CPA theory of diluted Heisenberg ferromagnet with random first- and second-neighbour exchange bonds

A coherent potential approximation (CPA) ansatz for a Heisenberg ferromagnet which possesses both nearest and next-nearest neighbour exchange interactions, is given. Spatial randomness caused by introduction of non-magnetic impurities, i.e. magnetic site dilution, is treated in the approximation where the bonds are taken to be independent. The occurrence of exchange bond fluctuations between pairs of magnetic atoms, is also considered. Numerical results are given for the coherent exchange integrals, spin-wave stiffness and the Curie temperature (which is derived within a random phase type of approximation) for a variety of situations.