Aaron Aguayo

Effects of Al doping on the structural and electronic properties of Mg 1 − x Al x B 2

We have studied the structural and electronic properties of Mg 1 - x Al x B 2 within the virtual crystal approximation (VCA) by means of first-principles total-energy calculations. Results for the lattice parameters, the electronic band structure, and the Fermi surface as a function of Al doping for 0≤x≤0.6 are presented. The ab initio VCA calculations are in excellent agreement with the experimentally observed change in the lattice parameters of Al-doped MgB 2 . The calculations show that the Fermi surface associated with holes at the boron planes collapses gradually with aluminum doping, and vanishes for x = 0.56. In addition, an abrupt topological change in the σ-band Fermi surface was found for x=0.3. The calculated hole density correlates closely with existing experimental data for T c (x), indicating that the observed loss of superconductivity in Mg 1 - x Al x B 2 is a result of hole band filling.

Effects of Co doping on the metamagnetic states of the ferromagnetic fcc Fe–Co alloy

The evolution of the metamagnetic states in the ferromagnetic face centered cubic (fcc) Fe(1-x)Co(x) alloy as a function of Co concentration has been studied by means of first-principles calculations. The ground state properties were obtained using the full-potential linear augmented plane wave method and the generalized gradient approximation for the exchange-correlation functional. The alloying was modeled using the virtual crystal approximation and the magnetic states were obtained from the calculations of the total energy as a function of the spin moment, using the fixed spin moment method. For ferromagnetic fcc Fe, the binding-energy curve shows metamagnetic behavior, with two minima corresponding to a small-volume, low-spin (LS) state and a large-volume, high-spin (HS) state, which are separated by a small energy (E(LS) ≲ E(HS)). The evolution of the magnetic moment, the exchange integral (J), and the binding-energy curve is analyzed in the whole range of Co concentrations (x). The magnetic moment corresponding to the HS state decreases monotonically from 2.6 μ(B)/atom in fcc Fe to 1.7 μ(B)/atom in fcc Co. In contrast, the exchange integral for the HS state shows a maximum at around x = 0.45. The thermal dependence of the lattice parameter is evaluated with a method based on statistical mechanics using the binding-energy curve as an effective potential. It is observed that the behavior of the lattice parameter with temperature is tuned by Co doping, from negative thermal expansion in fcc Fe to positive thermal expansion in fcc Co, through the modification of the energetics of the metamagnetic states.