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We report on magnetotransport properties on La_(0.7)Sr_(0.3)MnO_(3)/MgO/Fe tunnel junctions grown epitaxially on top of (001)-oriented SrTiO_(3) substrates by sputtering. It is shown that the magnetoresistive response depends critically on the MgO/Fe interfacial properties. The appearance of an FeOX layer by the interface destroys the 1 symmetry filtering effect of the MgO/Fe system and only a small negative tunneling magnetoresistance (TMR) (∼ −3 %) is measured. However, in annealed samples a switchover from positive TMR (∼ +25% at 70 K) to negative TMR (∼ −1 %) is observed around 120 K. This change is associated with the transition from semiconducting at high T to insulating at low T taking place at the Verwey transition (TV ∼ 120 K) in Fe3O4, thus suggesting the formation of a very thin slab of magnetite at the MgO/Fe interface during annealing treatments. These results highlight the relevance of interfacial properties on the tunneling conduction process and how it can be substantially modified through appropriate interface engineering.

This work probes the relevance of oxygen vacancies in the formation of local ferromagnetic coupling between Fe ions at octahedral sites in zinc ferrites. This coupling gives rise to a ferrimagnetic ordering with the Curie temperatures above room temperature in an otherwise antiferromagnetic compound. This conclusion is based on experimental results from x-raymagnetic circular dichroismmeasurements at the Fe L2,3 edges and magnetization measurements performed on zinc ferrites, nanoparticles, and films, with different cation distributions and oxygen vacancy concentrations. Our observations are confirmed by density-functional-theory calculations and indicate that the enhanced ferrimagnetic response observed in some nominally nonmagnetic or antiferromagnetic ferrites can be taken as a further example of the defect-induced magnetism phenomenon. Fil: RodrÍguez Torres, Claudia Elena. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina Fil: Pasquevich, Gustavo Alberto. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina Fil: Mendoza Zélis, Pedro. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina Fil: Golmar, Federico. CIC Nanogune; España. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Pérez, Silvia Inés. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Sólido; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Nayak, Sanjeev K.. Martin Luther University Halle-Wittenberg; Alemania Fil: Adeagbo, Waheed A.. Martin Luther University Halle-Wittenberg; Alemania Fil: Hergebert, Wolfram. Martin Luther University Halle-Wittenberg; Alemania Fil: Hoffmann, Martin. Martin Luther University Halle-Wittenberg; Alemania. Institut Max Planck of Microstructure Physics; Alemania Fil: Ernst, Arthur. Institut Max Planck of Microstructure Physics; Alemania Fil: Esquinazi, P.. University of Leipzig; Alemania Fil: Stewart, Silvana Jacqueline. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; Argentina

X-ray absorption near-edge and grazing incidence x-ray fluorescence spectroscopy are employed to investigate the electronic structure of ZnFe${}_{2}$O${}_{4}$ thin films. The spectroscopy techniques are used to determine the nonequilibrium cation site occupancy as a function of depth and oxygen pressure during deposition and its effects on the magnetic properties. It is found that low deposition pressures below 10${}^{\ensuremath{-}3}$ mbar cause iron superoccupation of tetrahedral sites without Zn${}^{2+}$ inversion, resulting in an ordered magnetic phase with high room-temperature magnetic moment.

We report here the unexpected observation of significant room-temperature ferromagnetism in a semiconductor doped with nonmagnetic impurities, Cu-doped TiO₂ thin films grown by pulsed laser deposition. The magnetic moment, calculated from the magnetization curves, resulted surprisingly large, about 1.5 μB per Cu atom. A large magnetic moment was also obtained from ab initio calculations, but only if an oxygen vacancy in the nearest-neighbor shell of Cu was present. This result suggests that the role of oxygen vacancies is crucial for the appearance of ferromagnetism. The calculations also predict that Cu doping favors the formation of oxygen vacancies., Facultad de Ciencias Exactas, Instituto de Física La Plata