Magnetic Coupling in the Ni-Pd Alloy System

The magnetic coupling properties of the Ni-Pd alloy system are considered in terms of a molecular-field solution based on a Heisenberg type of interaction for spin ½. The unpaired electrons in Ni are assumed to be part of a band of mainly $t_{2g}$ character in which the totality of interactions leads to an interatomic ferromagnetic coupling. The unpaired electrons in pure Pd are assumed to be part of a narrow, weakly interacting band with more nearly $e_g$ symmetry. In the alloy, the Pd band structure becomes progressively more like that of Ni. The magnetic energy is then calculated in terms of the probability of finding unpaired electrons in the overlapping orbitals of the Ni and Pd atoms. The probability is determined from the average moment curve on the assumption of $3d^9$ and $3d^{10}$ states only. A fit to the data on the Curie temperature and average magnetic moment then gives the following relative values for the exchange constants (molecular-field coefficients); $A$ for Ni-Ni, $\alpha A$ for Ni-Pd, and ${\alpha }^{2}A$ for Pd-Pd, with $\alpha \cong 0.8$.