José Luis Menéndez

Epitaxy, strain and morphology of low Ar pressure sputtered Pt thin films

Abstract We show that the use of low Ar pressures in the sputter deposition process leads to the growth of epitaxial Pt thin films on MgO(1xa00xa00) and (1xa01xa00) at moderate temperatures. This is due to the low thermalization of the Pt particles impinging on the substrate, and the subsequent generation of a high density of nucleation centers as well as an increase in the effective surface temperature. Despite the fact that the MgO lattice parameter is 7.4% larger than that of Pt, this higher particle energy distribution leads to a stronger film–substrate interaction and therefore to an unrelaxed, in-plane compressive strained growth. Distinct release of this compressive strain depending on growth temperature and crystalline orientation produces strong changes in the morphology of the films, leading to a 2D–3D transition for the (1xa01xa00) structures grown between 600 and 700°C.

Epitaxial Fe (001) micro tiling: Size and interaction effects

The magnetic properties of 200 A epitaxial Fe (001)/MgO (001) tiling are studied as a function of tile size and separation. For edge sizes above ∼3u200aμm, the individual tiles maintain the single domain behavior while below ∼3u200aμm the tiles break into domains due to demagnetizing effects. In addition, below an inter tile separation threshold of about 0.9 μm, a single tile magnetization switch provokes a reversal cascade in all the tiles; while, above this threshold, the individual tile’s magnetization switches independently. In this last case, we have experimental access to the distribution of nucleation sites. Thus, we have found a clear signature of the magnetic interaction between patterned epitaxial micro tiles with in-plane magnetization.

Transparent yttrium aluminium garnet obtained by spark plasma sintering of lyophilized gels

Lyophilized YAG gel, synthesized by the coprecipitation technique, has been sintered to transparency by spark plasma sintering method at 1500°C. Whereas conventionally dried gels show large agglomerates, over 1 µm, powders from lyophilized gels show no agglomeration with an average particle size below 100 nm. The absence of agglomerates affects on the optical properties of the sintered materials: conventionally dried powders are opaque after sintering, whereas 0.8 mm thick transparent YAG materials with in-line transmittances close to 60% at 680 nm and over 80% in the infrared range have been obtained for the lyophilized gels.

Structural characterization of Fe(1 1 0) islands grown on α-Al2O3(0 0 0 1)

Abstract The morphology, size distribution, epitaxial relationships and lattice distortion in nanometer sized Fe(1xa01xa00) oriented islands grown on Al2O3(0xa00xa00xa01) are studied and discussed as a function of the amount of deposited Fe. Combined scanning electron microscopy and transmission electron microscopy (TEM) as well as atomic force microscopy measurements show the dependence of these parameters with island size. For the smallest island sizes, an iron lattice distortion with respect to bulk values is found at their edges. Electron energy loss spectroscopy experiments demonstrate the absence of oxidation of the Fe islands either due to the high temperature deposition on Al2O3 or to the polishing procedure prior to TEM observations. From the analysis of the relative intensity for the L2 and L3 Fe peaks, a high spin ground state for the Fe atoms is deduced, probably correlated with the highly distorted regions at the edges of the islands.

Sintering to Transparency of Polycrystalline Ceramic Materials

There is currently a high demand for advanced materials for different types of applications (see Fig. 1.1) in which besides a high mechanical performance, a partial or total transparency in a given spectral range is required. Transparent ceramics become more and more important for applications in which materials are subject to extremely high mechanical and thermal stress in combination with optical properties. More recently, interest has focused on the development of transparent armor materials (ceramic) for both military and civil applications. Also, the development of new optoelectronic devices has extended the use of ordinary optical materials to new applications and environments such as temperature (IR) sensor, optical fiber communications, laser interferometers, etc. A considerable fraction of these new devices operates in aggressive environments, such as ovens, radiation chambers and aerospace sensors. In such cases, the sensitive electronic component must be preserved from the extreme external condition by a transparent window. Transparent and coloured ceramics are also often used as wear and scratch resistant parts such as bearings and watch glasses as well as for their aesthetic Properties in synthetic opals and rubies.

