Impact of local arrangement of Fe and Ni on the phase stability and magnetocrystalline anisotropy in Fe-Ni-Al Heusler alloys

Abstract

On the basis of density functional calculations, we report on a comprehensive study of the influences of atomic arrangement and Ni substitution for Al on the ground state structural and magnetic properties for Fe2Ni1+xAl1−x Heusler alloys. We discuss systematically the competition between five Heusler-type structures formed by shuffles of Fe and Ni atoms and their thermodynamic stability. All Ni-rich Fe2Ni1+xAl1−x tend to decompose into a dual-phase mixture consisting of Fe2NiAl and FeNi. The successive replacement of Ni by Al leads to a change of ground state structure and eventually an increase in magnetocrystalline anisotropy energy (MAE). We predict for stoichiometric Fe2NiAl a ground state structure with nearly cubic lattice parameters but alternating layers of Fe and Ni possessing an uniaxial MAE which is even larger than tetragonal L10-FeNi. This opens an alternative route for improving the phase stability and magnetic properties in FeNi-based permanent magnets.

Electronic and Vibrational Properties of Fe₂NiAl and Co₂NiAl Full Heusler Alloys: A First-Principles Comparison

“…However, Neibecker et al [41] recently suggested that layered crystal motives can be a strong competitor. This is corroborated by our recent efforts devoted to the firstprinciples study of structural stability and magnetic properties of these motifs in (Fe,Co) 2 Ni 1+x Al 1−x [42], [43]. In particular, we identified the T p -layered structure with nearly cubic lattice parameters as a potential ground state for Febased alloys at the stoichiometric composition and small Niexcess, whereas stoichiometric Co 2 NiAl has in turn an inverse tetragonal structure with c/a = 1.3.…”

“…Inverse XA structure: two X atoms are placed at non-equivalent 4a (0, 0, 0) and 4c (1/4, 1/4, 1/4) sites and have different environments, while Ni and Al are placed at 4b (1/2, 1/2, 1/2) and 4d (3/4, 3/4, 3/4) positions, respectively. T p , T c , and T # can be constructed from the inverse structure by intermixing [41], [42].…”

“…In case of Co 2 NiAl, we essentially observe a red-shift of the features in the energy range between 3 and 8 eV, which again results in a higher Such an increase in density at low energies is indicative of a softening of both structures, which also affects thermodynamic quantities, in particular an increase the lattice entropy, which might further stabilize the T p structure with increasing temperature. We therefore evaluated the free energy F in the harmonic approximation of the structures at finite temperatures in a similar fashion as in [42], taking into account the vibrational contributions derived within the harmonic approximation from the VDOS in Fig. 4 and the electronic contributions from the DOS in Fig.…”

“…4 and the electronic contributions from the DOS in Fig. 3 in terms of the Sommerfeld expansion, while the magnetic free energy is neglected according to the rather high Curie-temperatures in these compounds [42], [43]. The difference in the free energies ∆F (T ) = F T p (T ) − F inverse (T ) for both compounds as a function of temperature T is presented in Fig.…”

“…Here T p is close in energy, becoming more favorable with Ni excess [43]. In this work, we provide a comparative analysis of the electronic, vibrational and thermodynamic properties of stoichiometric Fe 2 NiAl and Co 2 NiAl, which both were predicted to show a large MAE in their T p structures: easy-axis and as large as 1.05 MJ/m 3 for Fe 2 NiAl [42] and easy plane with a value of −2.2 MJ/m 3 for Co 2 NiAl [43].…”