Ab initio study of magnetic impurities in rutile TiO2

L. A. ERRICO, M. WEISSMANN Y M. RENTERÍA.

Bonn, Alemania

Conferencia; XIII International Conference on Hyperfine Interactions & XVII International Symposium on Nuclear Quadrupole Interaction; 2004

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Integrating spin functionality into otherwise nonmagnetic materials has become a highly desirable goal in the last years. In particular, dilute magnetic impurities in semiconductors (DMS) produce novel materials appealing for spintronics and optoelectronics (see, e.g., refs. [1-2] and references therein). Magnetic dopants in nonmagnetic solids are assumed to couple with the electronic states of the host, but remaining magnetically active. For their practical applications, the DMS are required to have a high Curie temperature (TC). While most of the dilute magnetic semiconductors (like Ga1-xMnx) have a TC much lower than room temperature, room-temperature ferromagnetism has been observed in Mn-doped ternary compounds such as CdGeP2, ZnSnAs2, and ZnGeP2. Recently, Co-doped anatase and rutile TiO2 thin films were reported to be ferromagnetic even above 400 K [3]. These results have motivated intensive studies on the structural and electronic properties of these materials. However, due to intrinsic complexities, many questions remain regarding the precise location of Co in the host lattice and the underlying microscopic mechanism of long-range magnetic order.             As far as we know, in the case of rutile TiO2, the theoretical and experimental studies concerned only the Co impurities. In this contribution we present a set of density-functional-theory-based calculations in the systems RxTi1-xO2 (R = Mn, Fe, Co, Ni) for different impurity concentrations (x=0.5, 0.25, and 0.0625) and distributions. Calculations were performed with the Full-Potential Linearized-Augmented Plane Waves (FLAPW) method assuming that the magnetic impurities substitutionally replace the Ti atoms. Our results show that magnetism appears in all the systems studied, with the exception of NixTi1-xO2. We found that the impurities introduce contractions of the oxygen nearest-neighbours of the impurities. These contractions does not affect the magnetic properties of the systems.             Finally, from the study of different distributions of the impurities in the host, we calculate the coupling parameters (J). The tendencies to ferromagnetism or antiferromagnetism of these doped semiconductors are discussed and compared with previous calculations and experiments.    This research was partially supported by CONICET, Fundación Antorchas, ANPCyT (PICT 03-03727), Argentina, and TWAS, Italy. LAE (MW and MR) is (are) fellow (members) of CONICET.   [1]  H. Ohno, Science 281, 951 (1998). [2]  S. B. Ogale et al., Phys. Rev. Lett. 91, 77205 (2003). [3]  Y. Matsumoto et al,  Science 291, 854 (2001).