Lattice distortion and magnetic properties of nanometer size Fe(1 1 0) islands

Abstract Epitaxial Fe(1xa01xa00) islands of nanometric dimensions have been obtained on c-sapphire by triode sputtering. By ex situ X-ray diffraction (XRD) measurements we observe a strong dependence of the perpendicular lattice parameter as a function of the vertical island size and the deposition geometry. Deposition with the substrate facing the Fe target gives rise to almost relaxed islands. However, deposition with the substrate facing opposite to the Fe target yields to highly distorted islands (up to 5% along the growth direction). XRD rocking scans show two components in k parallel: a narrow one, independent of the island dimension, that it is correlated to the Fe/sapphire interface, and a broad one, related to its in-plane disorder, the width of which decreases when the island lateral dimension is increased. Correlation of the structural changes with the magnetization reversal processes and magneto-optical properties are studied.

Fabrication and magnetic properties of electron beam lithography patterned arrays of single crystals

The fabrication method of arrays of iron squares separated by submicrometric distances is presented. The magnetic elements are defined on a single crystal Fe(001) film by electron beam lithography followed by a ion beam etching process. The obtained Fe squares present smooth surfaces and homogeneous sizes over areas as large as 500 μm x 500 μm as analyzed by scanning electron and atomic force microscopies. The lateral size of the squares have been varied in the range 1–10 μm in order to analyze the size dependence. The magnetic properties of the single crystalline squares have been studied by magnetooptical Kerr effect. The results reveal an important change in the hysteresis loops as the square dimensions are reduced below a threshold of 3 μm.

Interplay between collective pinning and artificial defects on domain wall propagation in Co/Pt multilayers

The interplay between collective pinning on intrinsic structural defects and artificial pinning at a patterned hole is studied in magnetic multilayers with perpendicular anisotropy. The pinning strength of a patterned hole is measured through its efficiency to stop domain wall (DW) propagation into a consecutive unpatterned nanowire section (using antisymmetric magnetoresistance to detect the direction of DW propagation) whereas collective pinning is characterized by the field dependence of DW velocity. Close to room temperature, collective pinning becomes weaker than artificial pinning so that pinning at the hole compensates nucleation-pad geometry, blocking DW propagation across the nanowire.

Electronic transport in nanocrystalline iron: a low T magnetoresistance effect

Abstract We report on transport and magnetic measurements of islanded Fe (1xa01xa00) thin films. The electrical resistivity exhibits an anomalous increase at low temperatures, which disappears under the action of a magnetic field. Since such an anomaly completely disappears under the action of a magnetic field, it is inferred that it originates from spin-dependent scattering. We interpret the strong changes in the spin-dependent scattering in our films to be due to a low-temperature spin freezing of the island boundary magnetic regions that impedes ferromagnetic exchange between islands. A consequence of this magnetic behavior is the random arrangement of the individual magnetization, determined by the magnetocrystalline anisotropy of each island, resulting in an increase of the resistivity below the freezing temperature. Two main conclusions can be inferred from the thermal dependence of the magnetization and electrical resistivity: (i) Fe grain boundaries are not ferromagnetic at low temperatures, but behave as a reentrant spin glass; (ii) nanocrystalline Fe can be considered as low temperature giant magnetoresistance-like system.

Growth mode and matrix effects on the magneto-optical activity of ultrathin films

The magneto-optical activity of Fe nanoparticulate systems is studied in this work as a function of the matrix that surrounds the particles and the Fe concentration in the effective layer. It is shown that the magneto-optical activity is not linear or proportional to the amount of Fe even at low coverages when the matrix surrounding the particles is an insulator. As a conclusion, it is also demonstrated that the magneto-optical response of a magnetic ultrathin film system strongly depends on its growth mode, with modifications of up to 100% in its magneto-optical activity.