Efficient Current-induced Magnetization Switching In Ferromagnetic Multilayers

Spin-orbit torques(SOT)induced magnetization switching has been demonstrated as a promising technique to control the magnetization in the heavy metal(HM)/ferromagnetic metal(FM)heterostructures with perpendicular magnetic anisotropy(PMA).The charge current passing through the HM film with strong spin-orbit coupling(SOC)produces transversal pure spin current due to the spin Hall effect(SHE),which exerts torques on the magnetization of the FM film,and the sign and the magnitude of SOT are essentially determined by the spin Hall angle(SHA)of HM.On the other hand,when the FM atoms are in an environment with broken symmetry,SOT can also arise from the Rashba SOC,for which the accumulated spins at the interface can force the moments to change their direction by direct exchange coupling.To increase the magnitude and efficiency of SOT,a variety of approaches have been reported,by either generating a stronger spin current or modulating the spin current absorption to enhance torques exerted on the FM layers.In this thesis,the efficient SOT in ferromagnetic multilayers has been investigated through several effective approaches.In the first study,we demonstrated the robust zero-field SOT switching of a perpendicular FM layer where the symmetry was broken by a bilayer of HMs with the strong SOC.These two HM layers(Ta and Pt)are used to strengthen the SOC by linear combination.An enhancement of the strong SOC is seen in device which cannot be explained by a conventional understanding of the SHE.We also observed the similar SOT behavior after inserting Co2FeAl in the multilayer structure.In the second study,we have investigated the SOT in perpendicularly magnetized Pt/Co/W films,which is compared with the results in Pt/Co/AlOx films.Using the harmonic measurements,we have characterized the effective fields corresponding to the damping like torque and the field like torque.Theoretically,in the case of the asymmetrical Pt/Co/W trilayers with opposite sign of spin Hall angle,both damping like torque and field like torque due to the SHE and the Rashba effect will be enhanced,but we have found the dominancy of damping like torque in the Pt/Co/W films.It is much different from the results in the Pt/Co/AlOx films,in which both the damping like torque and the field like torque are evident.In the third study,we have investigated the oxidation effect on the SOT in Pt/Co/TiOx heterostructures through controlling the oxidation time of Ti.The SOT induced magnetization switching in Pt/Co/Ti heterostructures is weak.However,it becomes evident as the oxidation of the Ti layer goes above a threshold level,and the magnetization can be completely switched when the oxidation time of the Ti layer is 11 min.With excessive oxidation,the SOT in the Pt/Co/TiOx films finally disappears.The average valence of the Ti was confirmed by X-ray photoelectron spectroscopy spectra,which is about +3.619 for 11 min oxidation and the Ti2O3 will enhance the SOT efficiency.It is proposed that the oxidation will modulate the interfacial Rashba effect,the film qualities and the spin transparency at the interfaces,which will collectively dominate the SOT results.In the fourth study,the SOT in perpendicularly magnetized Pt/Co/AlOx/Co/Ta multilayers was investigated.The effective SOT field can be tuned by varying the thickness of AlOx as well as exchange coupling of the two FM layers.Reducing the thickness of the AlOx layer significantly affects the magnetic and electrical properties of the system.The relative magnitude of field-like effective field and damping-like effective field were determined.