Controllable Preparation And Magnetic Properties Of Parallel Aligned Ni Nanocone Arrays

On the nanoscale,the physical properties of a single nanostructure are greatly affected by its geometry.Hence,new geometries will give nanostructures exceptional properties not found in conventional bulk counterparts.For an assembly of single nanostructures,the overall properties and functional performances are further influenced by their mutual orientation.In recent years,one-dimensional nanomaterials have been widely used in optics,electricity,magnetism and mechanics.As a new structure,nanocone has aroused widely interest.Up to date,several methods have been developed to fabricate nanocones of various materials for pursuing the conical shapeenabled extraordinary properties,however,the fabrication methods for magnetic metallic nanocones are very limited in comparison with other materials.There are some problems such as non-strict conical shape,too small size,random orientation and so on,which lead to the averaging magnetic responses and consequently make their intrinsic properties elusive.In this thesis,arrays of magnetic metallic Ni nanocones have been firstly prepared by ion-track template method in combination with electrochemical deposition.The morphology and structure of the Ni nanocone arrays were investigated,and its static and dynamic magnetic properties were studied.Ni nanocones arrays were electrochemically deposited in ion-track templates containing conical nanochannels.According to the results of scanning electron microscopy(SEM),the Ni nanocone arrays have large aspect ratio,homogeneous shape and size,uniform distribution,controlled densities and parallel alignment.Transmission electron microscopy(TEM)and X-ray diffraction(XRD)characterizations show that the prepared nanocones are typical character of polycrystalline face cubic-centered Ni.The shape of the nanocone is perfectly conical and has very smooth contour.The static magnetic properties of the Ni nanocone arrays are studied by vibrating sample magnetometer(VSM).The results reveal that the Ni nanocone arrays have strong magnetic anisotropy,and the easy axis of magnetization is along the axial direction of the nanocone,which is mainly related to the shape anisotropy caused by the large aspect ratio.When the angle between the external magnetic field and the cone axis is low(?<70°),the reversal of magnetization is driven by a vortex domain wall mode.When the angle is high(?> 70°),it is driven by the nucleation and propagation of a transverse wall.The coercivity of the Ni nanocone arrays increases first and then decreases with the increase of angle,and reaches the maximum value at 70°.For comparison,we also prepared two samples of Ni nanowire arrays with the same size as the tip and the bottom of the nanocone by the same method,and measured their static magnetic properties.The results show that the coercivity of the nanocones is largely determined by the large size of cone bottom.In addition,due to the constraint of the conical shape on the edge magnetic moment,the Ni nanocone is more difficult to magnetize than the other two kinds of Ni nanowires.At last,we use the vector network analyzer(VNA)and Brillouin light scattering(BLS)to analyze the resonance peaks of Ni nanocone arrays in the high-frequency range of 2-17 GHz.This is the first observation of bulk spin wave by BLS in magnetic nanocone structures.The magnetic loss mechanism of the Ni nanocones is the contribution of natural resonance and exchange resonance.The natural resonance is caused by the magnetic moment of the cone base responding to the uniform precession of the external field.The exchange resonance is due to non-uniform precession response of the exchange coupled spin standing wave to the external field.This double resonance behavior indicates that the Ni nanocone can find applications in multi-band microwave absorption.