Controllable Fabrication And Magnetic Properties Of Co Nanocone Arrays Based On Ion Track Technology

As dimensions of the magnetic material get comparable to fundamental magnetic length scales,magnetic configuration and magnetization reversal become surprisingly unusual in emerging nontrivial magnetic structures,such as anomalous Bloch-point domain walls,vortices,and skyrmions.Previous studies have demonstrated that the magnetic configurations and magnetization reversal are largely affected by the geometrical shape and size of a magnetic nanostructure.For a magnetic nanowire,the magnetic configurations are mainly influenced by shape anisotropy,magnetocrystalline anisotropy,and exchange interaction.In contrast to the cylindrical shape of nanowire,the nanosized conical shape endows 1D structure with an additional freedom of modulation,which substantially stems from the unique size feature progressively diminishing along cone length.Similar to the shape effects,magnetocrystalline anisotropy is another pivotal factor in determining magnetic configurations.Thereafter,a large magnetocrystalline anisotropy can arm a magnetic structure with highly tuneable anisotropy via collaborative or competitive impact with shape anisotropy.Compared with other materials,HCP Co exhibits a large magnetocrystalline anisotropy energy constant with large magnetocrystalline anisotropy.And so far,the combination of perfect conical shape and large magnetocrystalline anisotropy has never been achieved.Therefore,the understanding of the magnetic structure and the magnetization reversal mechanism of conical nanostructures remain unclear.In addition,the combination of conical shape and large magnetocrystalline anisotropy will lead to unique microscopic effects that are expected to improve its microwave absorption properties.In this thesis,Co nanocone arrays with perfect conical shape and smooth cone walls were firstly prepared by the ion track template method combined with electrochemical deposition,and the effect of p H of the deposition solution on the texture of the nanocone arrays was investigated.The morphology and structure of nanocones were characterized by various analytical tools,and the static and dynamic magnetic properties were investigated by means of a vibrating sample magnetometer and vector network analyzer,and the mechanism related to the magnetic properties was investigated.In this thesis,the Co nanocone arrays and Co nanowire arrays were prepared using ion track template method combined with electrochemical deposition.Scanning electron microscopy(SEM)showed that the prepared Co nanocone arrays were perfectly conical shape with smooth cone walls.Transmission electron microscopy(TEM)and X-ray diffraction(XRD)characterized the structure of the prepared Co nanocones as a typical hexagonal structure(HCP)of pure metallic cobalt with tips polycrystalline structure.By controlling the p H of the deposition solution,it was found that the p H had a great influence on the texture of the Co nanocone arrays.In particular,when p H=6,the main texture in the Co nanocone is [001] texture.Static magnetic research on Co nanocones using vibrating sample magnetometer(VSM).The results show that the anisotropic magnetic response of Co nanocone arrays and Co nanowire arrays with the same texture exhibits intermediate characteristics,lying between thinner thicker nanowires.We also performed a first order reversal curve(FORC)test on Co nanocones using VSM.It was found that the small tail in the T shape distribution observed in the nanocone dominated by the [001]texture indicates the reversal process of the magnetization of the parallel-aligned magnetic moments in the tip part of the cone along the length direction.In the Co nanocone,the magnetic moments in the tip region are dominated by demagnetization energy,while the magnetic moments at the base of the cone are dominated by exchange interactions.For the cone base region of the nanocone,there may be static magnetic interactions between neighboring nanocones.In the nanocone of the [001]texture,the magnetocrystalline anisotropy interacts synergistically with the shape anisotropy.In contrast,magnetocrystalline anisotropy in the [100] texture competes with the shape anisotropy.And the [101] texture is in between.Finally,we investigated the microwave absorption properties of Co nanocone arrays with a specific texture at 2-18 GHz using a vector network analyzer(VNA).It was found that there is a significant polarization relaxation process in the nanocone,which is mainly due to the interfacial polarization of the nanocone.For the nanocone of [100] texture,the magnetic loss mechanisms are mainly eddy current loss,natural resonance,and exchange resonance.In contrast,the nanocone of [001] texture has only one resonance peak with a larger linewidth.The [001] texture nanocone with a matched thickness of 1.5 mm achieves a minimum reflection loss value of-30.1 d B and an effective bandwidth of 5.36 GHz.Its microwave absorption capability is significantly better than that of the [100] texture nanocone.This is mainly the result of the combination of magnetocrystalline anisotropy,magnetic loss enhanced by perfect conical shape,quarter-wavelength interference phase extinction,well-matched impedance,and Co nanocone interfacial polarization.