Spin polarization is the degree to which the spin, i.e., the intrinsic angular momentum of elementary particles, is aligned with a given direction. This property may pertain to the spin, hence to the magnetic moment, of conduction electrons in ferromagnetic metals, such as iron, giving rise to spin-polarized currents. It may refer to (static) spin waves, preferential correlationof spin orientation with ordered lattices (semiconductors or insulators). The circular polarization of electromagnetic fields is due to spin polarization of their constituent photons.