Quantized resonant tunneling effect in Josephson junctions with ferromagnetic bilayers

We study the Josephson effect in SF$_1$F$_2$S junctions, which consist of conventional s-wave superconductors (S) connected by two ferromagnets (F$_1$ and F$_2$). At low temperatures, the Josephson critical current displays periodic resonance peaks as exchange fields ($h_1$, $h_2$) and thicknesses ($d_1$, $d_2$) of the F$_1$ and F$_2$ layers vary, provided a potential barrier exists at the F$_1$/F$_2$ interface. These resonance peaks emerge under the quantization conditions $Q_{1(2)}d_{1(2)}=\left(n_{1(2)}+1/2\right)\pi$. Here, $Q_{1(2)} = 2h_{1(2)}/(\hbar v_F)$ represents a center-of-mass momentum carried by Cooper pairs, where $v_F$ is the Fermi velocity and $n_{1(2)} = 0, 1, 2, \cdots$. This critical current feature arises from a resonant tunneling effect induced by spin-triplet pairs with zero spin projection along the magnetization axis. At resonance, the Josephson current primarily originates from the first harmonic in both parallel and antiparallel magnetization configurations, whereas the second harmonic becomes more significant in perpendicular configurations. In cases where both ferromagnetic layers have identical exchange fields and thicknesses, the potential barrier selectively suppresses the current in the 0-state while maintaining it in the $\pi$-state for parallel configurations. Conversely, in antiparallel configurations, the current in the 0-state is consistently preserved.