Characterization Of Spin Torque Phenomenon In Magnetic Heterostructures

As the scaling down,CMOS-based memory and logic devices are facing significant challenges.While spin dependent transport phenomena based magnetic random access memories and magnetic logic devices have attracted interest due to their advantages of non-volatile,low power consumption and fast operation.The core component of the magnetic random access memory is the ferromagnetic tunnel junction,the read out mechanism is based on the tunnel magnetoresistance effect,and the writing method is in use of the spin torque induced by spin-polarized current to make magnetization switch.There are two forms of the torque,one is spin transfer torque generated by the spin angular momentum transfer effect;the other is the spin orbit torque generated on the spin Hall effect.Understanding the effects and mechanisms behind these two torques in the magnetization switching dynamics helps to optimize the structure of the ferromagnetic tunnel junction device and improve the efficiency of the spin torque and facilitate the design and construction of memory and logic devices with low power consumption.This thesis focus on the investigation of the application of spin transfer torque and spin orbit torque in new memory technology and logic device.The main results are as following:(1)Two types of Heusler based ferromagnetic tunnel junction structures were prepared,which were symmetric structure with Co2MnSi/MgO/Co2MnSi and asymmetric structure with Co2MnSi/MgO/CoFe.The room temperature tunneling magnetoresistance of the symmetric structure tunnel junction is as high as 88%.Further more,investigating the bias dependence of the anti-damping like and field-like spin transfer torque components in both symmetric and asymmetric structures based MTJs using spin transfer torque ferromagnetic resonance technique.One can find that while the damping like torque is linear with respect to bias for both MTJ structures,the asymmetric MTJ structure has an additional linear component to the ordinarily quadratic field like torque bias dependence,and that these results can be accounted for by a free electron tunneling model.Furthermore,these results suggest that the low damping and low saturation magnetization properties of Heusler alloys are more likely to lead significant improvements to spin torque switching efficiency rather than their half metallic character.(2)Magnetic multilayers were prepared with W(O)/CoFeB series where a thin insertion layer with negligible spin orbit coupling was inserted at the W(O)/CoFeB interface.This thesis investigated the mechanism of spin-orbit torque of the series of multilayer films using spin transfer torque ferromagnetic resonance technique.One can find that the placement of an insertion layer suppresses the measured effective spin hall angle in cases where oxygen in incorporated into the W(O)layer,which is different from the values estimated using the transparency formalism.This discrepancy between the bulk transparency formalism along with data that we have taken of the thickn ess dependence of the effective spin hall angle point towards an interfacial mechanism to the SOT in W(O)/CoFeB.The data is thus consistent with an interfacial SOT mechanism in the W(O)/CoFeB system.In addition,the SOT,as quantified by the spin Hall angle remains constant at-50%for W(O)thicknesses down to 2 nm.This is amongst the highest SOT efficiencies reported for a conventional materials system.(3)A novel three-terminal device was prepared by using W(O)as the spin current generated layer and Co40Fe40B20 as the magnetic layer of the ferromagnetic tunnel junction to investigate spin orbit torque phenomena in three terminal device.This thesis demonstrated a novel SOT scheme that exploits non-uniform micromagnetic states to achieve deterministic switching when the spin polarization and magnetic moment axis are non-collinear to one another in the absence of external magnetic field.Lastly,we illustrate how this scheme may potentially be useful for nanomagnetic logic applications.