Intrinsic defects and 4d/5d transition metal defects in Cr$_2$O$_3$: pathways to enhance the Néel temperature

Abstract: First-principles calculations are employed to explore avenues to increase the N\'eel 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. To assess the influence of dopant on $T{\mathrm{N}}$ of Cr$2$O$_3$, we calculate the enhancement of the exchange energy for the spin on the dopant site or on adjacent Cr site using the supercell method. Our study identifies isovalent Mo and W substitution on Cr site as the most promising candidates to increase N\'eel temperature due to the impurity-mediated enhanced exchange interaction for half-filled bands. Formation energy calculations indicate that Mo and W substitution on Cr are easier to form under Cr-rich conditions and a Fermi level near or slightly above the midgap renders a desirable neutral Mo and W defect. This is assisted by the formation of intrinsic Cr interstitial and O vacancy 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 furnish invaluable insights for directing subsequent experimental endeavors.