Structural, electronic, and magnetic properties of Co2HfZ (Z = Al and Sn) Heusler alloys for spintronic and caloric applications

Abstract

This report examines the structural, phonon, thermoelectric, and magnetocaloric properties of Co₂HfZ (Z = Al, Sn) full-Heusler alloys (FHA) through the application of density functional theory (DFT) and Monte Carlo simulations. Co₂HfAl and Co₂HfSn systems demonstrate half-metallic (HF) behavior with calculated lattice parameters measuring 6.0190 Å and 6.2199 Å, respectively. The minority spin bands exhibit an indirect band gap of 1.02 eV for Co₂HfAl and 1.61 eV for Co₂HfSn, whereas the majority spin bands persist in a metallic state. The estimated ferromagnetic (FM) transition temperature (T_C) is 170 K for Co₂HfAl and 405 K for Co₂HfSn. The phonon calculations reveal that both alloys exhibit dynamic stability and are devoid of soft modes, whereas thermoelectric evaluations indicate that for Co₂HfAl, the Seebeck coefficient (S) remains marginally positive near the Fermi level across the examined Chemical potential (μ) range and increases with temperature, indicating stable p-type behavior with improved performance at elevated temperatures. In contrast, Co₂HfSn displays a sign change in S with μ and temperature, trending toward more negative values at higher T, which signifies a shift toward enhanced n-type conduction. The research further evaluates the temperature-dependent magnetization and magnetic refrigeration (MR) properties. The maximum value of isothermal entropy change (−ΔS_m)_max varies from 0.25 to 2.85 Jkg^-1K⁻¹ for Co₂HfAl and from 0.1 to 1.8 Jkg^-1K⁻¹ for Co₂HfSn. The RCP measures 11.02 Jkg⁻¹ and 14.80 Jkg⁻¹ at H = 1 T for Co₂HfAl and Co₂HfSn, respectively, increasing to 235.58 Jkg⁻¹ and 312.79 Jkg⁻¹ for H = 17 T. The results highlight their promise for applications that prioritize energy efficiency in fields such as spintronics, thermoelectric, and solid-state refrigeration, offering considerable advantages for environmental sustainability.