Spin-torque-induced switching and precession in fully epitaxial Fe/MgO/Fe magnetic tunnel junctions

Abstract

We experimentally investigated current-driven oscillation in fully epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions (MTJs) to pave the way for a better understanding of why the linewidth (a few hundred MHz) of microwave oscillation in spin-torque nano-oscillators (STNOs) based on textured MTJs is much larger than that (smaller than 10 MHz) in STNOs based on current-perpendicular-to-plane giant-magnetoresistance junctions. The epitaxial Fe/MgO/Fe STNO is a model system for studying the physics of spin-transfer torque because it has a well-defined single-crystal barrier and electrode layers with atomically flat interfaces. In the Fe/MgO/Fe STNOs, clear spin-torque-induced switching and spin-torque-induced precession were observed in epitaxial MTJs. When the initial magnetic alignment was antiparallel and the bias current exceeded the threshold current, a state in which the spin-torque compensates for the damping, the STNOs showed a rapid increase in the peak intensity, a redshift of the peak frequency, and a minimum linewidth, all clear evidence of spin-torque-induced precession above the threshold current. The minimum linewidth of the STNOs was 200 MHz, which is comparable to that of textured CoFeB/MgO/CoFeB MTJs. This indicates that the origin of the large linewidth cannot be attributed to structural inhomogeneity in textured MTJs. When the initial magnetic alignment was parallel, the microwave spectrum showed a single peak, which has rarely been observed in textured MTJs without application of a perpendicular magnetic field. The mechanism of the single-peak oscillation can be explained by taking account of the induced perpendicular magnetic anisotropy in the 3-nm-thick Fe(001) free layer grown on the MgO(001) barrier layer.