Modifying the magnetic properties of Laves phase intermetallic multilayers and films by nano-patterning and ion implantation

摘要

Since the pioneering work of Kneller & Hawig and Skomski & Coey some 20 yearsago, the topic of exchange springs has received considerable attention. Exchangesprings, systems where thin hard and soft magnetic layers are alternately arranged inmultilayer stacks, provide great potential in improving the performance of a wide rangeof devices, from permanent hard magnets and microelectromechanical sensors andactuators, to magnetoresistive random access memory and permanent magnetic datastorage. Artificial structuring on the nano-scale will be beneficial in improving thefunctionality of exchange spring systems in all of these areas. In this work, twodistinctly different routes to nano-structuring in epitaxially grown rare earth – iron(REFe2) films and exchange spring materials are described. Namely i) electron beamlithography and Ar+ ion milling to define three-dimensional nano-scale structures, andii) ion implantation to directly alter the crystalline structure of the material at theatomic-scale. Nano-scale elements defined in REFe2 exchange spring materials arepresented, providing not only the first demonstration of nano-structuring in thesematerials, but also the successful implementation of electron beam lithography and Ar+ion milling on these novel systems. Nano-scale patterning confirms the suitability ofthe REFe2 exchange spring materials as excellent candidates for magnetic data storagemedia, since they remain relatively unaffected by nano-structuring, retaining theirthermal stability and comparatively small coercivity. Ar+ ion implantation is shown tobe effective at artificial structuring on the atomic-scale. In addition, energetic Ar+ ionshave been successfully used to accurately control the easy and hard axes ofmagnetization within epitaxial YFe2 and DyFe2 films and a DyFe2 / YFe2 exchangespring multilayer. At a fluence of ~ 1017 Ar+ ions cm-2, the magnetoelastic anisotropy(dominant at room temperature in the epitaxially grown films) is reduced to such anextent that the intrinsic magnetocrystalline anisotropy begins to dominate. Thus Ar+ ionimplantation serves to alter the easy and hard axes of magnetization, rotating themthrough 90°. Such behaviour is clearly evident in hysteresis loops obtained by both themagneto optical Kerr effect and vibrating sample magnetometry, and is furtherconfirmed by micromagnetic modelling. The reduction in magnetoelastic anisotropy isattributed to energetic Ar+ ions causing RE atoms to relax to their unstrained latticepositions, thereby relieving the strain responsible for the magnetoelastic anisotropy.This interpretation is confirmed by X-ray diffraction measurements.