Eva Céspedes

X-ray absorption study of the local order around Mn in Mn:ZnO thin films: the role of vacancies and structural distortions

This work reports an x-ray absorption near-edge structure spectroscopy (XANES) study at both Mn and Zn K edges in Zn(0.95)Mn(0.05)O thin films prepared with different sputtering gases (pure Ar, Ar/N(2) and Ar/O(2)). We have studied the local order around Mn in the films, with special emphasis on the role played by both oxygen and zinc vacancies and by the structural distortion associated with the substitution of Zn by Mn in the ZnO host. Our results indicate that the dependence of the magnetic properties of these Mn:ZnO thin films on the different sputtering gases used in the preparation is related to the structural distortion around Mn sites.

Effects of Au layer thickness and number of bilayers on the properties of Au/ZnO multilayers

Multilayered films of Au/ZnO were prepared by physical vapor deposition. Varying the Au thickness, tAu, and the number of bilayers, n, allowed us to investigate the role of these parameters on the sample structural and electronic properties. X-ray diffraction, X-ray absorption spectroscopy, grazing incidence small angle X-ray scattering and transmission electron microscopy experiments, have been combined to UV-visible and infrared spectroscopy to characterize the multilayers in the as-prepared state and after annealing. In the as-prepared state, the strong Au and ZnO lattice interaction leads to ZnO epitaxy on Au. Gold appears either as continuous layers or in form of nanoparticles. ZnO experiences a structural transformation from wurztite to rock salt monitored by the Au morphology. Annealing at 500 °C destroys the lattice matching. The electronic and optical properties of the systems are understood in line with the Au morphology and ZnO structural state.

Magnetic Structure and Electronic Study of Complex Oxygen-Deficient Manganites

Neutron diffraction and X-ray absorption near-edge structure (XANES) studies have been performed in La0.5Ca0.5MnO2.5, La0.5Sr0.5MnO2.5 and Nd0.5Sr0.5MnO2.5 oxygen-deficient perovskite compounds obtained by topotactic reduction. They all exhibit a brownmillerite structure with G-type antiferromagnetic ordering. Mn2+, Mn3+ and Mn4+ coexist at the octahedral sites, whereas only Mn2+ is placed in the tetrahedral positions. A magnetic moment of 1.6 microB has been detected at the tetrahedral layers, which can be explained by assuming Mn2+ is in a low-spin configuration.

Mn local order in room temperature ferromagnetic Mn/ZnO multilayers

The local environment of Mn atoms in (ZnO30 A/Mnt)n multilayers (ML) has been studied by means of x-ray absorption spectroscopy at the Mn K edge. A series of ML has been prepared by sputtering with approximately constant total amount of Mn (300 A) while the Mn layer thickness (t) has been varied from 60 to 1 A (modifying consequently the number of Mn–ZnO interfaces) in order to study the Mn–Zn–O system. Variation of the Mn layer thickness (t) has been directly associated with changes in the Mn oxidation states within the sample series. Absorption spectroscopy results for films with t≤15 A indicate mostly MnO2 formation and no signal of Mn in the ZnO lattice is found. Conversely, films with t≥30 A present two different Mn environments, that have been related to the coexistence of rocksalt and wurtzite phases, formed by Zn1−xMnxO mixed solid solutions. The magnetic behaviour of samples is correlated with structural properties. That correlation strongly points towards substitutional Mn in the wurtzite phase as the origin of the observed ferromagnetism above room temperature.

X-ray absorption and magnetic circular dichroism characterization of a novel ferromagnetic MnNx phase in Mn/Si3N4 multilayers

Ferromagnetism above room temperature has been observed in Mn/Si3N4 multilayered films for the first time. Characterization of the structural and electronic properties was performed to study the ferromagnetic behavior of this system. X-ray absorption spectroscopy at the Mn K and L edges, as well as x-ray magnetic circular dichroism, evidences the presence of divalent Mn in the films. X-ray absorption near edge structure measurements, which are compared to calculations, confirms the presence of a slightly distorted Mn3N2 phase that is proposed to be the origin of the ferromagnetism in this system.

