All-Electrical Manipulation Of Magnetization In Magnetic Heterojunction Films Via Spin-Orbit Torque Effect

The evolution of switching manner of the spins within the free layer of magnetic tunnel junction(MTJ)plays the key role in the generation-by-generation development of magnetic random access memory(MRAM),from first-generation field driven MRAM to second-generation MRAM based on spin transfer torque,then the third-generation spin-orbit torque(SOT)MRAM.The SOT enables us to manipulate the spins in higher speed and lower energy comsuption way.However,the indispensable in-plane bias field still remains obsessed when constructing perpendicular MTJ manipulated in all-electrical way driven by SOT.To solve this problem,this dissertation explores the principles of realizing all-electrical manipulation of perpendicular spins in magnetic heterojunction films via spin-orbit torque,and the main research and results are as following:(1)The film with the core structure of Pd/Co/IrMn,possessing perpendicular magnetic anisotropy(PMA),is prepared.It was then patterned into Hall bars via micro-nano fabrication technologies,so the anomalous Hall resistance could be examined to detect the average vertical moments of the Hall bars.Experiment results showed that the antiferromagnetic material IrMn could induce in-plane exchange bias field and spin Hall effect to realize magnetization switching of Co layer in the condition of zero external field.The exchange bias field was determined to be 1.8 mT and the critical current density was 2.2×10~7A/cm~2 during field-free switching processes.(2)In the synthetic antiferromagnet(SAF)of CoFeB/Ta/CoFeB where both CoFeB layers exhibited PMA and coupled with each other mediated by Ta spacer,we experimentally studied the switching behavior of the spins based on SOT.The magnetization of both CoFeB layer switched simultaneously in response to the applied current with the assistantance of in-plane bias field.Based on the results of harmonic measurements and switching phenomenon,we estimated the generation efficiency of the field-like field was about 0.74 mT/(MA/cm~2),while that of anti-damping-like field was determined to be approximately 0.60 mT/(MA/cm~2).That is to say,the generation efficiency of field-like torque is 1.24 times as large as that of anti-damping-like torque.Finally,we derived that the effective spin Hall angle of 1.3-nm-thick Ta layer is about-0.158.We also concluded that in this synthetic antiferromagnet field-free magnetization switching was ruled out by the absence of broken inversion symmetry.(3)In silimar structure of CoFeB/Ta/CoFeB in which the PMA CoFeB layer coupled to the other CoFeB layer with in-plane magnetic anisotropy(IMA),the PMA layer would be subjected to an effective field induced by IMA layer.The experiment results suggested that the IMA layer possessed a uniaxial easy axis(EA)while EA of the PMA layer titled against z axis by an angle of 10~o.We observed double types of magnetization switching mode in IMA-PMA coupling structure:type-T and type-Z mode.In type-T mode,the current was applied along the hard axis of IMA layer so that the current directed perpendicular to the PMA layer as well as in-plane effective field.Theoretical and experimental results showed that the applied current along hard axis of IMA layer could switch the spin of PMA and IMA layers simultaneously and the switching chirality was independent with external field.In type-Z mode,the current is along the EA of IMA layer,only the spins of PMA layer could be reversed during switching processes and the in-plane bias filed could determine the switching chirality.(4)The film structure of MTJ is substrate//Ta(8)/CoFeB(2)/Ta(1.3)/CoFeB(0.6)/MgO(2.5)/CoFeB(1.2)/Ta(5)/Ru(5 nm).After annealing,the two CoFeB layers separated by Ta spacer layer formed T-type magnetic configuration.Then the film was patterned into MTJs with ellipse pillars of the size of 5?m(short axis)-10?m(long axis)?5?m-15?m?5?m-20?m?10?m-20?m and 10?m-30?m.The tunneling magneticresistance ratio of the MTJs is about 43%~50%.Furthermore,all-electrical manipulation was realized without external field owing to the exchange coupling and the properties of magnetization switching was also systematically studied in the presence of external bias field.