| In amorphous magnetoelastic films,due to absence of magnetocrystallineanisotropy and presence of high magnetoelastic-coupling coefficient,their magneticproperties and magnetic domain structures are largely determined by anisotropy inducedby stress.Using its stress sensitivity,the amorphous magnetoelastic films can be appliedin the stress/strain sensors,adjustable filter and magnetic storage fields.For thesereasons,the influences of the stress on the magnetic properties of the amorphousmagnetoelastic films are investigated widely.In this dissertation,focusing on theFeCoSiB amorphous magnetoelastic films,the influences of the stress on staticmagnetic properties (such as magnetic domain structures,magnetization processes) anddynamic magnetic properties (such as permeability and impedance at high frequency) ofthe magnetoelastic films were investigated both theoretically and experimentally.The FeCoSiB amorphous films were prepared by DC magnetron sputtering.Theinfluences of the sputtering pressure on the composition,the surface morphologies,themagnetic domain structures and the magnetic properties were studied systematically.The results showed that with the increase of the sputtering pressure,the surfaceroughness of the FeCoSiB films increased.The magnetic contrast of the stripe domainincreased with the increase of the sputtering pressure and then decreased with furtherincrease of the sputtering pressure.The coercivity of the films increased while theremanence decreased with the increase of the sputtering pressure.To study the static magnetic properties of the stressed FeCoSiB films,stressedFeCoSiB films were prepared by a method of strain growth firstly.Magnetic forcemicroscopy (MFM) results showed that for the tensile stressed films,the root meansquare value of phase shift (RMS)ΔΦ decreased from 1.23 to 0.838 with the increase ofthe tensile strain from 0 to 0.2%,which meant that the magnetic contrast of the MFMimages decreased.It was observed that the unstressed sample had irregular domainswhile the stressed samples showed stripe domains.With the increase of the tensile strainfrom 0.15% to 0.18%,the angle between the stripe domain and the stress decreasedfrom 37°to 20°,which meant that the stripe domain of the stressed films tended to align parallel to the direction of the stress.In contrast,the MFM results showed that for thecompressively stressed films,the (RMS)ΔΦ decreased from 0.771 to 0.693 with theincrease of the compressive strain from 0.05% to 0.15%,which meant that the magneticcontrast of the MFM image increased too.When the compressive strain were 0.05%,0.15% and 0.15%,the angle between the stress and the stripe domain were 15°,35°and50°respectively,which meant that the stripe domain of the stressed films tended to alignperpendicular to the direction of the stress.Based on the Brown's equation,a stressdependent stripe domain model was proposed to interpret the experimental results.Computations based on this model showed that with the increase of the tensile stress,the critical film thickness increased while the critical magnetic domain width increasedtoo.The magnetic domain orientations rotated towards the direction of the tensile stress.Secondly,based on the mean field Stoner-Wohlfarth hysteresis model a magneticdomain rotation hysteresis model had been established when taking the randomanisotropy in the amorphous magnetoelastic films and the applied stress into account.The simulation showed that in the magnetic domain rotation magnetization processesthe tensile stress along the direction of the applied magnetic field resulted in theincrease of both the coercivity and remanence,while the compressive stress resulted inboth the coercivity and the remanence approaching zero.Also,the observedmagnetostriction was dependent on the applied stress.Based on the Jiles-Athertonmodel,considering the stress-induced anisotropy and the stress dependent pinningcoefficient,an anisotropic domain wall motion hysteresis model was developed.Thecomputation results showed that in the magnetic domain wall motion magnetizationprocesses,the tensile stress along the direction of the applied magnetic field resulted ina decrease of the coercivity,but the remanence increased.When the compressive stresshad been applied,the coercivity increased while the remanence decreased.Theexperimental results showed that in the FeCoSiB amorphous films,anisotropy wasinduced along the stress direction when tensile stress was applied.When the tensilestrain increased from 0 to 0.18%,the remanence ratio along the direction of the stressincreased from 0.324 to 0.735,while the remanence ratio perpendicular to the directionof the stress decreased from 0.319 to 0.118.However,with the increasing of thecompressive strain from 0 to 0.19%,the remanence ratio along the direction of thestress decreased from 0.236 to 0.194,while the remanence ratio perpendicular to the direction of the stress increased from 0.232 to 0.606.With the increasing of the tensilestress,the coercivity of the FeCoSiB films increased.But the coercivity of the FeCoSiBfilms increased with the increasing of the compressive stress and then decreased withthe further increasing of the compressive stress.The coercivity of the FeCoSiB filmswas largely dependent on the reversal magnetization processes of the films.Themagnetostriction coefficient had been deduced from the stress induced magneticanisotropic field.To study the dynamic magnetic properties of the stressed FeCoSiB films,a stressdependent high frequency permeability model was developed for ferromagnetic filmsbased on the Landau-Lifshitz-Gilbert (LLG) equation and the Maxwell equations.Theinfluences of the stress on the effective permeability and the stress impedance effect hadbeen studied.The results showed that the effective anisotropy varied with the appliedstress,which resulted in a variation of the permeability and cut-off frequency.With theincreasing of the tensile stress,the effective permeability increased and then decreasedwith the further increasing of the tensile stress.The effective permeability reached themaximum when the effective anisotropy is at the minimum.Remarkable stressimpedance effect could be observed in the films with thickness greater than the skindepth.And the stress impedance effect increased with the increase of the saturationmagnetization.A transverse anisotropy could improve the stress impedance effect whilethe magnitude of the anisotropy only shifted the impedance peak.The experimentalresults showed that the stress impedance effect in the FeCoSiB amorphous filmsincreases with the increase of the thickness of the films.In FeCoSiB films withthickness of 2μm,the maximum stress impedance effect up to 7.36% can be achieved.When the working frequency increased,the stress impedance effect increased.A higherstress impedance effect was obtained in the FeCoSiB films with stronger transverseanisotropy.Compared with the FeCoSiB films deposited under a bias magnetic field of65 Oe,the stress impedance effect of the FeCoSiB films deposited under the biasmagnetic field of 1000 Oe was improved about 89.1%.
|
PDF Full Text Request