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. Today’s 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.