Epitaxial Cr-doped Ti2O3 films show giant negative magnetoresistance up to −365% at 2K. The resistivity of the doped samples follows the behavior expected of spin (magnetic) polarons at low temperature. Namely, ρ=ρ0exp(T0∕T)p, where p=0.5 in zero field. A large applied field quenches the spin polarons and p is reduced to 0.25 expected for lattice polarons. The formation of spin polarons is an indication of strong exchange coupling between the magnetic ions and holes in the system.
Cr- and Fe-doped SnO2 films were deposited on Al2O3 substrates by pulsed-laser deposition. X-ray-diffraction patterns show that the films have rutile structure and grow epitaxially along the (101) plane. The diffraction peaks of Cr-doped samples exhibit a systematic shift toward higher angles with increasing Cr concentration. This indicates that Cr dissolves in SnO2. On the other hand, there is no obvious shift of the diffraction peaks of the Fe-doped samples. The magnetization curves indicate that the Cr-doped SnO2 films are paramagnetic at 300 and 5K. The Fe-doped SnO2 samples exhibit ferromagnetic behavior at 300 and 5K. Zero-field-cooled and field-cooled curves indicate superparamagnetic behavior above the blocking temperature of 100K, suggesting that it is possible that there are ferromagnetic particles in the Fe-doped films. The anisotropic magnetoresistance was not observed in any of the samples.
Pure HfO 2 and Gd-doped HfO 2 thin films have been grown on different single crystal substrates ͑silicon, R-Al 2 O 3 , and LaAlO 3 ͒ by pulsed laser deposition. X-ray diffraction ͑XRD͒ patterns show that the pure HfO 2 thin films are of single monoclinic phase. Gd-doped HfO 2 films have the same XRD pattern except that their diffraction peaks have a shift toward lower angles, which indicates that Gd dissolves in HfO 2. Transmission electron microscopy images show a columnar growth of the films. Very weak ferromagnetism is observed in pure and Gd-doped HfO 2 films on different substrates at 300 and 5 K, which is attributed to either impure target materials or signals from the substrates. The magnetic properties do not change significantly with postdeposition annealing of the HfO 2 films. In addition to the films, HfO 2 powders were annealed in pure hydrogen flow, and a ferromagnetic signal was not observed.
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