Alfonso Cebollada

Control of the axis of chemical ordering and magnetic anisotropy in epitaxial FePt films

Growth of epitaxial films of the L10 phase of FePt, with the tetragonal c axis along either the film normal or in‐plane, is described. Films were grown by coevaporation of Fe and Pt, under ultrahigh vacuum conditions, onto a seed film of Pt grown on MgO or SrTiO3 substrates. The perpendicular or in‐plane orientation of the c axis was controlled by selecting the (001) or (110) substrate plane, respectively. Nearly complete chemical ordering was achieved for growth at 500 °C for both orientations. Magnetic and magneto‐optical characterization of these films confirmed the huge magnetic anisotropy expected for this phase. In the most highly ordered films, anisotropy fields in excess of 120 kOe were measured.

Large magnetoresistance in Fe/MgO/FeCo(001) epitaxial tunnel junctions on GaAs(001)

We present tunneling experiments on Fe(001)/MgO(20 A)/FeCo(001) single-crystal epitaxial junctions of high quality grown by sputtering and laser ablation. Tunnel magnetoresistance measurements give 60% at 30 K, to be compared with 13% obtained recently on (001)-oriented Fe/amorphous-Al2O3/FeCo tunnel junctions. This difference demonstrates that the spin polarization of tunneling electrons is not directly related to the density of states of the free metal surface—Fe(001) in this case—but depends on the actual electronic structure of the entire electrode/barrier system.

Plasmonic Au/Co/Au nanosandwiches with enhanced magneto-optical activity.

Financial support from the Spanish Ministry of Science and Education (NAN2004-09195-C04-01 and MAT2005-05524-C02-01), and UE (NoE-Phoremost) are acknowledged.

Growth temperature dependence of long‐range alloy order and magnetic properties of epitaxial FexPt1−x (x≂0.5) films

The structural and magnetic properties of 1000‐A‐thick epitaxial FexPt1−x(001) alloy films, grown by molecular beam epitaxy with x≂0.5, have been studied as a function of growth temperature. X‐ray diffraction analysis showed that the long‐range order parameter increased from near zero for films grown at 100 °C to a maximum of 0.93 in films grown at 500 °C. Over this range the magnetic easy axis changed from in‐plane to perpendicular and the polar Kerr rotation increased strongly. Spontaneous long‐range ordering in these films has an activation energy ∼0.2 eV, consistent with an energy barrier for surface and step down diffusion.

Magnetoplasmonic nanostructures: systems supporting both plasmonic and magnetic properties

We present in this work our current understanding on magnetoplasmonic structures, that is, systems whose constituents exhibit simultaneously magnetic and plasmonic properties. We analyze both the influence of the plasmon resonance on the magneto-optical properties of the system and the ability of the magnetic field to modulate the plasmon properties. In particular we show how, in magnetoplasmonic systems sustaining localized or propagating surface plasmons, the associated electromagnetic field enhancement gives rise to an enhancement of the magneto-optical activity. On the other hand, we have analyzed the modulation of the propagating surface plasmon polariton wavevector in noble metal/ferromagnet/noble metal trilayers by an external magnetic field. These phenomena can be addressed as new concepts for the development of active plasmonic devices.

Au/Fe/Au multilayer transducers for magneto-optic surface plasmon resonance sensing

In this paper, we analyze the magnetoplasmonic MP features and sensing capabilities of Au/Fe/Au trilayer structures, as transducers of the magneto-optic surface plasmon resonance MOSPR biosensor. This biosensor, which can surpass the sensitivity of the standard SPR sensor, is based on a MP modulation technique generated by the simultaneous stimulation of the surface plasmon polaritons SPP and the transversal magneto-optical Kerr effect TMOKE. We study the magneto-optical activity of the trilayers as a function of the thickness and position of the Fe layer. We first demonstrate that this kind of structure allows modulating the SPP through an external magnetic field and moreover, induce a strong enhancement of the TMOKE effect. The modulation of the SPP is linearly proportional to the thickness of Fe layer and inversely proportional to the distance between the Fe layer and the external dielectric medium. Finally, we experimentally confirm a twofold increase in the MOSPR sensitivity with respect to the intensity-interrogated SPR biosensor in bulk refractive-index changes, keeping a similar chemical resistance and stability, unprecedented in other MP transducers, and biofunctionalization protocols.

Localized surface plasmon resonance effects on the magneto-optical activity of continuous Au/Co/Au trilayers

We study how the magneto-optical activity in polar configuration of continuous Au/Co/Au trilayers is affected by the excitation of localized plasmon resonances of an array of Au nanodiscs fabricated on top of them over a dielectric SiO(2) spacer. We show that the effect of the nanodiscs array is twofold. First, it optimizes the absorption of light at specific photon energies corresponding to the localized surface plasmon excitation of the array, modifying the reflectivity of the system (we define this effect as the purely optical contribution). Second, upon localized plasmon resonance excitation, the electromagnetic field in the whole system is redistributed, and an enhanced magneto-optical activity occurs at those energies where the electromagnetic field in the magnetic layer is increased (this effect is identified as the purely magneto-optical contribution of the nanodiscs array).

Enhanced gas sensing performance of TiO2 functionalized magneto-optical SPR sensors

Porous TiO2 thin films deposited by glancing angle deposition are used as sensing layers to monitor their sensing capabilities towards Volatile Organic Compounds both in a standard Surface Plasmon Resonance (SPR) sensor and in Magneto-Optical Surface Plasmon Resonance (MO-SPR) configuration in order to compare their sensing performances. Here our results on the enhanced sensing capability of these TiO2 functionalized MO-SPR sensors with Au/Co/Au transducers with respect to traditional SPR gas sensors are presented.

Enhancement of the magnetic modulation of surface plasmon polaritons in Au/Co/Au films

The deposition of a dielectric overlayer on top of Au/Co/Au multilayer films can significantly enhance the magnetic field induced modulation of the surface plasmon polariton (SPP) wave vector. This enhancement is analyzed as a function of the thickness of the dielectric overlayer and the associated SPP electromagnetic field confinement and redistribution. The decrease in SPP propagation distance is taken into account by analyzing an adequate figure of merit.

High magneto-optical activity and low optical losses in metal-dielectric Au/Co/Au-SiO2 magnetoplasmonic nanodisks

Metal-dielectric Au-Co-SiO(2) magnetoplasmonic nanodisks are found to exhibit large magneto-optical activity and low optical losses. The internal architecture of the nanodisks is such that, in resonant conditions, the electromagnetic field undertakes a particular spatial distribution. This makes it possible to maximize the electromagnetic field at the magneto-optically active layers and minimize it in the other, optically lossy ones.