Inverse Spin Hall Effect In Transition Metals

Spintronics,also known as magneto-electronics,is the study of the intrinsic spin of the electron and its associated magnetic moment,in addition to its fundamental electronic charge,in solid-state devices.Adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility,increasing data processing speed,decreased electric power consumption,and increased integration densities.Recently,pure spin current,i.e.,flow of angular momentum without accompanying net charge current,which is expected to be dissipationless,has attracted increasing attention and is a key ingredient in the field of spintronics.For the application of novel low power devices utilizing pure spin currents and their integration with present day charge-based technologies,the understanding of the inter-conversion between spin and charge currents is of crucial importance.The spin Hall effect(SHE)and its inverse effect,ISHE,have been intensively studied as they can achieve the spin and charge conversion in the absence of magnetic material and magnetic field.Overall,the inter-conversion can be described by Js=h/2e?SHJc×?(SHE)and Jc=2e/h?SHJs×?(ISHE),where Js(c)is the spin(charge)current,h is reduced Planck's constant,e is the electron charge,and ? denotes the direction of spin index.?sH is the spin Hall angle,which quantifies the conversion efficiency between charge and spin currents.Typically,the measurement of ?SH is entangled with another important material parameter,the spin diffusion length,ASd,which quantifies the decay behavior of the pure spin current during its propagation.Both ?SH and ?sd can be measured via nonlocal magneto-transport,ferromagnetic resonance(FMR)based spin pumping,or spin-torque(ST)-FMR.However,significant inconsistencies exist in the reported values of ?SH and ?sd,even when the same technique is used.This discrepancy is caused by the limitaion of the measurement methods and the ignorance of spin-orbit coupling at interface of current spin pumping theory.Because of the complexity of the interface effect,it is typically difficult to estimate the exact amplitude of the injected pure spin current with the first method.The third method is always entangled with unwanted signal related to the anisotropic magnetoresistance(AMR)and seebeck effect.The second method is of more advantage as the above difficulties can be removed with additional FMR measurements and special geometry design.In this thesis,we resolve the controversy over ?SH and ?sd measurements with systematic experimental studies of the spin-pumping-induced ISHE for a series of ferromagnet/nonmagnet(FM/NM)bilayer combinations(FM=Co,Py,or Co50Fe50,and NM=Pt or Pd)vs.layer thickness.We present measurements of the(i)effective spin mixing conductance,(ii)microwave photo-resistance,and(iii)ISHE voltage.Using an out-of-plane microwave magnetic field excitation,we demonstrate that one can disentangle the ISHE signal from the unwanted AMR effect with the designed geometries.As the injected spin current is proportional to the product of the in and out-of-plane precessing angles,it is important to measure the precessing angles for each individual sample.Through the microwave photoresistance measurements,we find the precessing angles depend on the Pd thickness and the detailed geometry even with the same input microwave power.We note that the ISHE generated charge voltage is proportional to the resistance of NM layer only,and independent of the FM layer thickness.Besides,the pumped spin current not only transmits and reflects at the FM/NM interface,but also suffers an interfacial spin loss(ISL)whose magnitude varies for different interfaces.By taking into account the ISL,we obtain consistent values for Pt(?SH=0.03,?sd = 8.0 nm)and Pd(?SH = 0.005,?scd = 7.7 nm)regardless of the ferromagnet used.Moreover,the obtained ?sd value is larger than mean free path for both Pt and Pd,consistent with general understanding.Our findings clarify the proper values of and approach to quantify ?SH and ?sd.In addition,utilizing epitaxial Co2Fe1-xMnxAl full-Heusler alloy films on GaAs(001),we address the controversy over the analysis for the split hysteresis loop which is commonly found in systems consisting of both uniaxial and fourfold anisotropies.Quantitative comparisons are carried out on the values of the twofold and fourfold anisotropy fields obtained with ferromagnetic resonance and vibrating sample magnetometer measurements.The most suitable model for describing the split hysteresis loop is identified.In combination with the component resolved magnetization measurements,these results provide compelling evidences that the switching is caused by the domain wall nucleation and movements with the switching fields centered at the point where the energy landscape shows equal minima for magnetization orienting near the easy axis and the field supported hard axis.