Ab-initio study of magnetic impurities in rutile TiO2.

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

Bonn

Conferencia; 13th International Conference on Hyperfine Interactions & 17th International Symposium on Nuclear Quadrupole Interaction; 2004

ISKP, Bonn, Germany

Integrating spin functionality into otherwise nonmagnetic materials has becomea highly desirable goal in the last years. In particular, dilute magnetic impurities insemiconductors (DMS) produce novel materials appealing for spintronics andoptoelectronics (see, e.g., refs. [1-2] and references therein). Magnetic dopants innonmagnetic solids are assumed to couple with the electronic states of the host, butremaining magnetically active. For their practical applications, the DMS are required tohave a high Curie temperature (TC). While most of the dilute magnetic semiconductors(like Ga1-xMnx) have a TC much lower than room temperature, room-temperatureferromagnetism has been observed in Mn-doped ternary compounds such as CdGeP2,ZnSnAs2, and ZnGeP2. Recently, Co-doped anatase and rutile TiO2 thin films werereported to be ferromagnetic even above 400 K [3]. These results have motivatedintensive studies on the structural and electronic properties of these materials. However,due to intrinsic complexities, many questions remain regarding the precise location ofCo in the host lattice and the underlying microscopic mechanism of long-rangemagnetic order.As far as we know, in the case of rutile TiO2, the theoretical and experimentalstudies concerned only the Co impurities. In this contribution we present a set ofdensity-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 PlaneWaves (FLAPW) method assuming that the magnetic impurities substitutionally replacethe Ti atoms. Our results show that magnetism appears in all the systems studied, withthe exception of NixTi1-xO2. We found that the impurities introduce contractions of theoxygen nearest-neighbours of the impurities. These contractions does not affect themagnetic properties of the systems.Finally, from the study of different distributions of the impurities in the host, wecalculate the coupling parameters (J). The tendencies to ferromagnetism orantiferromagnetism of these doped semiconductors are discussed and compared withprevious 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).