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

Abstract

Since the pioneering work of Kneller & Hawig and Skomski & Coey some 20 years ago, the topic of exchange springs has received considerable attention. Exchange springs, systems where thin hard and soft magnetic layers are alternately arranged in multilayer stacks, provide great potential in improving the performance of a wide range of devices, from permanent hard magnets and microelectromechanical sensors and actuators, to magnetoresistive random access memory and permanent magnetic data storage. Artificial structuring on the nano-scale will be beneficial in improving the functionality of exchange spring systems in all of these areas. In this work, two distinctly different routes to nano-structuring in epitaxially grown rare earth – iron (REFe2) films and exchange spring materials are described. Namely i) electron beam lithography and Ar+ ion milling to define three-dimensional nano-scale structures, and ii) ion implantation to directly alter the crystalline structure of the material at the atomic-scale. Nano-scale elements defined in REFe2 exchange spring materials are presented, providing not only the first demonstration of nano-structuring in these materials, but also the successful implementation of electron beam lithography and Ar+ ion milling on these novel systems. Nano-scale patterning confirms the suitability of the REFe2 exchange spring materials as excellent candidates for magnetic data storage media, since they remain relatively unaffected by nano-structuring, retaining their thermal stability and comparatively small coercivity. Ar+ ion implantation is shown to be effective at artificial structuring on the atomic-scale. In addition, energetic Ar+ ions have been successfully used to accurately control the easy and hard axes of magnetization within epitaxial YFe2 and DyFe2 films and a DyFe2 / YFe2 exchange spring 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 an extent that the intrinsic magnetocrystalline anisotropy begins to dominate. Thus Ar+ ion implantation serves to alter the easy and hard axes of magnetization, rotating them through 90°. Such behaviour is clearly evident in hysteresis loops obtained by both the magneto optical Kerr effect and vibrating sample magnetometry, and is further confirmed by micromagnetic modelling. The reduction in magnetoelastic anisotropy is attributed to energetic Ar+ ions causing RE atoms to relax to their unstrained lattice positions, thereby relieving the strain responsible for the magnetoelastic anisotropy. This interpretation is confirmed by X-ray diffraction measurements.

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