  A method for the generation of ultrarelativistic electron beams with high
spin polarization is put forward, where a tightly-focused linearly-polarized
ultraintense laser pulse interacts with a nonprepolarized
transverse-size-tailored solid target. The radiative spin polarization and
angular separation is facilitated by the standing wave formed via the incident
and reflected laser pulses at the overdense plasma surface. Strong electron
heating caused by transverse instability enhances photon emission in the
density spikes injected into the standing wave near the surface. Two groups of
electrons with opposite transverse polarization emerge, anti-aligned to the
magnetic field, which are angularly separated in the standing wave due to the
phase-matched oscillation of the magnetic field and the vector potential. The
polarized electrons propelled into the plasma slab, are focused at the exit by
the self-generated quasistatic fields. Our particle-in-cell simulations
demonstrate the feasibility of highly polarized electrons with a single 10 PW
laser beam, e.g. with polarization of 60% and charge of 8 pC selected at energy
of 200 MeV within 15 mrad angle and 10% energy spread.
