We studied the impact of different insertion layers (Ta, Pt, and Mg) at the CoFeBjMgO interface on voltage-controlled magnetic anisotropy (VCMA) effect and other magnetic properties. Inserting a very thin Mg layer of 0.1-0.3 nm yielded a VCMA coefficient of 100 fJ/V-m, more than 3 times higher than the average values of around 30 fJ/V-m reported in TajCoFeBjMgO-based structures. Ta and Pt insertion layers also showed a small improvement, yielding VCMA coefficients around 40 fJ/V-m. Electrical, magnetic, and X-ray diffraction results reveal that a Mg insertion layer of around 1.2 nm gives rise to the highest perpendicular magnetic anisotropy, saturation magnetization, as well as the best CoFe and MgO crystallinity. Other Mg insertion thicknesses give rise to either under-or over-oxidation of the CoFejMgO interface; a strong over-oxidation of the CoFe layer leads to the maximum VCMA effect. These results show that precise control over the Mg insertion thickness and CoFe oxidation level at the CoFeBjMgO interface is crucial for the development of electric-field-controlled perpendicular magnetic tunnel junctions with low write voltage.

Current-induced magnetization switching (CIMS) in low-resistance magnetic tunnel junctions was shown at average critical current densities (Jc=1.33×106 A/cm2). When large vertical currents pass through the junctions, spin-transfer torque, and vortex fields can rotate the magnetization of the free layer from the initial parallel state to a vortex state, resulting in 10.8% CIMS resistance change at zero-bias current, which is about half of the resistance change (22%) induced when switching is created by an external field. A micromagnetic simulation including the spin-transfer torque and the vortex field correctly predicts the critical negative-current-inducing switching from the parallel state into the vortex state, but fails to explain the reverse switching from the vortex state into the parallel state at an approximately symmetric positive critical current. Lead fields were analyzed and found to be not the cause of the observed switching. The very small dependence of the switching currents on an external magnetic field suggests the existence of hot-spots where local current densities may be much larger.

The current-induced spin-orbit torques (SOTs) in the perpendicularly magnetized Ir22Mn78/Co20Fe60B20/MgO structures are investigated. The damping- and field-like torques are characterized using a harmonic technique. The spin Hall angle of Ir22Mn78 is determined to be θSHE = +0.057 ± 0.002. The SOT-driven magnetization switching is also demonstrated with the assistance of an external in-plane field. Furthermore, the magneto-optical Kerr effect imaging experiments show that the magnetization switching is realized through domain nucleation and domain wall motion. These results may promise potential practical applications in high-performance SOT devices based on the antiferromagnetic materials.

Spin-torque-driven vortex dynamics are studied by micromagnetic modeling in a spin-valve pillar which contains a perpendicular polarizer and a vortex free layer. Two kinds of transient oscillations mediated by the vortex-core motion are observed. The oscillations are treated as the competition among the spin torque, gyroforce, Gilbert damping, and the restoring force, governed by the generalized Thiele equation [A. A. Thiele, J. Appl. Phys. 45, 377 (1974)]. The fundamental frequency is dominated by the gyrotropic motion, while the high-frequency oscillation is triggered by the balance of the spin torque and demagnetizing field. The polarity of the vortex core can be switched through a vortex-antivortex pair creation and annihilation process.