Nanostructured nickel oxide thin films grown by reactive RF Magnetron Sputtering

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Aplicada: Fecha de lectura:16-06-201

Thermal induced depletion of cationic vacancies in NiO thin films evidenced by x-ray absorption spectroscopy at the O 1s threshold

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in (Journal of Vacuum Science & Technology A 38.3 (2020): 033209) and may be found at (https://avs.scitation.org/doi/full/10.1116/6.0000080)The effects of thermal annealing on the concentration of cationic vacancies in p-type semiconducting NiO thin films have been studied by x-ray absorption spectroscopy at the O 1s threshold. This technique proves to be very sensitive to the amount of Ni vacancies through the intensity of a prepeak observed below the absorption threshold, associated with Ni ions in a high oxidation state. Samples with different vacancy concentrations were obtained by radio frequency magnetron sputtering with different O2/Ar ratios in the plasma. Thermal effects have been studied both during thin film growth and after postprocessing annealing. In both cases, the observed effects were very similar, showing a depletion of cationic vacancies with temperature. By changing the surface sensibility of the x-ray absorption spectroscopy measurements, the authors could find out that the transition to stoichiometric NiO begins at the surfaceThis work was partially supported by the Spanish MICINN, under Project No. ENE2010-21198-C04-04 and by the Comunidad de Madrid under Project No. NANOMAGCOSTCM-P2018/ NMT4321. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007–2013) under Grant Agreement No. 31228

X-ray absorption study of the local structure at the NiO/oxide interfaces

This work reports an X-ray absorption near-edge structure (XANES) spectroscopy study at the Ni K-edge in the early stages of growth of NiO on non-ordered SiO2, Al2O3 and MgO thin films substrates. Two different coverages of NiO on the substrates have been studied. The analysis of the XANES region shows that for high coverages (80 Eq-ML) the spectra are similar to that of bulk NiO, being identical for all substrates. In contrast, for low coverages (1 Eq-ML) the spectra differ from that of large coverages indicating that the local order around Ni is limited to the first two coordination shells. In addition, the results also suggest the formation of cross-linking bonds Ni - O - M (M = Si, Al, Mg) at the interfaceThis work was partially supported by the Spanish CONSOLIDER Project FUNCOAT CSD2008-00023, the ENE2010-21198-C04-04 and MAT2011-27573-C04-04 projects and by the Aragón DGA NETOSHIMA grant. We acknowledge the Spanish Ministerio de Ciencia e Innovación and CSIC for financial support and for provision of synchrotron radiation and we would like to thank the staff of SpLine at ESRF and KMC2 at BESSY II for technical support. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 2267

Coercivity and morphology in Fe/NiO films deposited on nanoporous Al2O3 membranes

[EN] The occurrence of ferro-antiferromagnetic (F/AF) interfaces in Fe/NiO bilayers brings about the possibility of controlling the Fe layer hysteretic properties through the unidirectional anisotropy induced by the presence of unbalanced, antiferromagnetic phase moments at that interface. In this work, we report on the Fe thickness and temperature dependencies of the hysteretic parameters measured in Fe/NiO bilayers deposited on AlO nanoporous membranes. The Fe/NiO bilayers grew in a six-fold columnar way around each one of the membrane nanopores. The in-plane hysteresis loops measured by field cooling the samples down to different temperatures below 300 K, evidenced the occurrence of coercive forces larger than that associated to the Fe anisotropy field and also of significant loop shifts, both of them clearly linked to the occurrence of exchange bias at the Fe/NiO interfaces. The magnetization reversal mechanism active in the Fe/NiO bilayers essentially differs from that responsible for the reversal occurring in continuous films incorporating lithographed antidots, due to the fact that in our samples it does not involve either domain wall propagation or pinning, and the reversal events are spatially restricted to the Fe-covered, hexagonally closely packed dot-like tops of the NiO columns.[ES] La presencia de intercaras ferro-antiferromagnéticas (F/AF) en bicapas de Fe/NiO permite modificar significativamente las propiedades de histéresis de la película de Fe mediante la anisotropía unidireccional, debido a la presencia de momentos de la fase antiferromagnética descompensados en la intercara. En este trabajo estudiamos la influencia del espesor de Fe y de la temperatura sobre los parámetros de histéresis de bicapas de Fe/NiO depositadas sobre membranas porosas de Al2O3. Dichas bicapas crecen con estructura columnar hexagonal en torno a los poros de las membranas. Los ciclos de histéresis, medidos tras enfriar con campo a varias temperaturas por debajo de 300 K, han mostrado campos coercitivos mayores que los esperables considerando el campo de anisotropía del Fe y la existencia de desplazamientos de los ciclos hacia campos negativos, efectos ligados a la existencia de «exchange bias» en las intercaras Fe/NiO. El mecanismo de conmutación activo en las bicapas difiere del que tiene lugar en películas continuas litografiadas con «antidots». Ello es debido a que en nuestras muestras no se producen propagación y enganches de paredes y, además, a que la inversión de la imanación está espacialmente localizada en las estructuras de Fe tipo «dot» que recubren las columnas de NiO.Work supported by the Spanish MINECO (Ministerio de Economía y Competitividad) under Projetcs MAT2013-47878-C2-1-R and MAT2013-47878-C2-2-R

Hexagonally-arranged-nanoporous and continuous NiO films with varying electrical conductivity

Nickel oxide (NiO) thin films have been prepared by magnetron sputtering, with different Ar/O2 ratios in the plasma, on several substrates, including hexagonally arranged nanoporous anodic alumina membranes (AAM). The obtained films exhibit columnar growth, which makes it possible to preserve the hexagonal order of the AAM substrates in the NiO thin films. X ray diffraction patterns show a polycrystalline structure with a crystallographic texture that depends on the plasma composition. Additionally, the NiO lattice parameter increases with the oxygen content of the plasma. The presence of oxygen during deposition is responsible for these structural changes, as well as for an oxygen enrichment in the NiO films, which leads to changes in their electrical properties. The electrical resistivity of the films decreases with the oxygen content of the plasma, which suggests p-type conductivity due to oxygen enrichment in the NiO lattice. Indeed, an analysis of the EXAFS oscillations at the Ni-K edge confirms the lattice expansion and a decrease of the Ni-Ni coordination number when the oxygen content of the plasma increases, which points towards an increasing presence of Ni vacancies for larger values of the O2/Ar ratio. © 2013 Elsevier B.V. All rights reserved.This work has been supported by the Spanish MICINN, under projects ENE2010-21198-C04-04, CSD2008-0023, and MAT2011-27573-C04-04.Peer Reviewe