Magnetization dynamics in spin torque nano-oscillators: Vortex state versus uniform state

Current-driven magnetization dynamics in spin torque nano-oscillators (STNOs) is intensely investigated because of its high potential for high-frequency (HF) applications. We experimentally study current-driven HF excitations of STNOs for two fundamental magnetization states of the free layer, namely, vortex state and uniform in-plane magnetization. Our ability to switch between the two states in a given STNO enables a direct comparison of the critical currents, agility, power, and linewidth of the HF output signals. We find that the vortex state has some superior properties, in particular, it maximizes the emitted HF power and shows a wider frequency tuning range at a fixed magnetic field.

Figure 1

(Color) (a) Giant magnetoresistance of a pillar structure as sketched in the inset. (b) Micromagnetic simulations of two decoupled disks of 230 nm diameter and thicknesses of 20 and 2 nm, respectively, separated by a distance of 6 nm (inset). The pairs of magnetizations patterns show the $m_y$ component of the thick (upper) and thin (lower) disks for the indicated fields, which are applied along the magnetic easy $x$ axis. The red curve shows the $x$ component of the reduced magnetization and the blue line the resulting resistance curve. The spike in the resistance at zero field is due to numerical errors in the calculation of the resistance.

Figure 2

(Color) Current-induced switching between the low-resistive vortex and high-resistive uniform state. Arrows on the curves indicate the current sweep direction. The black and purple curves start in the uniform state, all others in the vortex state. For clarity the graphs measured at 40 and 30 mT are offset by $+20$ and $+40\text{m}\Omega$, respectively, relative to the green curve.

Figure 3

(Color) Spin-transfer torque induced excitation of qualitatively different oscillatory modes in a nanodisk. (a) After preparation of a uniform state a standing-wave mode with a transition from blueshift to redshift is excited; measurement at 82 mT. (b) The gyrotropic mode is excited after preparation of the vortex state; measurement at 25 mT. Note that the microwave output power generated by the gyrating vortex for a given dc current in (b) is much higher than for the standing-wave mode in (a).