Over-room-temperature strain robust half-metallicity and variations in the co-oxidation/spin states in Ni-doped La2FeCoO6

Abstract

Half-metals (HM) hold immense potential for applications in modern nanoelectronics devices because of their 100% spin polarization (SP) at the Fermi level ($E_F$), which allows them to produce purely spin-resolved currents. Herein, the physical features of the La${}_2$FeCoO${}_6$ double perovskite oxide are investigated under the consequence of Ni-doping at the Fe/Co/FeCo-site (Ni@Fe/Ni@Co/Ni@FeCo) along with biaxial ([110]) strain using $ab$-$inito$ calculations. The “$-$” values of determined formation enthalpies ensure the thermodynamical stability of the structures. The pristine system is a ferromagnetic semiconductor owing an energy gap ($E_g$) of 2.04 eV. Strikingly, a half-metallic (100% SP) behavior is acquired for all Ni-doped structures, where Ni 3$d$ states significantly facilitate the conductivity of the spin-majority (N${}^{\uparrow }$) channels. Conversely, the spin-minority (N${}^{\downarrow }$) channels illustrate an insulating character owing a large $E_g$ of 2.55/1.55/1.82 eV for the Ni@Fe/Ni@Co/Ni@FeCo-doped system, which appears to be sufficient to inhibit spin-flipping and assure an effective spin-filtering response. Surprisingly, Co changes its oxidation state from ＋ 4 (high spin state) to ＋ 3 (intermediate spin state)/＋ 3 (high spin state) due to a significant increase in its partial spin moment $m_s$ from 0.32 to $\sim$2.0/3.09 ${\mu }_B$ in the Ni@Fe/Ni@FeCo-doped system. However, in the Ni@Fe, it remains in ＋ 4 (high spin state) with a spin moment of 0.36 ${\mu }_B$. Moreover, all the structures have $T_C$ above room temperature of 385/366/350/340 K in the pristine/Ni@Fe/Ni@FeCo-doped system, making them highly desirable candidates for device applications. Eventually, the robustness of the FM HM phase of the Ni-doped systems is analyzed, which remains preserved for a reasonable $\pm$5% strain. Thus, the present investigation is expected to trigger further experimental research to explore FM HM behavior among various DPOs via the combined influence of electron-doping and strain engineering.