Paolo Maria Eugenio Icilio Allia

Magnetic nanostructures and spintronics

Nanostructures featuring magnetic properties and devices for the elaboration and/or storage of information based on electron spin transport. The need of storing and elaborating binary information permeates everyday life. Todays computers are mainly charge-transfer devices; this means that inside a processing unit the information chain corresponds to a physical electron (hole) drift current, characterized by diffusion lengths and flow times that strongly limit the evolution toward faster computations and smaller sizes. Codifying binary information into a spin-polarized current could allow much faster logic operations, due to the possibility of reversing spins without stopping the carrier flow. The main difficulty of implementing spin-electronic (spintronic) devices is that typical spin diffusion lengths are well below the hundredth of nanometer for diamagnetic metals and between some tenths of nm and some nms for ferromagnetic metals and alloys, meaning that the typical device must be nanostructured. Furthermore, since spin-flip scattering must be prevented to process uncorrupted data, both interface quality and material purity must be extremely controlled. High and ultra-high vacuum deposition equipments are needed, and perfect control of geometries down to the nm scale requires extremely sophisticated lithographic steps. Such technological issues have slowed down the challenge of bringing spintronic devices into the core of our logic devices. On the contrary, commercial data storage still relies almost entirely on magnetic nanostructures: both granular media for hard disk drives and tunneling magnetic junction-based read/write heads are worldwide diffused. Due to their technological relevance, the most important effects on which commercial spintronic devices are based will be considered: Giant Magnetoresistance, Tunneling Magnetoresistance and Spin Torque. Two more subsections are addressed to particularly hot topics: Semiconductor Spintronics and Data Storage. In all these cases, physical systems and/ or devices are authentic nanostructures, that is, artificial, nanosized structures characterized by carefully controlled geometries in spite of their reduced dimensions. On the other hand, magnetic nanoparticles, although increasingly gaining attention because of their potential applications in different fields (ranging from ICTs to biomedicine) are excluded from this entry because the synthesis processes, even when characterized by extremely narrow size distributions, do not allow complete control on the assembly geometries, not (yet) resulting in the fabrication of bonafide ordered nanostructures. A subsection related to magnetic anisotropies at the nanoscale represents an essential premise to the rest of the work

Kinetic analysis of structural relaxation of FeNi based amorphous alloys by means of dsc and electrical resistivity measurements

Abstract Kinetic analysis of processes distributed over an activation energy spectrum is used to study the structural relaxation in Fe34Ni36Cr10P14B6 amorphous ribbons prepared at different quenching rates. A comparison of the activation energy spectra obtained by differential scanning calorimetry and resistivity measurements shows possible correlations existing between variations of different properties related to structural relaxation processes. The spectra obtained for the thinner ribbons (i.e. prepared with the higher quenching rates) appear to be shifted towards lower energies with respect to those determined for the thicker ribbons. The overall shape of the spectrum is in any case quite independent of the quenching rate.

Mechanical spectroscopy and analytical TEM of structural transformation in the Fe73.5Cu1Nb3Si13.5B9 alloy

Abstract Measurements of the dynamic elasticity moduli and of the Δ E effect coupled to transmission electron microscopy observations were performed on nanocrystalline FeCuNbSiB alloys obtained by melt spinning. The results from measurements on specimens prepared at different quenching rates and submitted to subsequent different thermal treatments allow one to evidence a connection between the magnetic properties of the nanophase with some details of the nanocrystallization and of the topological disorder in the precursor amorphous phase.