Magnetic skyrmions are nanoscale topological spin structures offering great promise for next-generation information storage technologies. The recent discovery of sub-100-nm room-temperature (RT) skyrmions in several multilayer films has triggered vigorous efforts to modulate their physical properties for their use in devices. Here we present a tunable RT skyrmion platform based on multilayer stacks of Ir/Fe/Co/Pt, which we study using X-ray microscopy, magnetic force microscopy and Hall transport techniques. By varying the ferromagnetic layer composition, we can tailor the magnetic interactions governing skyrmion properties, thereby tuning their thermodynamic stability parameter by an order of magnitude. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of two and ten, respectively. We thus establish a platform for investigating functional sub-50-nm RT skyrmions, pointing towards the development of skyrmion-based memory devices.

Spin injection and accumulation are key phenomena supporting a variety of concepts for spin-electronic devices. These phenomena are expected to be enhanced in nanoparticles over bulk structures due to their discrete energy levels and large charging energies. In this article, precise magnetotransport measurements in the single-electron tunnelling regime are performed by preparing appropriate microfabricated devices containing cobalt nanoparticles. Here we provide experimental evidence for characteristic features of spin accumulation in magnetic nanoparticles, such as oscillations of the magnetoresistance with a periodical sign change as a function of bias voltage. Theoretical analysis of the magnetoresistance behaviour clearly shows that the spin-relaxation time in nanoparticles is highly enhanced in comparison with that in the bulk.

In this paper we review studies on spin-dependent transport in systems containing ferromagnetic nanoparticles. In a tunnel junction with a nanometer-scaleisland, the charging effect leads to an electric current blockade phenomenon in which a single electron charge plays a significant role in electron transport, resulting in single-electron tunneling (SET) properties such as Coulomb blockade and Coulomb staircase. In a tunnel junction with a ferromagnetic nano-island and electrode, it was expected that the interplay of spin-dependent tunneling (SDT) and single-electron tunneling (SET), i.e., spin-dependent single-electron tunneling (SD-SET), would give rise to remarkable tunnel magnetoresistance (TMR) phenomena. We investigated magnetotransport properties in both sequential tunneling and cotunneling regimes of SET and found the enhancement and oscillation of TMR. The self-assembled ferromagnetic nanoparticles we have employed in this study consisted of a Co-Al-O granular film with cobalt nanoparticles embedded in an Al-O insulating matrix. A Co 36 Al 22 O 42 film prepared by a reactive sputtering method produced a TMR ratio reaching 10 % and superparamagnetic behavior at room temperature. The TMR ratio exhibited an anomalous increase at low temperatures but no indication of change with bias voltage. In Ch. 4, we show that the anomalous increase of the MR provided evidence for higher-order tunneling (cotunneling) between large granules through intervening small granules. We emphasize that the existence of higher-order tunneling is a natural consequence of the granular structure, since broad distribution of granule size is an intrinsic property of granular systems. In Ch. 5, we concentrate on SD-SET properties in sequential tunneling regimes. We fabricated two types of device structures with Co-Al-O film using focused ion-beam milling or electron-beam lithography techniques. One had a granular nanobridge structure: point-shaped electrodes separated by a very narrow lateral gap filled with the Co-Al-O granular film. The other had a current-perpendicular-to-plane (CPP) geometry structure: a thin Co-Al-O granular film sandwiched by ferromagnetic electrodes with the current flowing in the direction perpendicular to 1 the film plane through a few Co particles. We found the enhancement and oscillation of TMR due to spin-dependent SET in sequential tunneling regimes. In Ch. 6, we report experimental evidence of a spin accumulation effect in Co nanoparticles leading to the oscillation of TMR with alternate sign changes. Furthermore, we discovered that the spin relaxation time in the nanoparticles is unprecedentedly enhanced up to the order of more than hundreds of nanoseconds, compared to that evaluated from the spin-diffusion length of ferromagnetic layers in previous CPP-GMR studies, i.e.,the order of tens of picoseconds.