Study On The Curvature-driven Magnetic Properties Of Magnetic Nanostructures

With the rapid development of information technology,the demand for capacity,density,speed,and stability of data storage is increasing.Therefore,nanoscale spintronic devices are increasingly attractive.As the beginning of large capacity data storage devices,the development of spin-valve type giant magnetoresistance(GMR)devices has greatly promoted the development of spintronics in which the degree of freedom of electronic spin is mainly manipulated.In spin-valve GMR devices,the exchange-bias field produced by ferromagnetic-antiferromagnetic exchange coupling plays an important role in pinning the magnetic moment of the ferromagnetic layer.Although the exchange bias effect has been extensively studied in various material systems,there is still controversy over its micro-physical mechanism,and there is no unified theory so far.Recently,a new asymmetric phenomenon has attracted attention,where the descending branch of the hysteresis loop in patterned ferromagnetic/antiferromagnetic exchange-biased nanostructures exhibits a gap while the ascending branch appears round,which is different from the traditional asymmetry where one branch is square-shaped and the other is rounded.In addition,the size and shape of nanostructures can greatly affect their magnetic properties.For example,magnetic vortex states can be observed in nanodiscs or hollow spheres.Therefore,the structure-performance relationship of nano-magnetic materials has always been a focus of research.Based on this,in this thesis,we firstly introduce a curvature structure by using a polystyrene(PS)bead array as a template and sputtering ferromagnetic/antiferromagnetic exchange-coupled thin films on its surface.Then we systematically study the curvature-driven effect on magnetic properties(such as exchange bias effect,magnetic vortex state,etc.),and reveal the inherent correlation between the magnetic vortex ground state and the new asymmetry of hysteresis loops.The main research content and conclusions of this thesis are summarized as follows:(1)A PS-bead array was successfully self-assembled via gas-liquid interface,and then bilayer thin films of Ni/Co O were sputtering on the PS-bead array template,and finally a nanocap array with curvature was obtained.The hysteresis loop of the nanocap-array film exhibits a new type of asymmetry with a gap in the descending branch.By separating the nanocap array from the spacer film among PS beads,it is found that the new asymmetry of the hysteresis loop is only related to the nanocap and has nothing to do with the spacer film.As a result,the new asymmetry of the hysteresis loop can be controlled by curvature.Changing the temperature and cooling field direction indicates that the new asymmetry of the hysteresis loop is also closely related to the ferromagnetic-antiferromagnetic exchange coupling of Ni/Co O.(2)First-order reversal curve(FORC)and its spectrum analysis show that this new type of asymmetry may be related to the existence of magnetic vortex states in the nanocap.The existence of the vortex state is confirmed by the waist-like feature of the zero-field-cooled hysteresis loop.Further changes in the strength of the cooling field have been used to achieve the evolution from the vortex state to the new asymmetry with a gap in the descending branch,revealing the inherent correlation between the magnetic vortex ground state and the new asymmetry of hysteresis loops.