Theory of superconducting SFS and SF junctions involving a ferromagnetic metal at nearly critical temperatures

A microscopic theory of $SFS$ and $SF$ junctions is constructed for temperatures near $T_C$ in the clean and “dirty” limits. A fundamental feature of $SFS$ and $FS$ junctions is demonstrated: If the spin-orbit scattering is negligibly small ($l_{s0}\gg\max\{\xi_0,v_0/h\}$), the parameter of the superconducting order in the $F$ region vanishes strictly at a certain distance from the $SF$ boundary. As a result, phase correlation of the superconducting borders of the $SFS$ junction is impossible if the thickness of the ferromagnetic barrier exceeds a definite critical value $d_C$. (In the pure limit $d_C=\frac{\pi\theta}4\frac{v_0}{h}$, $(h/E_F)^{1/2}<\theta\leqslant1$, and in the “dirty” limit $d_C=\frac{5\pi}4(D/h)^{1/2}$ if $h\gg T_C$ and $d_C=\frac{\sqrt3\pi}2(l/\xi_0)^{1/2}v_0/h$ if $h\ll T_C$.) If there is strong spin-orbit scattering in the $F$ region ($l_{s0}\ll v_0/h$) this feature disappears.