Grain structure and magnetic relaxation of self-assembled Co nanowires

The magnetic relaxation of Co nanowires assemblies embedded in CeO(2)/SrTiO(3)(001) epilayers has been investigated by magnetization decay measurements. Two different samples were studied, with nanowires having distinct crystallographic structures and diameters of 3 and 5 nm. The structure of the nanowires was derived from high-resolution transmission electron microscopy analysis. The 3 nm diameter nanowires are made of hcp Co grains with the c-axis pointing along one of the four <111> directions of the CeO(2) matrix, separated by fcc Co regions. In the 5 nm diameter nanowires, the grains are smaller and the density of stacking faults is much higher. The magnetic viscosity coefficient (S) of these two systems was measured as a function of the applied field and of the temperature. Analysis of the variation of S and of the activation volume for magnetization reversal reveals distinct behaviors for the two systems. In the nanowires assembly with 5 nm diameter, the results can be described by considering an energy barrier distribution related to shape anisotropy and are consistent with a thermally activated reversal of the magnetization. In contrast, the anomalous behavior of the 3 nm diameter wires indicates that additional sources of anisotropy have to be considered in order to describe the distribution of energy barriers and the reversal process. The distinct magnetic behaviors observed in these two systems can be rationalized by considering the grain structure of the nanowires and the resulting effective magnetocrystalline anisotropy.

Mechanism of localization of the magnetization reversal in 3 nm wide Co nanowires

The mechanism of magnetization reversal has been studied in a model system of self-assembled cobalt nanowires with a 3 nm diameter. The structure, orientation and size of grains within the nanowires could be determined by high resolution transmission electron microscopy. The magnetic properties were probed using static and dynamic magnetization measurements. Micromagnetic modeling based on the structural analysis allows us to correlate the structure and the magnetic behavior of the wires, revealing competition between shape anisotropy, magnetocrystalline anisotropy and exchange in the localized reversal within Co hcp oriented grains. These results provide direct experimental evidence of the link between anisotropy fluctuations and reversal localization in nanowires.

Ferromagnetism induced by oxygen and cerium vacancies above the percolation limit in CeO2

We studied the structural, chemical and magnetic properties of non-doped ceria (CeO(2)) thin films electrodeposited on silicon substrates. Experimental results confirm that the observed room temperature ferromagnetism is driven by both cerium and oxygen vacancies. We investigated ceria films presenting vacancy concentrations well above the percolation limit. Irradiation experiments with neon ions were employed to generate highly oxygen defective CeO(2-δ) structures. X-ray photoelectron spectroscopy and x-ray absorption near-edge structure spectroscopy were used to estimate the concentration of Ce(3+) sites in the films, which can reach up to 50% of Ce(3+) replacing Ce(4+), compared to a stoichiometric CeO(2) structure. Despite the increment of structural disorder, we observe that the saturation magnetization continuously increases with Ce(3+) concentration. Our experiments demonstrate that the ferromagnetism observed in ceria thin films, highly disordered and oxygen-deficient, preserving the fluorite-type structure only in a nanometer scale, remains intrinsically stable at room temperature.