Room temperature ferromagnetism in polycrystalline Co(x)Ce(1-x)O(2-δ) (0.001≤x≤0.10) bulk samples has been investigated. Annealing in the forming gas transformed the as-prepared paramagnetic into a ferromagnetic insulating material with over two orders of magnitude enhancement (from 3.7 × 10(-2) to 1.24 μ(B)/Co) in the magnetization. Structural characterization of both the as-prepared and H(2)-treated samples showed a single phase material. The incorporation of Co with the formation of oxygen vacancies in the oxide lattice was revealed by x-ray photoelectron spectroscopy (XPS). The presence of oxygen vacancies is indicated by the existence of mixed valence states of cerium (Ce(4+) and Ce(3+)) in the high resolution XPS 3d spectrum. The role of the donor defects (oxygen vacancies) has been verified through the removal of oxygen vacancies. The ferromagnetic insulating ground state has been explained in terms of the interaction of the F(+) center and 3d magnetic cations. The connection between magnetic properties, electronic structure of the magnetic impurity and donor defect has been established. First principle calculations have been performed using the full potential linearized augmented plane wave method within the density functional theory (DFT) framework; these support our experimental findings. Both the experiment and calculations reinforced the crucial role of oxygen vacancies.

The low-frequency resistance noise in sputtered-deposited magnetic tunnel junctions with MgO barriers has been measured as a function of annealing time at different annealing temperatures. The noise has a 1 / f spectrum and it is quantified by a Hooge-like parameter ␣ given in units of m 2. Unannealed devices have the highest noise levels and their ␣ parameters exhibit a pronounced dependence on the voltage bias across the junction. A significant increase in tunneling magnetoresistance ͑TMR͒ is observed for short annealing times ͑on the order of minutes͒ at high temperatures and it is correlated with a large reduction in noise and in its bias dependence. The maximum TMR and minimum noise levels are reached at a later time that depends on temperature, being shorter at higher annealing temperatures. Devices annealed at 380 and at 430°C exhibit the same minimum noise levels, ␣ Ϸ 2 ϫ 10 −10 m 2. The origin of the resistance noise, its annealing time evolution, and its bias dependence are discussed and they are attributed to vacancy defects in the MgO barriers.

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We report the crystallization study of CoFeB/MgO/CoFeB magnetic tunnel junctions using in-situ, time-resolved synchrotron-based x-ray diffraction and transmission electron microscopy. It was found that the crystallization of amorphous CoFeB electrodes occurs on a time scale of seconds during the postgrowth high temperature annealing. The crystallization can be well fit by the Johnson-Mehl-Avrami model and the effective activation energy of the process was determined to be 150 kJ/mol. The solid-state epitaxy mode of CoFeB was found to involve separate crystallization at different locations followed by subsequent merging of small grains, instead of layer-by-layer growth of CoFeB film along the MgO template.

Magnetic 1/f noise is compared in magnetic tunnel junctions with electron-beam evaporated and sputtered MgO tunnel barriers in the annealing temperature range 350-425 C. The variation of the magnetic noise parameter (a mag) of the reference layer with annealing temperature mainly reflects the variation of the pinning effect of the exchange-bias layer. A reduction in a mag with bias is associated with the bias dependence of the tunneling magnetoresistance. The related magnetic losses are parameterized by a phase lag e, which is nearly independent of bias especially below 100 mV. The similar changes in magnetic noise with annealing temperature and barrier thickness for two types of MgO magnetic tunnel junctions indicate that the barrier layer quality does not affect the magnetic losses in the reference layer. V C 2012 American Institute of Physics.