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3 páginas, 3 figuras.-- et al., We report on continuously graded anisotropy. During deposition, a compositional gradient is achieved by varying the Cu concentration from Cu-rich (Fe53Pt47)70Cu30 to Cu-free Fe53Pt47. The anisotropy gradient is then realized after annealing using the composition dependence of the low-anisotropy (A1) to high-anisotropy (L10) ordering temperature. The critical role of the annealing temperature on the resultant anisotropy gradient is investigated. Magnetic measurements support the creation of an anisotropy gradient in properly annealed films which exhibit both a reduced coercivity and moderate thermal stability. These results demonstrate that an anisotropy gradient can be realized, and tailored, in single continuous films without the need for multilayers., Support from The Swedish Foundation for strategic Research (SSF), The Swedish Research Council (VR), the Göran Gustafsson Foundation, and the Knut and Alice Wallenberg Foundation is gratefully acknowledged. J.N. thanks the Catalan DGR (Grant No. 2009SGR1292) and the Spanish MICINN (Grant No. MAT2010-20616-C02) projects for partial financial support.

3 páginas, 2 figuras.-- et al., The reversal mechanisms of graded anisotropy FePtCu films have been investigated by alternating gradient magnetometer (AGM) and magneto-optical Kerr effect (MOKE) measurements with first-order reversal curve (FORC) techniques. The AGM-FORC analysis, which clearly shows the presence of soft and hard components, is unable to resolve how these phases are distributed throughout the film thickness. MOKE-FORC measurements, which preferentially probe the surface of the film, reveal that the soft components are indeed located toward the top surface. Combining AGM-FORC with the inherent surface sensitivity of MOKE-FORC analysis allows for a comprehensive analysis of heterogeneous systems such as graded materials., Support from The Swedish Foundation for strategic Research (SSF), The Swedish Research Council (VR), the Göran Gustafsson Foundation, and the Knut and Alice Wallenberg Foundation is gratefully acknowledged. V.B. thanks the Blanceflor Boncompagni–Ludovisi Foundation. J.N. thanks the Catalan DGR (Grant No. 2009SGR1292) and the Spanish MICINN (Grant No. MAT2010-20616-C02) projects for partial financial support. J.Å. is a Royal Swedish Academy of Sciences Research Fellow supported by a grant from the Knut and Alice Wallenberg Foundation.

The physics of nanomagnetic elements has become one of the main areas of research in the last few decades. Their magnetic characterization is fundamental in order to understand the magnetic reversal mechanisms and to control the magnetic switching precisely in view of device applications. In such small elements, the magnetic behaviour is strongly determined by the interplay of magnetic anisotropy with the geometric shape. Moreover, in closed packed arrays, the effects of the inter-element interactions become important and reveal often unresolved issues regarding the strength of these couplings and their dipolar or exchange mediated origin. In this thesis three different cases have been discussed about the mechanisms of interactions among the magnetic elements of very densely packed systems. The experimental section opens with the study of the magnetization reversal process in magnetostatically interacting permalloy (an alloy of nickel and iron) nanowires, using the vectorial magneto-optical Kerr effect magnetometry. The measured in-plane magnetization components parallel and perpendicular to the applied field show a transition from coherent rotation to inhomogeneous reversal mode over and above a determined value of the wires thickness. A strong dependence on wires thickness is also observed in the analysis of effects of dipolar interactions. Successively, the motion and pinning of domain walls have been investigated in vertical and horizontal chains of permalloy triangular microrings and compared with the isolated rings case. Using longitudinal and diffracted magnetooptic Kerr effects, magnetic force microscopy and micromagnetic simulations, the field dependence of the spin structure into the ring has been determined. This investigation has allowed to observe how the dipolar inter-element interaction changes qualitatively the manner in which reversal occurs. Considering that the magnetic domain walls displacement can also be controlled by using magnetic fields, such structures could be viewed as a preliminary approach for the development of magnetic logic applications. Finally, static and dynamical properties have been studied in patterned arrays of pseudo spin-valves in which a layer of permalloy has been coupled with a layer of cobalt, through an interspacer of non-magnetic copper. Magnetic hysteresis loops have been measured by longitudinal magneto-optic Kerr effect and by X-ray resonant magnetic scattering, whereas the dynamical properties have been investigated using the Brillouin light scattering. The results show a complex magnetization reversal process determined by the interplay between the interlayer dipolar interaction and the different reversal nucleation fields in the two ferromagnetic layers. Reducing the thickness of the copper spacer, an interlayer magnetic exchange coupling between the permalloy and cobalt layers emerges, in addition to the dipolar interaction, introducing further possibilities for manipulating the magnetic behaviour of the system.

In hard X-ray applications that require high detection efficiency and short response times, such as synchrotron radiation-based Mössbauer absorption spectroscopy and time-resolved fluorescence or photon beam position monitoring, III–V-compound semiconductors, and dedicated alloys offer some advantages over the Si-based technologies traditionally used in solid-state photodetectors. Amongst them, gallium arsenide (GaAs) is one of the most valuable materials thanks to its unique characteristics. At the same time, implementing charge-multiplication mechanisms within the sensor may become of critical importance in cases where the photogenerated signal needs an intrinsic amplification before being acquired by the front-end electronics, such as in the case of a very weak photon flux or when single-photon detection is required. Some GaAs-based avalanche photodiodes (APDs) were grown by a molecular beam epitaxy to fulfill these needs; by means of band gap engineering, we realised devices with separate absorption and multiplication region(s) (SAM), the latter featuring a so-called staircase structure to reduce the multiplication noise. This work reports on the experimental characterisations of gain, noise, and charge collection efficiencies of three series of GaAs APDs featuring different thicknesses of the absorption regions. These devices have been developed to investigate the role of such thicknesses and the presence of traps or defects at the metal–semiconductor interfaces responsible for charge loss, in order to lay the groundwork for the future development of very thick GaAs devices (thicker than 100 μm) for hard X-rays. Several measurements were carried out on such devices with both lasers and synchrotron light sources, inducing photon absorption with X-ray microbeams at variable and controlled depths. In this way, we verified both the role of the thickness of the absorption region in the collection efficiency and the possibility of using the APDs without reaching the punch-through voltage, thus preventing the noise induced by charge multiplication in the absorption region. These devices, with thicknesses suitable for soft X-ray detection, have also shown good characteristics in terms of internal amplification and reduction of multiplication noise, in line with numerical simulations.