ИСТИНА

The tuning of physical properties of solids to realize a desired functionality is always a challenge. In case of semiconductors the last decades were devoted to the creation, understanding and manipulation of ferromagnetism in doped semiconducting materials. In spite of great advances in recent years in this area [1] the origin of above and below room temperature ferromagnetism in different DMS and oxides remains a controversial issue [2] in majority of cases. A number of models of a long-range ferromagnetic order in different DMS systems are reported in the literature (see e.g. [3] and references therein). In the case of 3d doped titanium dioxide the most popular but competitive points of view on origin of RTFM are carrier-mediated and defect-induced models. Many recent published results show the strong dependence of RTFM on the preparation method, so different types of structural defects have to be taken into account for the explanation of this phenomenon. Nevertheless, most of authors are concentrated only on the importance of oxygen vacancies [4] and their complexes or the type of the crystal structure (anatase or rutile). We discovered recently [5] that negatively charged structural defects in V- doped TiO2 (1at%) play an important role and should be considered separately from an influence of oxygen vacancies. A ratio of saturation magnetizations in semiconducting and insulating films (a difference in resistivities is about 13 orders of magnitude) was found to be 0.7 while a ratio of defects concentrations in both films was estimated from Positron Annihilation Spectra (PAS) spectra as 0.67. For both films neither MO spectra nor AHE has been found. So, defect-induced model of RTFM is assumed for Ti0.99V0.01O2-δ without involvement of free charge carriers or the local magnetic moment of V ions. Contrary to that samples with 3at% of V doping exhibit the strong MO signal and the shape of MO spectra is conductivity dependent. We suppose that this is the evidence of new spin-polarized states appeared inside the band-gap due to the incorporation of V ions into the octahedral complex of oxygen. Also, a tiny signature of AHE signal has been found for one of these samples. So, we expect that RTFM in 3at% V doped TiO2 is related somehow with a local magnetic polarization of V ions that could create a spin-polarization of the carriers. The results of PAS are also discussed. Support by a German-Russian joint research group HRJRG-314 & RFBR 12-02-91321-SIGa is acknowledged. [1] H. Ohno, J. Appl. Phys. 113, 136509 (2013) [2] T. Dietl, Nat. Mater. 9, 965 (2010) [3] J.M.D. Coey, M. Venkatesan and C.B. Fitzgerald, Nature 4, 173 (2005) [4] J. Lu, K. Yang, H. Jin et al., J. Sol. St. Chem. 184, 1148 (2011) [5] O. Yildirim at el., accepted to PSS, will be published in 2014