Self-Organization Growth And Physical Properties Of Magnetic Metal Nanostructures

Much attention has been paid on low dimensional magnetic nanostructures,because of its application prospects in magnetic memory and logic devices.Magnetic structures can be controlled by designing different sizes,shapes and magnetocrystalline anisotropy of the magnetic nanomaterials.People usually use top-down microfabrication method to fabricate magnetic nanomaterials,with complex craft and high cost.What is more,frequently-used microfabrication method such as electron beam evaporation can be used to prepare magnetic polycrystalline materials,which has no magnetocrystalline anisotropy.Ultrathin layer electrochemical deposition system has been used to achieve the fabrication of single-crystalline and polycrystalline filaments here.The crystalline,magnetic properties and growth mechanism in these structures are also investigated.Main results are included as following:1.We successfully fabricate single-crystalline cobalt nanowire arrays by using ultrathin layer electrochemical deposition system without template.The branching rate of the nanowires can be greatly suppressed when using this space-limited system,because of the suppression of convective noise in front of the deposit.On this basis,through optimizing the growing conditions,we achieve the fabrication of single-crystalline cobalt nanowire arrays.Periodic anti-parallel and compressed vortex magnetic domain structures are measured by MFM.Statistical result indicates that the former is corresponding to the thin and wide areas of nanowires,while the latter is corresponding to the thick and narrow areas,which is also confirmed by micro-magnetic simulation.We realized a kind of stable periodic antiparallel magnetic domain pattern at remanence after the sample being exposed to a 0.6T magnetic field perpendicular to the plane.(This work has been published in Physical Review B,93,054405(2016).)2.For the first time we fabricate magnetic metal micro helix arrays using self-organized electrochemical method,and propose a self-template growth mechanism of these helical filaments.In our ultrathin layer electrochemical deposition system,the interface between ice and solution is not so rigid.The Joule heat would be released while filaments growing.As a result,ice would be melted into some holes in front of the growth tip when the driving voltage is sufficiently high.We has realized the fabrication of cobalt helix arrays by using this system through optimizing the system temperature and driving voltage.SEM images show the morphology of cobalt helix array and the detail of a single helical nanowire.We find that a single helix is composed of periodic stairs on the inner side like a micro helical staircase and is depositing smoothly on the outer side.TEM analysis indicates the wire is polycrystalline with a preferred orientation.In situ measurement shows an ice local melt phenomenon in front of the tip when helix growing.Combining these characterizations and the earlier research achievement of our group,we propose a self-template growth mechanism of the helical filaments.What is more,magnetoresistance measurement has been done and shows two extremums when external magnetic field increasing,because of the angle between the external field and the current direction in helix.This result is quite different from straight nanowires.