Magnetic-Field-Effect On The Formation Of Nickel Film And Other Magnetic Materials

Similar to temperature and pressure,magnetic fields could have a strong influence on the structures and properties of materials.The objective of this dissertation is to investigate the magnetic domain structure and physical properties of materials such as Ni thin films formed under magnetic fields.More details are summarized below:1.A one-step electroless deposition method was developed to synthesize copper film with favorable mechanical adhesion on silicon substrate.The as-prepared copper film is polycrystalline and composed of copper nanoparticles with a diameter ranged from 45 to 60 nm,thickness of which is approximate 2μm.The film possesses the special sponge-like structure.Due to its special structure,the as-prepared copper film has higher resistivity(300μΩcm)and lower Young's modulus(0.46 GPa)than the bulk copper and copper films prepared by other methods.This work can lay the foundation of our following research on prepare nickel.2.Based on the results of copper film,we designed a simple hydrothermal approach during the course of which a 0.30 T external magnetic field could conveniently applied to prepare the nickel film and study the magnetic effect on as-prepared film. The prepared nickel film possesses shiny-silver and plain surface in macroscopic scale and strong binding ability between the film and the silicon surface.The magnetic field induces the increase of the thickness of the film:the film prepared in an external magnetic field applied is 1.35μm,while that of the film synthesized without an external applied field is 0.80μm.By analysing of MFM,it can be concluded that an external magnetic field applied can organize the domain structure in regular patterns in the film,which will result in the improvement of magnetic properties of the film including Ms and Hc.Magnetic measurement indicates that saturation magnetization and the coercivity of the sample prepared with a 0.30 T magnetic field is 495 emu/cm^-3and 65 Oe,which are higher than those of the other sample without a magnetic field applied(338 emu/cm^{-30 and 48 Oe,respectively}.Based on the discussion above,it is expected that an applied magnetic field could be employed as an important method to control magnetic properties of ferromagnetic film materials. Furthermore,the method in this paper during the course of which an external magnetic field can be conveniently applied at relatively low reaction temperature is useful to synthesize other ferromagnetic films,such as cobalt film,etc.3.A distinctive type of nickel micro-leaf dendrite with length about 20μm has been synthesized via a simple hydrothermal route without any surfactant.Upon detailed studies,it is revealed that the stem firstly grows along[111],one unilateral branch grow along[-111]and then the other unilateral branch grows along[100],respectively. The orientation of the stem is the magnetic easy axis of a cubic nickel crystal,which could possibly result in the decreased Hc and reduced remanence Mr/Ms for the sample.And the good crystallinity of the nickel dendritic micro-leaf would improve value of Ms.A new model called magnetic-induced aggregation and limited diffusion is proposed to illustrate the growth of ferromagnetic nickel micro-leaves formed in equilibrium hydrothermal system.Parts of nickel ions in solution were firstly reduced to nickel nuclei.These ferromagnetic nuclei generate magnetic field which could induce paramagnetic nickel ions to aggregate among the nuclei.With the aggregation of nickel ions,the concentration of nickel ions increased,resulting in the enhancement of electrode potential,which made Ni(â…¡)ions easier to be reduced by NaH₂PO₂.With the low concentration of nickel,which leaded to limited diffusion and supply,stem along[111]orientation initiated fast growth,and then generated a pole.The nickel ions among the nuclei were then reduced and transported from the bottom to the top along the stem.With the transportation of nuclei,the branches began to grow.All of the branches become bigger and thicker and finally interconnected to form the dendritic micro-leaf structures.The present model could explain the formation of other dendritic ferromagnetic materials in equilibrium hydrothermal system which has been reported before.4.To make sure an external magnetic field can affect the cross of reaction,we designed hydrothermal approach with a low-reaction rate and low temperature to synthesize nickel under different magnetic fields(0 T,0.15 T,0.30 T).Varying the intension of applied magnetic fields could change the structure of the prepared products.It was observed that new nanosea-pumpkin structure composed of nickel nanorods was formed under a 0.30 T magnetic field.While under a 0.15 T external magnetic field,both the nanourchin structure and nanosea-pumpkin structure can be observed.A formation process has been proposed to illustrate the growth of nanosea-pumpkin.Firstly,the nickel ions were reduced to nickel nuclei.Then the nickel nuclei were induced to array along the magnetic line of force of an external magnetic field.With the controlling of CTAB,the nickel nuclei grew to nickel nanorods.Then,the nanosea-pumpkin structures were formed at last.Magnetic measurement shows that with the increased intension of applied magnetic fields,the Ms for as-prepared products become high,they are 16.06 emu/g,42.58 emu/g,46.25 emu/g,respectively.The increased crystallinity could explain this phenomenon. However,Hc for the products become lower(298 Oe,216 Oe,194 Oe,respectively), which might be associated with the comparatively ordered and stable nanosea-pumpkin hierarchic structures.