Low-Symmetry Nickel Nanocone Arrays:Controlled Fabrication And High Performance Microwave Absorption

Geometric size decreasing and symmetry reduction are important ways for magnetic nanostructures to generate novel magnetic phenomena and enhanced performance for applications.Magnetic nanostructure arrays with special magnetic configurations have important applications in information storage,biomedicine,energy environment,etc.In conical nano-shape structure with asymmetric size feature progressively diminishing along cone length,exchange interactions,and magnetostatic interactions contribute to produce different static magnetic properties,which can be used as a unique research platform for physical studies of magnetism.Nanocone arrays also have potential applications in microwave absorption because of their special micro/nano structures and good dispersion.However,there are few studies on magnetic nanocones with different geometries with ideal conical structures because of the complexity and poor controllability in fabrication,which further leads to a lack of clear explanation about the geometry-related static magnetism and applications of magnetic nanocone arrays.Here we have successfully prepared Ni nanocone arrays with near-perfect conical shape using an advanced ion-track template method and electrochemical deposition.We have investigated the static magnetic properties and microwave absorption properties of Ni nanocone arrays with different cone angles.Our studies includes the following three main aspects of work:（1）Ni nanocone arrays with different cone angles and different lengths were fabricated by using advanced ion-track templates,asymmetric chemical etching and subsequent electrochemical deposition.SEM images showed that the prepared Ni nanocone arrays have a near-perfect conical shape and random distribution,and TEM and XRD results indicated that the Ni nanocone arrays are polycrystalline FCC structure.（2）In this thesis,the effects of cone angle,substrate and array density on the static magnetism of Ni nanocone arrays are investigated by means of hysteresis loops,angular-dependence remanence curves and first-order reversal curves.The results show that Ni nanocone arrays with cone angles of 1°and 3°exhibit weak magnetostatic interaction,and the static magnetic properties of the arrays are dominated by shape anisotropy;Ni nanocone arrays with cone angles of 5°and 7°have strong magnetostatic interaction in the arrays as a result of the close distance between the cones,and there is even a pinned effect caused by the cone-base overlapping,leading to the deviation of the easy axis.The first-order inversion curves show that in the Ni nanocone length direction,the high-density 3×10⁷ cm^-2 Ni nanocone arrays with different cone angles exhibit multi-domain conformation with the reversible switching events;the low-density 5×10⁶ cm^-2 Ni nanocone arrays exhibit quasi-single-domain conformation with the irreversible switching events.（3）The microwave absorption performance of Ni nanocone arrays at different cone angles is studied to achieve the modulation of microwave absorption performance and high-performance microwave absorption by cone angle.The mechanism of microwave absorption performance related with cone angle is investigated further.The results show that the Ni nanocone array has excellent microwave absorption performance,especially the Ni nanocone array with a cone angle of 1°,and the minimum reflection loss is-54.1 d B with an effective bandwidth of 5.21 GHz（12.79-18 GHz）when the thickness is 1.31 mm.When the thickness is 1.5 mm,the maximum absorption bandwidth can reach 6.09 GHz（11.91-18 GHz）.The excellent performance of both broadband microwave absorption（EAB）and high reflection loss per unit thickness（RL/d）is achieved.Magnetic loss,dielectric loss,and good impedance matching are the keys to the excellent microwave absorption performance of Ni nanocone arrays.The excellent magnetic loss of Ni nanocone arrays with 1°cone angle is mainly attributed to the small geometry of the cone structure,where the eddy current loss is suppressed in the cone tip region and the exchange resonance is enhanced.Dielectric loss is mainly caused by the interfacial polarization which is enhanced by micro-capacitance effect affected by the average spacing between nanocones.