Effect Of Yb Doping And Spin-pumping Effect On Damping Of Magnetic Thin Films

Spintronics devices,which can control the spin of electrons,have attracted much attention because they have one more degree of freedom than microelectronic devices which can only precisely control the charge.The performance of magnetic thin film materials used in spintronics devices determine the efficiency of the spin freedom control.The damping factor of the magnetic thin films used in spintronics devices is an important parameter,which determines the power consumption and data writing rate of magnetic storage devices based on the spin transfer torque.If the damping factor is too large,the critical current density for generating spin transfer torque to overcome the damping torque will be very high,so the power consumption will also be very large.However,if the damping factor is too small,it will take a long the time for the magnetization reverse,and the time transfered from one stable state to another stable state is very long,resulting in the low write rate of the device.Therefore,the research on the regulation method and mechanism of the damping factor is a research hotspot,which has important research value and significance.Damping factor can be divided into intrinsic and extrinsic terms.Among them,the intrinsic term of the damping factor is usually related to the strength of the spin-orbit coupling effect in the materials.Regulating the strength of the spin-orbit coupling to realize the regulation of the intrinsic damping factor is an important means for the devices which efficiently reverse the magnetization intensity by utilizing the spin orbital torque.The spin pumping effect in ferromagnetic/heavy metal heterojunctions is also an important extrinsic mechanism for the damping factor of magnetic films.The research on this term can reflect the transmission efficiency of the spin current at the interface,and it is also of great significance to realize the reversal of magnetization by using the spin orbital torque.In this thesis,we have done two aspects of research work.First,we studied the influence of rare earth element Yb doping on the intrinsic damping factor of Fe₆₅Co₃₅ thin films,and then studied the influence of the spin current flowed through the interface of ferromagnetic/non-magnetic metal heterojunctions on the effective damping factor through the spin pumping effect.In the first parts,since the occupation of 4f electrons of Yb element are completely filled,its orbital magnetic moment is zero,so theoretically it should have relatively small intrinsic damping factor with the alloy formed with the magnetic metal.To test this idea,we prepared(Fe₆₅Co₃₅)_?1-x?Yb_x samples and measured the composition,structure,static and dynamic magnetism of the samples.The results showed that with the increasing Yb content,the crystal structure of the samples gradually changed from polycrystalline state to amorphous state,the saturation magnetization and uniaxial anisotropy constant decreased monotonically with the increasing Yb content,but the damping factor increased sharply.When the temperature changes from 100 K to 400 K,the saturation magnetization of the samples decreases monotonically and satisfies Bloch's T^3/2 law.The anisotropy constant of the samples also decreases monotonically.Meanwhile the damping factor of the samples changes nonmonotonically in the temperature range of 100-400 K with a minimum around 200 K.This result is qualitatively consistent with the theoretical calculation results based on Kambersky's torque-correlation model.The influence of Yb doping on the damping factor of Fe₆₅Co₃₅ thin films and the physical mechanism behind were studied by studying the dependences of the damping factor and the anisotropic constant on the content of Yb and the temperature.In the second aspect of research work,firstly,in order to study whether Cr can be used as a material to effectively absorb the spin current,we prepared Cr(t_Cr)/Co samples,and measured the static and dynamic magnetic properties of the samples at room temperature by means of vibrating sample magnetometer and electronic spin resonance spectroscopy.By analyzing the dependence of the effective damping factor on the Cr layer thickness,we obtained that the spin-mixing conductivity of the Cr/Co interface is 5.88?10¹⁹ m^-2,and the spin diffusion length of the Cr layer is 4.69 nm.Secondly,in order to investigate whether the Al insertion layer can reduce the magnetic proximity effect of Pt in ferromagnetic/Pt heterojunction,FeCoB/Pt(t_Pt)and FeCoB/Al(t_Al)/Pt samples were prepared.And the static and dynamic magnetic parameters of the thin films were also measured at room temperature.By analyzing the dependence of effective damping factor on t_Pt,we obtained that the spin-mixing conductivity of FeCoB/Pt interface is 1.63?10¹⁸ m^-2.In the FeCoB/Al(t_Al)/Pt system,the effective damping factor of the samples rapidly decreased from 0.028 to 0.016 as the t_Al increased from 0 to 0.5 nm.This downward trend indicates that the Al layer weakens the interlayer exchange coupling and thus inhibits the magnetic proximity effect.This also indicates that it is not comprehensive to regard Pt as a general spin current absorbing material without considering the magnetic proximity effect.Finally,in order to determine whether the oxide inserting layer can improve the spin mixing conductivity in Co/Pt and Co/Ta samples,we have grown Co/CuO_x/Pt(t_Pt),Co/CuO_x/Ta(t_Ta)samples,and two reference sample systems:Co/Cu/Pt(t_Pt)and Co/Cu/Ta(t_Ta).The composition,layered structure,static and dynamic magnetism of the samples were tested.By fitting the dependence of the effective damping factors of the four groups of samples on t_Pt and t_Ta,we obtained the values of spin mixing conductivity and spin diffusion length,and found that the introduction of CuO_x insert layer could increase the spin mixing conductivity by 1.5 times.In addition,similar phenomena were observed in YIG/CoO_x/Pt(t_Pt)samples,indicating that the introduction of CuO_x and CoO_x oxide layers can enhance the transmission of interfacial spin current,providing an idea for the design of efficient spintronics devices.