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-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-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-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-C of these samples allows us to identify several different mechanisms influencing T-C and causing an experimental maximum in T-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.