Magnetic Anisotropy And Strain Tunable High-frequency Magnetic Properties Of Stretchable CoFeB Films

Microwave soft magnetic films with small volume,high permeability and high working frequency are important to improve the power density and reduce power loss of high-frequency electronic devices,such as filters,planar inductors and antenna,etc.,so as to improve the integration of electronic systems.It is necessary to realize high ferromagnetic resonance frequency in microwave soft magnetic films in order to improve the working frequency of microwave devices.Magnetic field annealing,exchange bias,oblique deposition and patterning method are usually used to significantly improve the anisotropy of the magnetic films,so as to improve ferromagnetic resonance frequency.However,the above methods are complex to operate,and cannot be dynamically regulated.In recent years,stretchable electronic devices with good mechanical flexibility,low manufacturing cost,simple fabrication process,and light weight have attracted a lot of attention.The microwave soft magnetic films fabricated on flexible substrates have good mechanical flexibility,and their high-frequency magnetic properties can be continuously tuned by applying strain,which is of great significance for the development of high-frequency electronic devices.Based on the above research background,this thesis mainly focuses on how to fabricate microwave soft magnetic films with ideal tensile properties and high-frequency magnetic properties,and tunning their high-frequency magnetic properties by applied tensile strain.The main research works are as follows:Firstly,the uniaxial stretchable magnetic film with surface periodic wrinkling morphology is successfully fabricated on the PDMS by adhering the flexible CoFeB film to the asymmetric biaxial pre-strained elastomer substrate.All samples exhibit uniaxial magnetic anisotropy,and their high-frequency magnetic properties can be adjusted by transverse pre-strain and applied strain.We realized the positive tensile dependence of ferromagnetic resonance frequency of stretchable CoFeB films by applying transverse pre-strain to the elastic substrate.With the increase of the applied strain from 0 to 200%,the ferromagnetic resonance frequency increases monotonically from 3.92 to 5.26 GHz.The residual stress analysis model of CoFeB film can qualitatively explain the regulation of the strain on its high-frequency magnetism.The fatigue test results of 2000 times show that the stretching dependence of high-frequency magnetic properties has a good repeatability.Secondly,we applied the pre-strain of 50% x-direction and 0 to 50% y-direction to the elastic substrate,and then adhered the flexible CoFeB film on it.After releasing the pre-strain,a series of stretchable CoFeB films were obtained.Optical microscope images and static magnetic characterization results show that the surface morphology of samples gradually tends to "zigzag" structure,and its magnetic anisotropy first decreases,and then tends to be stable with the increase of y-direction pre-strain.Static and dynamic magnetic tests on 50%×50% and100%×100% stretchable samples were used to investigate the magnetic properties of biaxially stretchable films.Static magnetic characterization shows that all samples exhibit uniaxial magnetic anisotropy,and the direction of easy axis deviates from the x-axis.The results of dynamic magnetic characterization show that the samples have biaxial tensile properties and the positive biaxial tensile dependence of ferromagnetic resonance frequency is realized.