ИСТИНА

A continues search for systems that could be considered as Diluted Magnetic Semiconductors (DMS) has been launched by the theoretical prediction of T. Dietl et al in 2000 [1] and for this purpose a lot of compounds have been already studied [see 2-3, and references therein]. Among them the systems on the basis of titanium dioxide thin films doped by 3d impurities attracted a lot of attention already since 2001 when Y. Matsumoto with co-authors published his article [4], where the above room-temperature magnetism in this dioxide has been discussed. Nevertheless, up to now there is no one suitable theory that could explain all experimental facts observed in such systems: the large value of magnetic moment per impurity atom, high Curie temperature or specific features in magnetic, transport and magneto-optical properties. Moreover, the role of different defects – oxygen vacancies, their complexes and defects of matrix – is still not satisfactory clear. We report about our last results of joint studies of several DMS systems: TiO2:Co(x at%) and TiO2:V(x at%) thin films (~300nm, x = 1÷3) on LaAlO3 (001) substrates prepared by rf magnetron sputtering technique in argon-oxygen atmosphere at reduced oxygen partial pressure and TiO2:Co(y at%) thin films with amorphous, polycrystalline and epitaxial structures (~300nm, y = 0.5÷4) on SrTiO3 substrates prepared by ion-implantation. All samples reveal the room-temperature ferromagnetic order confirmed by SQUID or VSM magnetometry; the absence of parasitic ferromagnetic phases has been checked by EDX analysis. The rather strong magneto-optical signal measured by Transversal Kerr Effect spectroscopy has been found for all films doped by Co, meanwhile the MO response from films doped by V was at least one order of magnitude lower even for samples with the same level of magnetic moment per impurity atom. Positron Annihilation Spectroscopy (PAS) was used to check the point and open volume defects by depth-resolved Doppler broadening spectroscopy at the slow-positron-beam SPONSOR [5] at ELBE accelerator at HZDR campus with positron energies 27 eV…36 keV. The possible difference in exchange mechanisms in TiO2:Co and TiO2:V systems is under discussion.