Appearance of room-temperature ferromagnetism in Cu-doped ${\mathrm{TiO}}_{2-\delta }$ films

We report here the unexpected observation of significant room-temperature ferromagnetism in a semiconductor doped with nonmagnetic impurities, Cu-doped ${\mathrm{TiO}}_2$ thin films grown by pulsed laser deposition. The magnetic moment, calculated from the magnetization curves, resulted surprisingly large, about $1.5{\mu }_B$ per Cu atom. A large magnetic moment was also obtained from ab initio calculations, but only if an oxygen vacancy in the nearest-neighbor shell of Cu was present. This result suggests that the role of oxygen vacancies is crucial for the appearance of ferromagnetism. The calculations also predict that Cu doping favors the formation of oxygen vacancies.

Figure 1

X-ray diffraction pattern for Cu-doped ${\mathrm{TiO}}_{2-\delta }$ film deposited by PLD on a ${\mathrm{LaAlO}}_3$ substrate.

Figure 2

Room-temperature hysteresis curve of the Cu-doped ${\mathrm{TiO}}_{2-\delta }$ film (points) in the sample as cast and after a thermal treatment (TT) at 800 °C for 30 min in a oxygen-rich atmosphere. Solid lines are the best least-squares fits of Eq. (1) to the data. The inset compares the hysteresis loops measured with the external magnetic field applied parallel (PA) and perpendicular (PP) to the film plane for the as cast sample.

Figure 3

Density of states of ${\mathrm{Ti}}_3{\mathrm{CuO}}_7$, with the oxygen vacancy near Cu. Positive values are for spin up and the negative ones for spin down. Energies are referred to the Fermi level, $E_F$. The minority spin feature inside the gap but close to $E_F$ is 50% of Cu 3$d$ and 50% of oxygen and Ti character.