“Ferromagnetic oxides, a novel kind of magnetism?”

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

La Plata

Workshop; Hyperfine Interactions at La Plata-International Workshop; 2005

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 TiO2 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-oxygen vacancies is crucial for the existence of high Tc ferromagnetism in Co-doped TiO2 [4]. We present here the structural and magnetic characterization of a set of pulsed laser deposited thin films with nominal composition Ti0.9R0.1O2-? (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 TiO 2, with and without oxygen vacancies. We found that  magnetic moments appear for Mn, Fe, and Co, but not for Ni and Cu when no oxygen vacancies are cons idered. The magnetic ordering results antiferromagnetic for Mn and also for Fe in some geometrical distributions but ferromagnetic for Co. When oxygen vacancies are introduced our results show that the energetically preferred vacancy location is that of nearest neighbor to the impurity. Their presence decreases the energy required to introduce the impurities in the host lattice and reciprocally, the presence of impurities is related to a higher vacancy concentration. The pairs impurity-nearest neighbor oxygen vacancy seem to be the energetically preferred structures and to produce the highest local magnetic moments. Ni and even Cu impurities acquire magnetic moments in this environment. These results support the idea that oxygen vacancies play an important role in the origin of magnetism in doped TiO 2. The results  for Cu impurities indicates that magnetic ions are not essential to obtain ferromagnetism and also that is not due to impurity clustering, since none Cu nor Cu-cluster is magnetic.