Study On Exchange Bias Effect And Magnetic Anisotropy Induced By Curvature

Study on magnetic materials with nanoscale dimension is the basis for the development of future spintronics devices toward miniaturization and multifunctionality.Developing new magnetic nanomaterials,uncovering their physical properties and its intrinsic correlations with their microstructures,and exploring their potential applications are hot research topics in this field.In recent years,enhanced coercivity and remanence have been found in magnetic materials with bent nanostructures,such as macroporous films,nano-bowls and hollow spheres.Simultaneously,Some theoretical calculations suggest that large curvature of domain wall may lead to an increase in coercivity.Based on previous studies in other groups and our own group,in this thesis,we have fabricated two types of thin film systems with nanocap structure on closely-packed single-layer polystyrene(PS)-bead arrays.one is a ferromagnetic/antiferromagnetic(FM/AFM)exchange-coupled double-layer film system,and the other is a ferromagnetic/heavy-metal thin film system.And systematically investigated the impact of PS-bead surface curvature on the microstructures and magnetic properties of these two thin film systems.The main research contents and innovations are summarized as follows:(1)Closely-packed single-layer PS-bead arrays were prepared through self-assembling of PS beads at air-water interface,and used as substrates for subsequent deposition of thin films.The FM/AFM double layer film system of Ni/Co O with nanocap structure was deposited on the PS-bead arrays by DC-sputtering.The effects of the thickness of the FM layer(t _FM)and the AFM layer(t _AFM)on the FM-AFM exchange bias were investigated.It is found that in the case of Ni(t _FM)/Co O(15 nm),the exchange bias field increases nearly proportionally to 1/t _FMand reaches a maximum of 700 Oe when 1/t _FM=0.125 nm^-1(i.e.,t _FM=8 nm);when1/t _FM>0.125 nm^-1,the exchange bias field decreases.In the case of Ni(8nm)/Co O(t _AFM),when the thickness of the antiferromagnetic layer t _AFM changes,the exchange bias field and coercivity gradually decrease and tend to stabilize with the increase of t _AFM.However,when t _AFM>15 nm,the left and right branches in the hysteresis loop of the nanocap show unique asymmetry in shape.Further study shows that temperature and the AFM layer thickness affect the significance of the asymmetry,and the direction of the cooling field changes the direction of the asymmetry.This result indicates that the unique asymmetry is closely related to the exchange coupling at the FM-AFM interface.Further,since this kind of asymmetric hysteresis loop does not exist in the flat film prepared under the same conditions,the unique asymmetry should also be attributed to the microstructures of the nanocaps induced by the PS-bead surface curvature.Further experimental and theoretical studies need to be carried out to explore the underlying physical mechanism of such asymmetric hysteresis loops.(2)Closely-packed single-layer PS-bead arrays were prepared by spin coating and used as substrates for subsequent deposition thin films.The tantalum/cobalt/platinum(Ta/Co/Pt)three-layer film system with nanocap structures was deposited on the PS-bead arrays by DC-sputtering.In this film system,Ta is used as a buffer layer,Co is a FM layer,and Pt is a heavy-metal layer with strong spin-orbit coupling.The effects of the thickness of the Co layer and Pt layer on the effective magnetic anisotropy of the nanocaps have been systematically investigated.It is found that when both the thickness of the Co layer and the Pt layer are 5 nm,the effective magnetic anisotropy(K_eff)reaches the maximum value of 1.74×10⁷ J/m³.Replacing Co with iron(Fe)or replacing Pt with palladium(Pd)results in the reduction of the effective magnetic anisotropy of the three-layer film system.The latter may be due to the fact that the spin-orbit coupling strength of Pd is smaller than that of Pt.