Ab initio investigation of magnetic ordering and spin-glass transition in Cu-rich Cu–Mn systems

Manganese-noble metal alloys exhibit a large variety of magnetic structures as a function of Mn concentration. Ab initio studies of these structures are practically absent due to difficulties in handling simultaneously atomic and magnetic ordering effects in disordered alloys. In this work we use the generalized perturbation method (GPM) A.V.Ruban, S. Shallcross, S.I. Simak H.L. Skriver, Phys. Rev. B 70 (2004) 125115 in the framework of the KKR method and the coherent potential approximation (CPA) in order to determine both effective chemical interactions and magnetic exchange interaction parameters of the classical Heisenberg Hamiltonian in Cu-Mn alloys. The effective chemical interactions have been used in Ising Monte Carlo simulations to determine the equilibrium distribution of atoms on the FCC lattice at the experimental aging temperatures. The obtained atomic short-range order for Cu-Mn alloys is in excellent agreement with the existing experimental data. The underlying atomic distribution has subsequently been used in Heisenberg Monte Carlo simulations with the exchange interaction parameters calculated for the corresponding alloy composition by the GPM method. For alloys with low Mn content, up to 20 at%, we find a transition into a spin-glass state with a specific magnetic short order.