Characterization of two dimensional self-organized Au nanoparticles embedded in Si3N4

Au/Si3N4 films have been prepared by gold and silicon nitride sequential sputtering with different Au thicknesses. The goal is to investigate the Au growth mode and to identify the regime where Au nanoparticles (NPs) can be prepared. The number of Au atoms, the bilayer thickness, the Au environment, the size, and the organization of the particles have been determined. Gold appears in metallic form. The thickest Au layer (8 nm) is continuous whereas the thinnest ones (with average values of 1–3 nm) are formed by nearly spherical Au NPs with diameters in the 5–7 nm range. Au NPs spatial self-organization is observed for average thickness of tAu=2.9 nm. These growth modes are discussed and compared with those observed for other metal/Si3N4 systems prepared in similar conditions.

Magnetic properties and short-range structure analysis of granular cobalt silicon nitride multilayers

The magnetic properties and local order of cobalt/silicon nitride metal-insulator multilayered system have been studied. Magnetization characterization reveals an evolution of the magnetic features by varying the metal layer thickness. Results show that multilayers with larger metal thickness (t) present a pure ferromagnetic character, whereas samples with t < 2 nm exhibit a granular superparamagnetic behavior, as it corresponds to discontinuous metal-insulator materials. An important decrease in the magnetization values for the clustered samples has also been observed. X-ray absorption near edge spectroscopy (XANES) and extended x-ray absorption fine structure (EXAFS) at the Co K-edge have been used to determine the local order and electronic configuration around cobalt. Results reveal the formation of an intermediate CoN phase likely located at the Co/Si3N4 interface. Further insight in the electronic structure of cobalt atoms has been investigated through ab-initio XANES calculations based on Green fun...

Microstructural properties and local order around iron in granular metal–insulator Fe/Si3N4 systems prepared by magnetron sputtering

[Fe/Si3N4] multilayers have been prepared by magnetron sputtering with a layer thickness of 3 nm for silicon nitride and the iron layer ranging from 1 to 10 nm. A variety of techniques for determining the microstructure, such as x-ray reflectivity, transmission electron microscopy and Rutherford backscattering spectrometry techniques, have been performed to unveil the microstructure of such samples. Also, an analysis of the local environment around iron, by means of extended x-ray absorption fine structure and x-ray absorption near-edge structure spectroscopies performed at the Fe K-edge, confirms the presence of two phases forming the iron layers, the metallic Fe and the FeN phase. Special emphasis has been placed on the x-ray absorption analysis for multilayers with smaller Fe layer thicknesses, which present features of granular systems.

Correlation between Mn oxidation state and magnetic behavior in Mn/ZnO multilayers prepared by sputtering

Compositional, microstructural, and magnetic characterization of [ZnO(30 A)/Mn(x)]n multilayers prepared by sputtering is presented to study the observed ferromagnetism in the Mn-ZnO system. The nominal Mn layer thickness, x, is varied from 3 to 60 A, while the number of bilayers, n, is increased to maintain the total amount of Mn constant. Microstructure information was deduced from x-ray reflectivity, Mn oxidation state was determined by x-ray absorption spectroscopy, and magnetic properties were measured over a temperature range of 5–400 K. Magnetic behavior of these samples is found to be related to the Mn layer thickness (x). Multilayers with x≥30 A exhibit ferromagnetism with a Curie temperature above 400 K, while mostly paramagnetic behavior is obtained for x<15 A. Magnetic behavior is discussed in terms of electronic and structural parameters of samples. Mn-ZnO interface effect is related to the ferromagnetic order of the samples, but it is not a sufficient condition. The essential role of the Mn ...

Interplay between metal nanoparticles and dielectric spacing layers during the growth of Au/Si3N4 multilayers

Competition between ordering and disordering processes during growth of granular Si3N4/[Au(t)/Si3N4]n thin films by vapor deposition is tracked in a simple, unconventional and nondestructive way by probing the infrared response of the dielectric matrix. Si3N4 crystallization occurs as a consequence of the presence of Au nanostructures. The average Si3N4 crystalline order is improved upon increasing the thickness tAu of deposited Au per bilayer. On the contrary, crystalline order is destroyed when the number n of bilayers overcomes a threshold value that varies with t. Additional information provided by conventional measurements (x-ray diffraction, visible absorption spectroscopy) and by transmission electron microscopy observations suggests that the Si3N4 atomic ordering results from seed-induced crystallization on the Au(111) planes. Disordering is tentatively attributed to roughness accumulation upon stacking, whose magnitude is ruled by the roughness of the first deposited Au granular layer.