Effect of electron correlations on the electronic and magnetic structure of Ti-doped<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>α</mml:mi></mml:math>-hematite

We study the electronic and magnetic structure of $\ensuremath{\alpha}$-hematite and $\ensuremath{\alpha}$-hematite doped with Ti using density-functional theory. We use both the local spin-density approximation (LSDA) and the local spin-density approximation with Coulomb correlation $(\mathrm{LSDA}+U)$ approximations. We find that as the value of the parameter U is increased, $\ensuremath{\alpha}$-hematite(Ti) changes from a magnetic half metal with a single relatively delocalized d electron per Ti to an insulator which has the electron localized on a particular Fe site neighboring the Ti impurity. In contrast to Ti-doped $\ensuremath{\alpha}$-hematite, LSDA and LSDA+U are in qualitative agreement for the undoped system, although $\mathrm{LSDA}+U$ predicts values of the structural parameters, band gap, and magnetic moments on the Fe sites which are closer to experimental estimates. In general, $\mathrm{LSDA}+U$ appears to be better suited for this type of material.