Exploring the intrinsic limit of the charge-carrier-induced increase of the Curie temperature of Lu- and La-doped EuO thin films

Raising the Curie temperature ${T}_{\mathrm{C}}$ of the highly spin-polarized semiconductor EuO by doping it with rare-earth elements is a strategy to make EuO more technologically relevant to spintronics. The increase of ${T}_{\mathrm{C}}$ with free carrier density $n$ and the surprisingly low dopant activation $p$, found in Gd-doped EuO thin films [Mairoser et al., Phys. Rev. Lett. 105, 257206 (2010)], raised the important question of whether ${T}_{\mathrm{C}}$ could be considerably enhanced by increasing $p$. Using a low-temperature growth method for depositing high-quality Lu-doped EuO films we attain high dopant activation ($p$) values of up to 67%, effectively more than doubling $p$ as compared to adsorption-controlled growth of Lu- and Gd-doped EuO. Relating $n, p$, and lattice compression of La- and Lu-doped EuO films grown at different temperatures to the ${T}_{\mathrm{C}}$ of these samples allows us to identify several different mechanisms influencing ${T}_{\mathrm{C}}$ and causing an experimental maximum in ${T}_{\mathrm{C}}$. In addition, scanning transmission electron microscopy in combination with electron energy loss spectroscopy measurements on La-doped EuO indicate that extensive dopant clustering is one, but not the sole reason for dopant deactivation in rare-earth doped EuO films.