Tuning Of Magnetic Anisotropy And Magnetization Dynamics Of Co-Fe-Based Soft Magnetic Films

In recent years,with the development of wireless communications,new requirements for the development of magnetic materials have been emerged,in the direction of miniaturization,integration and high frequency.Thin film of magnetic material is the basis of realizing high-frequency micro device.The Co-Fe-based soft magnetic thin films with high saturation magnetization,controllable magnetic anisotropy field and high resistivity,are promising in the field of high-frequency applications.Magnetic anisotropy and magnetization dynamics parameters,including spin wave characteristics and magnetization relaxation,are crucial for the high-frequency performance of magnetic thin films.As such,it is of great significance to design new high frequency devices by manipulating the magnetic anisotropy and magnetization dynamics of Co-Fe-based soft magnetic thin films.In this dissertation,the effects of strain,film thickness and annealing temperature on the microstructure,in-plane magnetic anisotropy,and magnetization relaxation,particularly the spin wave behavior in Co₂FeSi,Co FeB,FeCo monolayers or multilayers were systematically investigated by using ferromagnetic resonance technique.The time-resolved magneto-optical Kerr effect,vibrating sample magnetometer and X-ray magnetic circular dichroism were also employed as assistant research tools.The main conclusions are as follows:(1)Strain-controlled spin wave excitation and Gilbert damping are realized in the flexible Co₂FeSi/Ta films by using ferromagnetic resonance and femtosecond pump-probe technique.We demonstrate an effective approach for the excitation of the dipole-interaction-induced magnetostatic surface spin wave(MSSW)and exchange-dominated perpendicular standing spin wave(PSSW)by applying strain to the Co₂FeSi/Ta film grown on flexible polyimide(PI)substrates.Considering the strain distribution across the film thickness,the PSSW excitation is attributed to the inhomogeneous distribution of magnetic anisotropy field in the Co₂FeSi films arising from the non-uniform strain distribution across the film thickness,while the excitation of the MSSW mode is ascribed to the non-uniform in-plane distributed anisotropy field along the strain direction owing to the ultrafast heating of atomic lattice.Moreover,we reveal that the Gilbert damping of the Co₂FeSi film manifests opposite dependences on the compressive and tensile strains.Using X-ray magnetic circular dichroism measurements,we exclude that spin-orbit coupling is the source of strain regulated damping factor.Further first principles calculations show that the compressive strain decreases the density of states N(E_F±)near the Fermi level of Co₂FeSi,while the tensile strain increases it,which is consistent with the variation of damping with strain.Therefore,it is proved that the source of strain-controlled damping is the effect of strain on N(E_F±)of Co₂FeSi films.(2)Manipulation of magnetic anisotropy and magnetization dynamics in Co FeB/Ta/Mica thin film(i)The strain effects on the static and dynamic magnetic properties of Co FeB/Ta/Mica films with different thickness were systematically studied.We found that the saturation magnetization of Co FeB films decreases with the increase of compressive strain and increases with the film thickness.The uniaxial magnetic anisotropy field increases to about 175 Oe with the increase of compressive strain and film thickness.Using the FMR technique,we demonstrate that the PSSW can be effectively excited by an appropriate magnitude of strain.More interestingly,the critical strain required to excite spin waves is inversely scale with the thickness of the magnetic film.To explain this phenomenon,a model based on the strain-induced inhomogeneous magnetic anisotropy field confined within the flexible films has been established.Besides,the fitting results of the frequency dependence of linewidth show that the effective damping factor of Co FeB film decreases to 0.005 with the increase of strain and film thickness.(ii)We found that by annealing the Co FeB/Ta/Mica thin film,the saturation magnetization of the thin film was significantly enhanced,and an in-plane uniaxial magnetic anisotropy of about 50 Oe was induced in the 400?annealed film.Considering the effect of annealing on the structure and interfacial atom diffusion,the enhancement of saturation magnetization of thin films can be attributed to the removal of residual stress and the improvement of atomic ordering and boron atom diffusion in the annealing process.The origin of in-plane uniaxial magnetic anisotropy may be stems from the anisotropic thermal expansion of mica substrate induced by high temperature annealing.Moreover,we demonstrate that by annealing the Co FeB/Ta/Mica thin films,the PSSW can be effectively excited using ferromagnetic resonance,which can be attributed to the annealing-induced volume magnetic inhomogeneity.Furthermore,by fitting the frequency-dependent FMR linewidth of all the samples,the Gilbert damping constants are found to increases by about23.3%with the increase of annealing temperature,and the spin-orbital coupling and two-magnon scattering mechanism are discussed as the possible reasons.(3)Manipulation of Non-collinear magnetic anisotropy and damping in(FeCo/IrMn)₃exchange bias multilayer(i)By doping non-magnetic Mg O in the anti-ferromagnetic layer of exchange-biased multilayer(FeCo/IrMn)₃,the exchange bias field H_eb increases from 185 Oe to 300 Oe.A misalignment angle?between the unidirectional and uniaxial anisotropy axes in the exchange-biased system can be quantified by the in-plane angular-dependent FMR measurements,which is attributed to the noncollinear spin structure in ferromagnetic layer.By comparing the variation trend of?and H_ebwith Mg O doping,it is found that they are opposite.The reason is that a large H_eb signifies a strong pining effect which weakens the nonlinearity of spin structure in the ferromagnetic layer and hence leads to a small?.(ii)With mechanical strain applied to the(FeCo/IrMn)₃/Ta films grown on flexible polyimide substrates,H_eb is enhanced from 20 Oe to 200 Oe by increasing compressive strain,while this is not observed for the case of tensile strain.More importantly,the underlying physics of the anomalous dependence of H_eb on strain is revealed by a systematic in-plane FMR study,and the H_eb behavior is ascribed to the misalignment angles between different anisotropies due to the strain in the exchange-biased multilayers.Besides,the damping parameters of the(FeCo/IrMn)₃/Ta/PI exchange bias multilayer show unidirectional anisotropic behavior in the direction of H_eb,and the damping anisotropy constantincrease significantly with the increase of the compressive strain,but remained almost constant with the increase of the tensile strain,which is consistent with the distinct change of H_eb.