Ferromagnetic oxides, a novel kind of magnetism?

DUHALDE S.; VIGNOLO M. F.; GOLMAR F.; CHILIOTTE C. E.; RODRÍGUEZ TORRES C. E.; ERRICO L. A.; CABRERA A. F.; RENTERÍA M.; SÁNCHEZ F. H.; WEISSMANN M.

La Plata

Workshop; HFILP2005, International Workshop “35th Anniversary of Hyperfine Interactions at La Plata”,; 2005

Laboratorio de Espectroscopía Nuclear e Interacciones Hiperfinas, FCE, UNLP e IFLP (CONICET)

Dilute magnetic impurities in semiconductors (DMS) produce novel materials that may be ferromagnetic at room temperature (RT) and are therefore appealing for spintronics [1]. While most of the DMS have a Curie temperature (TC) much lower than RT, Co-doped TiO 2 thin films were reported to be ferromagnetic even above 400 K [2]. These results have motivated intensive studies on the structural and electronic properties of Co-doped TiO2. However, many questions remain open regarding the underlying microscopic mechanism of long-range magnetic order. Carrier-induced interaction between the magnetic atoms was first suggested as the important ingredient underlying ferromagnetism [3]. Subsequent reports have raised concerns about the intrinsic nature of ferromagnetism in these materials, due to possibility of ferromagnetic metal clustering [2]. It has been also suggested that the interaction impurities-oxygenvacancies is crucial for the existence of high Tc ferromagnetism in Co-doped TiO 2 [4].We present here the structural and magnetic characterization of a set of pulsedlaser deposited thin films with nominal composition Ti0.9R0.1O2-d (R = Mn, Fe, Co, Ni,Cu), in order to study the role of dopants in the origin and significance of ferromagnetism in these systems. Room temperature ferromagnetism were found in the cases of Fe, Co, Ni and even Cu impurities, but not in the case of Mn doping. For the interpretation of the experimental results we performed ab initio calculations on doped TiO2, with and without oxygen vacancies. We found that magnetic moments appear for Mn, Fe, and Co, but not for Ni and Cu when no oxygenvacancies are cons idered. The magnetic ordering results antiferromagnetic for Mn andalso for Fe in some geometrical distributions but ferromagnetic for Co. When oxygenvacancies are introduced our results show that the energetically preferred vacancylocation is that of ne arest neighbor to the impurity. Their presence decreases the energyrequired to introduce the impurities in the host lattice and reciprocally, the presence ofimpurities is related to a higher vacancy concentration. The pairs impurity-nearestneighbor oxygen vacancy seem to be the energetically preferred structures and toproduce the highest local magnetic moments. Ni and even Cu impurities acquiremagnetic moments in this environment. These results support the idea that oxygenvacancies play an important role in the origin of magnetism in doped TiO2. The resultsfor Cu impurities indicates that magnetic ions are not essential to obtain ferromagnetismand also that is not due to impurity clustering, since none Cu nor Cu-cluster is magnetic.[1] H. Ohno et al., Nature 408 944 (2000).[2] Matsumoto et al, Science 291, 854 (2001).[3] H. Munekata et al, Phys.Rev.Lett. 63, 1849 (1989).[4] R. Suryanarayanan et al., Solid State Comm. 133, 439 (2005).Keywords: DMS, ab initio, ferromagnetism, oxygen vacancies, metallic impurities ,spintronics , oxide, FP-LAPW