Correlation between spin polarization and magnetic moment in ferromagnetic alloys

The correlation between the spin polarization of the tunneling current in ferromagnet/Al2O3/Al tunnel junction experiments and the magnetic moment in ferromagnets has been a mystery [Phys. Rev. B 16, 4907 (1977) and Phys. Rep. 238, 173 (1994)]. In this study, an attempt is made to explain this correlation. By assuming that the s electrons are responsible for the tunneling, the tunneling currents are proportional to the s density of states at the Fermi level [Phys. Rev. B 8, 3252 (1973)]. A tight-binding coherent potential approximation model for itinerant magnetism is applied to calculate the band structure of Fe- and Ni-based alloys [Rev. Mod. Phys. 46, 465 (1974)]. The Slater-Pauling curve for the bulk magnetic moment is recovered, and the spin polarization of the s electrons at the Fermi level is found to correlate well with the magnetic moment. The calculation is carried out in a model where the approximate band structure of an alloy is calculated by using two extra tight-binding parameters besides those of the host. As the average number of electrons is changed by the impurities, the total Coulomb energy and hence the splitting between the up and down spin bands has to be modified self-consistently. The modified band structure is then used to obtain the magnetic moment and the spin polarization. The relation between the magnetic moment and spin polarization can be understood by noting that within approximately 1.5 eV of the Fermi level, the density of states of the s band for either spin is roughly an increasing function of energy and therefore of the number of electrons. For a fixed total number of electrons, the magnetic moment increases with the number of spin-up electrons, corresponding to a larger spin-up density of states at the Fermi level. Thus, the spin polarization increases as the magnetic moment increases. (C) 1999 American Institute of Physics. [S0021-8979(99)00213-3].