Measurement And Study Of Spin Orbital Torque In Magnetic Heterostructures

Nowadays,the explosive growth of information shows higher requirements for information processing and storage technology,but the microelectronic devices and integrated circuits based on the charge properties of electrons have approached their physical limits.The development of new control means of electronic devices has become an urgent requirement for the development of electronic information technology.Spintronic devices are electronic devices based on controlling another basic attribute of electron-spin.Its emergence not only improves the area density of traditional magnetic recording,but also opens a new research field of condensed matter physics and microelectronics.Especially in magnetic random access memory(MRAM),the spin torque effect induced by current is used to replace the external magnetic field as the source of reversalling magnetic moment in magnetic heterostructures,which makes the kind of memory has the advantages of high density and low power consumption.The third generation MRAM,spin orbit torque magnetic random access memory(SOT-MRAM),using the spin orbit torque effect of spin current generated by non-magnetic layer as the way of data reading and writing,which makes it have the advantages of faster reading and writing speed and stronger stability,has become the most promising magnetic storage technology at present.The accurate measurement,physical origin and control of spin current of SOT in magnetic heterostructures are the key to the development of SOT devices,they have also become the focus of spintronics research.In this paper,we prepared heavy metal/ferromagnetic(HM/FM)and ferromagnetic/non-magnetic/ferromagnetic(FM/NM/FM)heterostructures by magnetron sputtering technology.The SOT in these systems was measured by the second harmonic measurement based on magnetoresistance and Hall effect and the loop shift method,we also discussed its physical origin.The main innovative results are as follows:1.A complete SOT second harmonic measurement method is established.The second harmonic method previously developed is mainly based on the second harmonic of Hall effect to measure the SOT effective field generated by the spin current with specific polarization in the plane.This method is not applicable to the system with weak anomalous Hall effect(AHE);In addition,with the further development of spintronics,the discovery of spin currents in different polarization directions also requires the further development of second harmonic technology.In this paper,firstly,we theoretically give the harmonic signals generated by various Hall effects and magnetoresistance effects of spin current with different polarization directions,then obtain the angular relationship of the second harmonic signals from the hall end and resistance end in different planes.According to the dependence about the two torques of the second harmonic signals on the direction of magnetic moment and the size of magnetic field,the corresponding effective field can be separated from the second harmonic signals.This work broadens the original second harmonic measurement technology based on Hall effect and lays a foundation for the accurate measurement of SOT in magnetic heterostructures.2.The effective field of SOT in Pt/NiFe structure is measured by the second harmonic measurement method of magnetoresistance.The feasibility of measuring the effective field of SOT by the second harmonic method at the resistance end is verified experimentally.It is found that the second harmonic signal(sin3?+sin?)is obtained by in-plane rotation operation;The second harmonic signal obtained by out of plane(xz plane)rotation operation presents sin2?_M/[H_Kcos?_M+Hcos(?_H-?_M)],which is consistent with the theoretical expectation.The independent measurement of field like SOT(FL-SOT)and damping like SOT(DL-SOT)in Pt/NiFe structure is realized by fitting the angular dependence of the second harmonic signals with different plane rotation.On this foundation,we further study the dependence of SOT effective field with Pt thickness in this structure.Fitting it by spin diffusion model,we get that the spin Hall angle of Pt is 0.09,which is basically consistent with the result measured by other methods.This work realizes the accurate measurement of SOT in the system with weak anomalous Hall effect(AHE),and it widens the application of the original second harmonic measurement.3.The second harmonic method of Hall effect is used to measure the size and the thickness dependence of the two kinds of SOT effective fields in HM/CoFeB/MgO system.The experimental results show that in Pt(Ta)/CoFeB/MgO series samples,when the ferromagnetic layer is thin to a certain thickness,the FL-SOT effective field deviates from the 1/t_FM dependence and shows an obvious enhancement;The efficiency of DL-SOT has no obvious dependence on the thickness of ferromagnetic layer.We believe that the main reason for this phenomenon is the spin reflux effect at the CoFeB/MgO interface;Due to the obvious dead layer effect in Ta/CoFeB structure,the effective thickness of CoFeB is reduced,resulting in the further enhancement of spin reflux effect.In addition,in Ta/Pt/CoFeB/MgO structure,DL-SOT and FL-SOT show basically the same change trend with the change of Pt thickness,and can be fitted by spin diffusion theory,which shows that spin Hall effect is the main source of spin current in this structure4.The SOT effect and the magnetization reversal in ferromagnetic/non-magnetic/ferromagnetic structure are studied.L1₀-Fe Pt(110)/Cu/Fe/SiN heterostructure was epitaxially grown on single crystal Sr Ti O₃(110)substrate.It was found that when the current direction and Fe Pt easy axis meets the orthogonal configuration,magnetization reversal of Fe layer driven by the current can be realized.This phenomenon can not be explained by the Oersted field generated by current,which confirms the spin current and SOT effect are generated by AHE effect in ferromagnet;We further measured the effective field and efficiency of SOT in this system by loop shift method and second harmonic method.This work shows important value for expanding the material,studying mechanism of spin current and controlling the polarization direction of spin current.