Quasipolaron Surface Polarization in Bismuth Ferrite

With the introduction of a not fully screened polarization that leads to three magnetic subdomains nested in one ferroelectric domain, we identify it as the quasipolaron surface polarization in the electrode effect of bismuth ferrite. The remanent polarization at 40 Hz is about 439 μC/${\mathrm{cm}}^2$ in the dielectric hysteresis loop of a reverse S shape, accompanying the antiferromagnetic one of an S shape, which reveals that the surface is an indispensable part in spintronics.