Studies On Spin-orbit Torques In Epitaxial Magnetic Heterostructures

In the field of magnetic random storage,nano-oscillators,and spin logic devices,achieving fast and low-power control of magnetic moments is the mainstream research interests of spintronics.The current-driven spin-orbit torques,as a new effective means to manipulate magnetic moment,has set off another new development of spintronics.The purpose of studying the physics mechanism in spin-orbit torques is to control the spin polarization direction of the spin current and improve the spin-charge conversion efficiency.In this paper,we have epitaxially grown in-plane magnetized V/Fe0.5Co0.5/AlOx heterostructures on a(001)-oriented MgO substrate.The harmonic voltage measurement has done to investigate the system's spin-orbit torques for basic research.The conclusions studied are as follows:1.The second harmonic voltage formula in disturbance of spin-orbit torques is obtained by magnetization free energy model of ferromagnetic films.Cubic magneto-crystalline isotropy energy is used to study the magnetization dynamics theory driven by the spin-orbit torques.Combined with the harmonic voltage theory,the magnetic resistance under spin-orbit torques of the single crystal magnetic heterostructure thin-film system is derived.The effective field expressions of the longitudinal and transverse voltage signals provide a theoretical basis for the measurement of the spin-orbit torques.2.The origin of the spin-orbit torques in the V/Fe0.5Co0.5/AlOx heterostructure is systematically studied by using a second-harmonic planar Hall voltage analysis.Compared with the traditional system with heavy metals as non-magnetic layers,the spin-orbit coupling of the bulk materials can be ignored due to the substitute of 3d transition metal V.The V and V/Fe0.5Co0.5 interface dominate the contribution of spin-orbit torques.We found that Field-like torque is dominated while the Damping-like torque is weak.Besides,with the increase of nonmagnetic-layer thickness,the sign of Field-like torque changed and the amplitude gradually increased.We suggest the spin current caused by the interface spin-orbit coupling field be the main source of this spin-orbit torque in this systems.3.The planar Nernst effect mixed in second harmonic signals is analyzed.When studying the second-harmonic planar Hall voltage,we observed a sin2?H signal of unknown origin(?H represents the angle between the external magnetic field and the direction of the current density).Considering the temperature gradient caused by the uneven resistance distribution of the device and the angle-dependent planar Nernst effect that satisfies(m·x)(m·y),we use two Hall-bars with axisymmetric position for verification.It is further confirmed that the source of that term is the planar Nernst effect.4.The behavior of spin-orbit torques related to crystal orientation in the V/Fe0.5Co0.5/AlOx heterostructure is studied.By using Hall-bars with different crystal orientations,harmonic Hall measurements were carried out.It was observed that there was a phase change in both 1st harmonic signals and 2nd harmonic signals,and the phase was related to the crystal orientation.We found the spin-orbit torque efficiency of V[110]crystal orientation is larger than other orientations in this heterostructures.