Intrinsic defects and 4d/5d transition metal defects in Cr${}_2$O${}_3$: pathways for enhancing the Néel temperature

Abstract

First-principles calculations are employed to explore avenues to increase the Néel temperature ($T_{\mathrm{N}}$) of the magnetoelectric antiferromagnet Cr${}_2$O${}_3$ through doping. Employing the hybrid functional method, we calculate the formation energy of intrinsic defects and transition metal dopants (Mo, W, Nb, Ta, Zr, and Hf) to assess their likelihood of formation. Intrinsic defect calculations indicate that Cr interstitials and oxygen vacancies dominate under Cr-rich conditions, whereas Cr vacancies prevail under O-rich conditions. Notably, under Cr-rich conditions, the Fermi level can be pinned slightly above mid-gap due to the formation of Cr interstitials and oxygen vacancies. Additionally, we identify a split Cr vacancy configuration that is more energetically stable than a normal Cr vacancy in corundum Cr${}_2$O${}_3$. To assess the influence of dopants on $T_{\mathrm{N}}$ of Cr${}_2$O${}_3$, we calculate the enhancement of the exchange energy for the spin at the dopant site or at an adjacent Cr site using the supercell method.Our study identifies isovalent Mo and W substitutions at Cr sites as the most promising candidates for increasing the Néel temperature because of impurity-mediated enhancement of the exchange interaction for half-filled bands. Formation-energy calculations indicate that Mo and W substitutions at Cr sites form more readily under Cr-rich conditions and that a Fermi level near or slightly above midgap stabilizes the desirable neutral charge states of the Mo and W defects. This is assisted by the formation of intrinsic Cr interstitials and O vacancies under Cr-rich conditions. These findings offer a route to utilize defects for higher $T_{\mathrm{N}}$and enhanced performance of Cr${}_2$O${}_3$ in magnetoelectric devices and provide useful guidance for future experimental studies.

arXiv comment12 pages, 11 figures