Single electron spintronics

Nanostructure-induced L10-ordering of twinned single-crystals in CoPt ferromagnetic nanowires

L1₀-ordered ferromagnetic nanowires with large coercivity are essential for realizing next-generation spintronic devices. Ferromagnetic nanowires have been commonly fabricated by first L1₀-ordering of initially disordered ferromagnetic films by annealing and then etching them into nanowire structures using lithography. If the L1₀-ordered nanowires can be fabricated using only lithography and subsequent annealing, the etching process can be omitted, which leads to an improvement in the fabrication process for spintronic devices. However, when nanowires are subjected to annealing, they easily transform into droplets, which is well-known as Plateau-Rayleigh instability. Here, we propose a concept of "nanostructure-induced L1₀-ordering" of twinned single-crystals in CoPt ferromagnetic nanowires with a 30 nm scale ultrafine linewidth on Si/SiO₂ substrates. The driving forces for nanostructure-induced L1₀-ordering during annealing are atomic surface diffusion and extremely large internal stress at ultrasmall 10 nm scale curvature radii of the nanowires. (Co/Pt)₆ multilayer nanowires are fabricated by a lift-off process combining electron-beam lithography and electron-beam evaporation, followed by annealing. Cross-sectional scanning transmission electron microscope images and nano-beam electron diffraction patterns clearly indicate nanostructure-induced L1₀-ordering of twinned single-crystals in the CoPt ferromagnetic nanowires, which exhibit a large coercivity of 10 kOe for perpendicular, longitudinal, and transversal directions of the nanowires. Two-dimensional grazing incidence X-ray diffraction shows superlattice peaks with Debye-Scherrer ring shapes, which also supports the nanostructure-induced L1₀-ordering. The fabrication method for nanostructure-induced L1₀-ordered CoPt ferromagnetic nanowires with twinned single-crystals on Si/SiO₂ substrates would be significant for future silicon-technology-compatible spintronic applications.

Long spin lifetime and large barrier polarisation in single electron transport through a CoFe nanoparticle

We have investigated single electron spin transport in individual single crystal bcc Co30Fe70 nanoparticles using scanning tunnelling microscopy with a standard tungsten tip. Particles were deposited using a gas-aggregation nanoparticle source and individually addressed as asymmetric double tunnel junctions with both a vacuum and a MgO tunnel barrier. Spectroscopy measurements on the particles show a Coulomb staircase that is correlated with the measured particle size. Field emission tunnelling effects are incorporated into standard single electron theory to model the data. This formalism allows spin-dependent parameters to be determined even though the tip is not spin-polarised. The barrier spin polarisation is very high, in excess of 84%. By variation of the resistance, several orders of magnitude of the system timescale are probed, enabling us to determine the spin relaxation time on the island. It is found to be close to 10 μs, a value much longer than previously reported.

New directions in spintronics

Conventional microelectronics exploits only the charge degree of freedom of the electron. Bringing the spin degree of freedom to bear on sensing, radio frequency, memory and logic applications opens up new possibilities for 'more than Moore' devices incorporating magnetic components that can couple to an external field, store a bit of data or represent a Boolean state. Moreover, the electron spin is an archetypal two-state quantum system that is an excellent candidate for a solid-state realization of a qubit.