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

Abstract

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.

Lattice distortion induced anomalous ferromagnetism and electronic structure in FCC Fe and Fe-TM (TM = Cr, Ni, Ta and Zr) alloys

“…In a range of volumes near equilibrium, the ferromagnetic (FM) state is found to have the lowest energy in BCC Fe while in FCC Fe the antiferromagnetic (AFM) state has the lowest energy [10]. While magnetic states in alpha have been heavily investigated, the magnetic states of FCC Fe and Fe-based alloys have not, through they are as equally intriguing and complex [10][11][12] having been found to include the 2γ-state with low-spin-low-volume states (LS) and high-spin-high-volume states (HS). In addition, both the room-temperature ferromagnetism in FCC nano particles of Fe [4], and an AFM response in FCC Fe thin films (~18Å) below 80K [13] and other structures [14,15] have been observed.…”