【作者】 叶婧；

【导师】 陈乾旺；

【作者基本信息】 中国科学技术大学 ， 无机化学， 2008， 博士

【摘要】 像温度和压力一样,磁场有可能发展成为调控材料结构和性能的重要参数。本论文致力于在磁场下制备磁性Ni薄膜等材料,研究磁场如何影响生长习性、磁畴结构和磁性能,得到了一些结果,详细内容归纳如下:1.探索在低温溶剂热条件下沉积薄膜的工艺技术。成功发展了一步化学法在硅基底上沉积金属铜薄膜工艺,制备的铜膜厚度约为2.2μm,是由大小为45～60 nm的铜纳米粒子组成的多晶膜,铜膜具有海绵状多孔的特殊结构。我们制备的铜膜具有较高的电阻率和较低的杨氏模量,分别为300μΩcm和0.46 GPa,研究发现这与其特殊的海绵状结构有关。该工作为进一步制备镍薄膜奠定了实验基础。2.基于沉积铜膜的工作基础,我们设计了一个低温水热反应来沉积磁性镍膜,并在反应体系中引入了磁场强度为0.30 T的外加磁场,研究磁场对磁性薄膜结构及性能的影响。制备的镍膜具有银色金属光泽,肉眼观看平整致密,且与硅基底结合紧密。其它条件完全相同的情况下,施加外磁场能使薄膜的厚度显著增加:磁场下合成的薄膜厚度约为1.35μm,而没有磁场条件下制备的镍膜厚度仅约为0.80μm。磁力显微镜分析发现外加磁场可以诱导膜内磁畴趋于有序,呈线性排列,不同于无磁条件下杂乱无章的磁畴结构。磁畴结构的显著不同导致了磁性能的变化:外加磁场条件下合成镍膜的饱和磁化强度和矫顽力分别为495 emu/cm~3,65 Oe,比无磁场条件下合成的样品的数值要高(分别为338 emu/cm~3,48 Oe)。这些结果说明外加磁场也可以引入磁性薄膜的合成中,并对磁性薄膜的磁畴结构和磁性能产生影响。在反应过程中施加磁场的合成方法也可以推广用于其它铁磁性薄膜的制备,如钴薄膜等。3.在简单水热体系中,合成了大小约为20μm的磁性镍的树叶状枝晶结构。研究发现枝晶具有独特的取向,主干首先沿[111]方向生长,然后一侧分枝沿[-111]方向生长,而另一侧分枝则沿[100]方向生长。因为枝晶的主干生长方向[111]是立方镍晶体的易磁化轴方向,这可能导致了样品矫顽力和剩磁比较低,分别为77 Oe和0.10。不过,枝晶结构所具有的高结晶性提高了样品的饱和磁化强度(48emu/g)。对于在水热这样一个平衡体系中镍枝晶是如何形成的,我们提出了一个新的模型——磁诱导扩散受限生长的机制来解释。在溶液中,部分镍离子首先被还原生成镍核,这些铁磁性镍核产生的磁场可以诱导溶液中剩余的顺磁性镍离子聚集在镍核周围,镍核附近的镍离子浓度较高,导致电极电势增大而使得镍核附近更容易发生还原反应。由于前驱物溶液中的镍离子浓度很低,因此镍不能源源不断的供给,而是受限的。因为镍供给的受限,导致镍核首先沿着一个方向生长,形成主干,镍核表面后被还原的镍沿着主干由底部往上输运,提供给主干的生长,侧枝随之也开始生长,最后形成了树叶状枝晶结构。有报道称其它磁性材料也能在平衡体系中形成枝晶结构,用我们的模型也能给予很好的解释。4.为了使外加磁场对反应过程产生影响,我们设计了一个低温并且缓慢进行的水热体系,在不同磁场强度下(0 T,0.15 T,0.30 T)合成金属镍。发现在磁场强度较弱的情况下,得到的产物不全是纳米海参分级结构,还有部分纳米海胆结构,而当磁场强度足够大时(如0.30 T),则可以诱导次级结构成纳米海参分级结构。在这里我们提出了一个可能的纳米海参分级结构的磁场诱导组装机理:首先溶液中的镍离子被还原形成一个个的镍核,其次,在磁场作用下,被还原的镍核沿外加磁场的磁力线方向排列,并在CTAB的控制下,镍核逐渐一维生长成短棒,这些短棒互相交错最终形成了分级结构。对在不同磁场强度下(0 T,0.15T,0.30 T)得到的产物进行磁性能的测量,发现随着磁场强度的增加,所得产物的饱和磁化强度增大,分别为:16.06 emu/g,42.58 emu/g,46.25 emu/g,这可能因为磁场作用导致产物的结晶性变好。然而,随着磁场强度的增加,样品的矫顽力却随之降低,分别为:298 Oe,216 Oe,194 Oe,但都高于对应的体材料的矫顽力大小(100 Oe),磁场诱导较为有序和稳定的分级结构的形成可能是导致矫顽力降低的原因。更多还原

【Abstract】 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（Ⅱ）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.更多还原