First-principles theory of short-range order, electronic excitations, and spin polarization in Ni-V and Pd-V alloys

The short-range order (SRO) and long-range order (LRO) of Ni-V and Pd-V alloys are studied theoretically by a combination of first-principles calculations of Ising-like interaction energies (Jf) with a Monte Carlo solution of the Ising Hamiltonian. We find the following: (i) There are several compositions in these alloys for which the dominant wave vectors of LRO and those of SRO do not coincide, indicating that the low-temperature (T) LRO may not always be inferred from the high-T SRO. (ii) In Ni3V and Pd3V, the density of states at the Fermi level, n(εF), is much larger in L12 than in the stable D022 structure. This has two consequences: (a) thermal electron-hole excitations across εF are energetically more favorable in the L12 structure and lead to a T dependence of Jf, and (b) magnetization stabilization is larger in L12, so spin polarization affects structural stability. As a result, (iii) calculations using T-dependent Jf's are needed to obtain quantitative agreement with experimental measurements of LRO, SRO, and transition temperatures in Ni0.75V0.25, Ni0.67V0.33, and Pd0.75V0.25. (iv) We provide predictions of the SRO patterns where there is currently no experimental evidence for Pd0.67V0.33, Ni0.6V0.4, Pd0.6V0.4, and Pd0.5V0.5. (v) For Ni3V and Pd3V, discrepancies are found between the total-energy differences of the L12 and D022 structures as determined by T=0 first-principles calculations and those inferred from diffuse neutron scattering measurements at high T. By performing temperature-dependent self-consistent local-density-approximation calculations, we find that electronic excitations are responsible for reducing the discrepancy by ∼25% and the combination of spin polarization and electronic excitations reduce the discrepancy by ∼30-50 %. Thus, electronic excitations and spin polarization are not fully responsible for the T dependence of Jf used in the SRO calculations.