR) and electrochemical impedance spectroscopy (EIS) techniques. The results showed that at higher potentials, the nucleation/growth process of CoNiFe ternary alloy followed 3D instantaneous mechanism, while in the case of lower potentials it followed 3D progressive mechanism. With potential increasing, charge transfer resistance reduced, which leads to increasing of nucleation rate and much finer of the grains.Finally, Nanocrystalline CoNiFe soft magnetic thin films electrodeposited from the baths with boron/phosphorus containing additive have been further investigated to optimize soft magnetic properties and corrosion resistance. The results showed that both additives had an affect on the microstructure of the deposite. For both systems, the boron containing additive improved the magnetic properties (with a B_s value of 1.94T and H_c of 7.6Oe) and the corrosion resistance of the deposit, while Sodiumhypop hosphite as additive, we failed to obtain an excellent CoNiFe alloy film (with a low B_s value of 1.25T and a high H_c of 156.9Oe), and the inclusion of P sacrificed the magnetic and anticorrosion properties.