Field-free synthetic-ferromagnet spin torque oscillator

We study the magnetization dynamics of spin valve structures with a free composite synthetic ferromagnet (SyF) that consists of two ferromagnetic layers coupled through a normal metal spacer. A ferromagnetically coupled SyF can be excited into dynamical precessional states by an applied current without external magnetic fields. We analytically determine the stability of these states in the space spanned by the current density and SyF interlayer exchange coupling. Numerical simulations confirm our analytical results.

Figure 1

A spin valve structure with an SyF free layer, where FM${}_0$ is the fixed layer and FM${}_{1,2}$ layers are (anti) ferromagnetically coupled.

Figure 2

Dynamical phase diagram in the parameter space of currents and RKKY coupling strengths. (a) Phase diagram calculated analytically by Eq. (5); none of the four states $\uparrow \uparrow ,\uparrow \downarrow ,\downarrow \uparrow ,\downarrow \downarrow$ is stable in the white region. (b) Phase diagram calculated by numerically solving the LLGS Eq. (3). The purple are the STO phase, and the white one is the canted state. (c) The time evolution of the polar angles ${\theta }_{1,2}=\angle ({\mathbf{m}}_{1,2},\widehat{\mathbf{z}})$ at the six different points indicated in the phase diagram. In the third subfigure of (c), the solid and dashed lines correspond to different sets of initial conditions.

Figure 3

Power spectrum for ${\mathbf{m}}_0·{\mathbf{m}}_1$ (left) and ${\mathbf{m}}_1·{\mathbf{m}}_2$ (right) as a function of current density $j$ and frequency $f$ at $J=0.25$ mJ/m${}^2$, corresponding to the black line in the top-right panel of Fig. 2.