Half-Metallic Ferrimagnetism in CoFeNbZ (Z = Al, Si, Ge, Sn) Quaternary Heusler Alloys: a DFT Study

First principles calculations based on density functional theory is used to study the structural, electronic, and magnetic properties of novel CoFeNbZ (Z = Al, Si, Ge, Sn) quaternary Heusler alloys with the full-potential linearized augmented plane wave (FP-LAPW) method. The exchange-correlation effects are treated using generalized gradient approximation (GGA) to study the structural properties. With GGA as the exchange-correlation potential, CoFeNbAl and CoFeNbSn are half-metallic with 100% spin polarization, whereas CoFeNbSi and CoFeNbGe are nearly half-metallic with high spin polarization of 90.06 and 98.28% respectively. For CoFeNbAl and CoFeNbSn, the spin-flip gap increases with the use of modified Becke-Johnson potential. Using this approximation, CoFeNbGe is a half-metal with a spin-flip gap of 0.303 eV. The magnetic properties indicate that all the compounds follow the M_t = Z_t - 24 Slater-Pauling rule. The half-metallic behavior is maintained over a relatively wide range of lattice constants against hydrostatic strain. The transport properties of half-metallic CoFeNbZ (Z = Al, Ge, Sn) compounds are reported. Our present work has the potential to trigger the design of quaternary Heusler alloys with applications in spintronic devices.