Stress Release And High Frequency Magnetic Stability Of Flexible Ribbon-patterned Wrinkle Films

Nowadays,great progresses have been made in flexible electronic devices due to their wearable and implantable applications with the advantages of light weight,enhanced durability,mechanical flexibility,and installability on uneven surfaces.In contrast to traditional silicon-based electronic devices,the deformation of flexible magnetic films,including bending and stretching,may change the magnetic anisotropy and the high-frequency properties owing to the inverse magnetostrictive effect,which is not conducive to its application in electronic devices attached to human skin.The key to achieve the stability of magnetic properties under tensile strain is to release the biaxial internal stress caused by tensile strain,and it can be achieved by combining ribbon patterned method with pre-stretching method.In this paper,we reported two methods to release residual biaxial stress in selfassembled wrinkle structures.First,Co Fe B wrinkled films with different ribbon widths were prepared on PDMS substrate by combining the pre-stretching method with ribbon patterned method.It is found that the PDMS spacer formed by ribbon patterned can release the residual tensile stress perpendicular to the pre-tensile direction well,so that the ribbon patterned films have fewer natural cracks than the continuous film.Moreover,the reduction of residual stress leads to the decrease of magnetic anisotropy,and finally the magnetic film obtains the resonance frequency stability under tensile strain.In addition,the shadowing effect of the mask will lead to uneven film thickness at the edge of the ribbon,which will reduce the stable range of the high frequency magnetism of the narrow ribbon patterned films.Under the influence of the stress release effect of the PDMS spacer region and the shadowing effect of the mask,the 200-μm-wide ribbon-patterned Co Fe B film show stable ferromagnetic resonance frequency of about3.17 GHz under 5%-25% tensile strain,and it can maintain the high frequency magnetic stability under 2000 cyclic stretching.In addition,based on the idea that the central layer only subject to weak stress under bending strain,we investigate the effect of the central layer effect on the magnetic stability of ribbon-patterned wrinkle films by changing the thickness of Ta layer in Ta/Co Fe B/Ta structure.It is found that the increase of the thickness of Ta layer can effectively reduce the natural cracks on the surface of the film,and reduce the bending amplitude of the magnetic layer,so that the stress distribution is more uniform and the peak stress is smaller.This will eventually lead to smaller variations in magnetic anisotropy and high-frequency magnetic performance of magnetic films located near the central layer.