How to Define and Calculate the Degree of Spin Polarization in Ferromagnets

Different ways to define and calculate the degree of spin polarization in a ferromagnet are discussed, particularly with respect to spin-polarized tunneling and Andreev reflection at the boundary between superconductor and ferromagnet. As an example, the degree of spin polarization for different experiments in Fe and Ni is calculated in the framework of the local spin density approximation and used to illustrate the differences between various definitions of spin polarization.