A set of TiO₂ thin films with different Co nanoparticle content was prepared by reactive co-sputtering onto Si (100) substrates. The films thicknesses were in range of 200 to 330 nm. Electron probe microanalysis showed the Co content (x) of the Ti_1-xCo_xO₂ samples was 2.2, 3.0, 4.4, 6.5, and 8.5%. X-ray diffraction measurements revealed polycrystalline Rutile structure of the samples. Magnetization measurements showed hysteresis behavior for all samples with the coercive field in range of 55 - 65 Oe. The saturation magnetization at room temperature ranged from 7 emu/cm³ (2.2% Co) to 28 emu/cm³ (8.5% Co).

ESR measurements at X-band (9.5 GHz) were performed as a function of the sample orientation and temperature. For the magnetic field parallel to the film plane very strong temperature dependence of ESR spectra has been observed. The room temperature signals decreased with cooling, and vanished at lowest temperatures. A very broad signal appeared with cooling, dramatically intensified at about 50-80K, and then decreased at lowest temperature (10K). A kind of phase transition in the temperature behavior of the intensity was observed near the temperature of 50K. We attribute this behavior to unconventional spin-glass-like ordering realized in the system. The results received are important for understanding the tunnel magnetoresistance behavior of such systems.

LRT thanks grants UR.01.01.061 and NIOKR/ANT 06-6.2-47/2002(F) for partial support. The work at MIT is supported by ONR grants N00014-J-92-1847 and N00014-01-1-0832.