Micromagnetic modeling of magnetization switching and oscillation modes in spin valve with tilted spin polarizer

Abstract

Materials with perpendicular magnetic anisotropy have been intensively investigated due to their potential applications in the nonvolatile magnetic memory and spin-torque oscillators. Hear in this paper, we report a special interesting spin-transfer-driven magnetic behavior in perpendicularly magnetized (Co/Ni) -based spin-valve nano-pillars due to the reduced symmetry of easy axis in the free layer. The micromagnetic simulations indicate that a dip in the average magnetization curve can take place due to the reduced symmetry such as tilt of the magnetic field as well as the easy axis of the free and polarizer layers. In order to further clarify the physics mechanism of the dip, we carry out a series of new simulation studies. In our simulations, we consider a spin-valve nano-pillar with perpendicular anisotropy free layer and a 3 tilted polarizer layer. A negative perpendicular magnetic field and a positive perpendicular current are both applied simultaneously. In the average magnetization curves mz as a function of the magnetic field with various currents, three dips are observed. Note that although the spin-transfer torque is essential to the appearance of the dips, the position of the dips is less affected by the current in a certain current range. For three dips, we notice that the mz values are almost identical at a special magnetic field for different currents. At this special magnetic field, the magnetization oscillation modes in the free layer are similar to each other for different currents. The corresponding frequency spectra show that the amplitude of the main frequency peak decreases with the increasing of current due to the enhanced spin-transfer torque. In addition, the frequency shows a blue-shift with the increasing of applied current. Our simulations show that the main frequency f1 corresponding to the highest peak is approximately equal to the precession frequency of the local magnetization in the free layer. Several high-order frequency peaks are also observed in the frequency spectrum with fn=nf1, where n is an integer. Therefore the periodic oscillation of mz is a harmonic oscillation. Further simulations indicate that the dip appearance is also affected by the thickness of free layer. The spin-transfer torque effect decreases with the thickness of the free layer increasing. As a consequence, the dips shift to a low magnetic field range with the increase of the thickness. And for larger thickness t=8.0 nm, no dip appears. This result suggests that the spin-transfer torque is necessary for the dip, rather than the unique effect factor, to occur. In the dip region, the magnetic oscillation modes of the free layer show interesting frequency spectrum characters:harmonic frequency or inter-harmonic frequency. As a consequence, the periodic oscillation of the free layer is accompanied by the harmonic waves.