First-principles-based investigation of half-metallicity in new half-Heusler alloys HfYAs (Y = Fe, Mn)

In the present work, we report a detailed first-principles investigation of the structural, electronic, magnetic, mechanical, phonon dynamics and thermodynamic properties of the newly proposed half-Heusler alloys HfYAs (Y = Fe, Mn) using density functional theory within the generalised gradient approximation as implemented in the Quantum ESPRESSO package. The calculated electronic band structures reveal clear half-metallic behaviour, with a finite band gap in the majority spin channel and metallic character in the other. The obtained energy band- gaps are 1.36 eV for HfFeAs and 1.02 eV for HfMnAs with half-metallic gaps of 0.10 and 0.22 eV, respectively. The total magnetic moments are found to be 1.00 μB and 2.00 μBper formula unit, respectively, in accordance with the Slater–Pauling rule, indicating spin polarisation at the Fermi level. Importantly, the half-metallic nature is found to be robust under lattice strain, which highlights the stability of the spintronic characteristics against structural distortions. Phonon dispersion calculations further confirm dynamical stability through the absence of imaginary (negative) frequencies throughout the Brillouin zone. The computed elastic constants satisfy the Born’s mechanical stability criteria for cubic systems and suggest a ductile mechanical response. Overall, the combined evidence of thermodynamic, mechanical, dynamical and strain stability establishes HfFeAs and HfMnAs as promising candidates for future spintronic applications.