High Frequency Magnetism And Damping Mechanism Of Iron-based Soft Magnetic Films

With the rapid development of communication and information technology,the requirements for components and functional materials in electronic equipment are increasingly miniaturized,high frequency and integrated.Soft magnetic thin film materials have attracted wide attention in device applications due to their excellent magnetic properties such as high magnetic permeability,saturation magnetization,and resonance frequency.Besides of the traditional devices,spintronics devices have been widely researched and developed recently,such as magnetic tunnel junctions?MTJ?,spin valves,spin-orbit nano-oscillators,etc.Thin film materials are important and even indispensable components in these devices.The current mainstream communication transmission frequency band has reached the level of GHz,and researching and adjusting the high-frequency magnetic properties of soft magnetic thin films has become one of the problems to be solved urgently.In view of this,many research groups have made detailed studies on the high-frequency performance of soft magnetic thin film materials.However,the magnetic damping mechanism has always been ignored or extremely simple to deal with.In many practical device application,the damping of soft magnetic films often plays an important role.Therefore,understanding and regulating soft magnetic materials damping is very important.This paper focuses on the high-frequency magnetism and damping mechanisms of iron-based soft magnetic thin films,and analyzes the internal mechanism of damping,high-frequency response and microstructure information.Various methods were tried to adjust the damping constant,and experimental conclusions were drawn through establishing models,data analysis to make the high-frequency magnetic and damping of iron-based soft magnetic films become adjustable,controllable,and predictable.The main research contents of this paper are as follows:Soft magnetic thin film samples were prepared using the vacuum magnetron sputtering system and laser direct writing instrument.Vibrating sample magnetometer?VSM?,X-ray diffraction spectrometer?XRD?,magnetic force microscope?MFM?,and scanning electron microscopy?SEM?,Vector network analyzer?VNA?,electronic spin resonance spectrometer?ESR?,variable temperature ferromagnetic resonance?FMR?and other means to characterize and analyze the magnetic properties of the sample.1.N-element doping was used to adjust the soft magnetic properties of Permalloy?Py?films.Due to the extremely high electrical conductivity,Py generates extremely eddy current loss in high frequency application.Therefore,N element doping is used to reduce the conductivity and eddy current loss.The static and dynamic magnetic properties of the samples were adjusted by changing parameters such as the partial pressure ratio of nitrogen,power and substrate temperature during the thin film sputtering process.The results show that changing the substrate temperature during sputtering can adjust the static and dynamic magnetic characteristics of the FeNiN soft magnetic thin film effectively and obtain the best soft magnetic properties.Compared with samples prepared at other temperatures,the hysteresis loops of the samples prepared at the substrate temperature of 200 ^oC show that they have the smallest coercivity and the largest remanence ratio.The dynamic magnetic properties of the sample can also be obtained from the magnetic spectrum and ESR measurement.In this paper,various important magnetic parameters of the sample are studied,such as saturation magnetization?4?M_s?,uniaxial anisotropic field?H_u?.The relationship between the natural resonance frequency?f_r?and substrate temperature changes also shows that the FeNiN soft magnetic thin film prepared at a substrate temperature of 200^oC has the best soft magnetic properties.2.By changing the partial pressure of N₂ in the sputtering chamber gas,a series of FeNiN thin film samples with different N content were prepared.The hysteresis loops show the saturation magnetization of the sample gradually decreases as the increase of partial pressure of N₂.The other static magnetic parameters,such as coercivity,do not change much.For the magnetic spectrum measurement of high N content of the samples,one failed to obtain the magnetic spectrum,which may be due to the huge effective damping.The resonance field and resonance line width of the sample were measured by ESR.The experimental data were fitted using theoretical formulas of intrinsic damping,two-magnon scattering and inhomgeous broadening.The results show that the doping of the N element will lead to an increase in the intrinsic damping parameter of the sample.The more important is this process leads to a sharp increase in the two-magnon scattering intensity,which increases the effective damping of the samples.3.Magnetron sputtering system,laser direct writing instrument and Ar ion etching technology were used to prepared the Py matrix rings with the same outer diameter?30?m?and different inner diameters?10?m,15?m,20?m and 25?m?.The results of the hysteresis loops show that the patterning process has small effect on the static magnetic properties of the sample.On the other hand,the hysteresis loops also show that the sample has the structure of stripe domain,which was proved by the MFM results.The out-of-plane ESR measurements was used to measure the resonance spectrum of the sample,wchich show double resonance peaks.We refer to these two resonance modes as low-field mode and high-field mode.As the inner diameter of the rings decreases,the resonance field of the high-field mode decreases and its intensity increases,meanwhile,the resonance field of the low-field mode increases and its intensity decreases.Micromagnetic simulations prove that the two resonance modes occour at different positions of the rings.These two resonance models have the evolution process of"multi-peak-fusion-split"when the magnetic field rotated from in-plane to out-of-plane.On the other hand,the micromagnetic simulation also shows more high-order spin standing wave modes in the high-field mode.The fitted data by using the Kittel equation is in good agreement with the experimental results.The radial demagnetization field in the fitted results is a very important parameter.As the inner diameter of the ring increases,the radial demagnetization field increases accordingly,which shows the demagnetizing model of Py rings is accurate.4.Py stripe with different widths were prepared by using magnetron sputtering system and laser direct writing instrument.The hysteresis loops of the samples were measured by VSM.The results show that the coercivity gradually decreases with the rise of stripe width,which is due to the magnetic moment pinning effect of the strip edges.On the other hand,the uniaxial anisotropy field decreases with increasing strip width,which is due to the decrease in the shape anisotropy of the sample.The relationship between the applied magnetic field and the resonance frequency was fitted by the VNA-FMR method,which prove the demagnetization analysis of the stripe sample.The effective damping of the sample is divided into two parts of the continuous film and the additional damping due to the patterning process.The damping variation law of the prepared sample is explained by the model of the gradual damping.5.The 30 nm Py thin film was prepared by magnetron sputtering method.The low temperature resonance properties of the sample were measured and analyzed using a broad-frequency FMR system to obtain the magnetic parameters and damping.The related formulas were used to fit the relationship between the resonant magnetic field and the microwave frequency.The results show that as the measured temperature decreases,the uniaxial anisotropy and effective saturation magnetization of the sample gradually increase,which is due to the thermal disturbance from the rising temperature.On the other hand,it is found that the damping parameter and inhomogeous broadening of the sample,which is derived from the linear relationship between resonance linewidth and the microwave frequency,increase with decreasing temperature.As the temperature decreases,the two magnon scattering effect alse increases.The enhanced two magnon scattering effect at low-frequency and low-temperature make the relationship between resonance linewidth and the microwave frequency no longer correspond to the linear relationship.Thus,the damping parameter and inhomogeous broadening obtained by the simple linear fitting curve will be enhanced.6.A series of Co/IrMn exchange bias systems with different thicknesses of Co were prepared using magnetron sputtering system.The anisotropy,exchange bias field and damping parameter of the samples were measured by a variable temperature broad-frequency FMR device.By analyzing the relationship between the resonance field and the microwave frequency,it is found that the exchange bias field of the sample decreases with the thickness of the Co layer,which agree with the basic law of the exchange bias system.The measured damping parameter of the sample magnetized in the direction of the exchange bias field H_EB is smaller than that along the-H_EB,which indicates that the exchange bias effect suppresses the diffusion of the spin current from the Co layer to the IrMn layer.As the measurement temperature decreases,the exchange bias effect of the sample becomes stronger and the suppression of spin current form Co layer to the adjacent layer becomes stronger.