A Skyrmion crystal typically arises from helical spin structures induced by the Dzyaloshinskii-Moriya interaction. Experimentally its physical exploration has been impeded because it is a rarity and is found only within a narrow temperature and magnetic field range. We present a method for the assembly of a two-dimensional Skyrmion crystal based upon a combination of a perpendicularly magnetized film and nanopatterned arrays of magnetic vortices that are geometrically confined within nanodisks. The practical feasibility of the method is validated by micromagnetic simulations and computed Skyrmion number per unit cell. We also quantify a wide range in temperature and field strength over which the Skyrmion crystal can be stabilized without the need for any intrinsic Dzyaloshinskii-Moriya interactions, which otherwise is needed to underpin the arrangement as is the case in the very few known Skyrmion crystal cases. Thus, our suggested scheme involves a qualitative breakthrough that comes with a substantial quantitative advance.

We report the creation of an artificial skyrmion crystal, which is configurable reliably at room temperature. The samples are fabricated by embedding lithography-patterned arrays of micron-sized Co disks onto Co/Pt multilayer films that have perpendicular magnetic anisotropy. Kerr microscopy and magnetic force microscopy reveal that the disks are in the vortex state with controllable circulation. Via comparison of measured hysteresis loops and calculated ones, we find that the sample can be configured into either a skyrmion or a non-skyrmion state. The reproducible and stable skyrmion crystal at room temperature opens the door to direct exploration of their unique topological properties, which has deservedly caused a flurry of theoretical activity.

We utilize a nanoscale magnetic spin-valve structure to demonstrate that current-induced magnetization fluctuations at cryogenic temperatures result predominantly from the quantum fluctuations enhanced by the spin transfer effect. The demonstrated spin transfer due to quantum magnetization fluctuations is distinguished from the previously established current-induced effects by a nonsmooth piecewise-linear dependence of the fluctuation intensity on current. It can be driven not only by the directional flows of spin-polarized electrons, but also by their thermal motion and by scattering of unpolarized electrons. This effect is expected to remain non-negligible even at room temperature, and entails a ubiquitous inelastic contribution to spin-polarizing properties of magnetic interfaces.

We address the long-term controversy on the fundamental question of the role of the surface state on the Kondo effect with Co adatoms on a Ag(111) surface. The width of the Kondo resonance oscillates with the same period of half Fermi wavelength of the surface state. But the amplitude increases for a Co adatom placed next to another Co adatom, at the vicinity of a step edge, and quantum confined within nanocorrals. A greater than three times enhancement of the resonance width can be achieved when quantum confinement is introduced. The experimental results are described quantitatively utilizing an analytical model where the contributions of the bulk and surface states are weighted by their exchange values with the magnetic impurity. These findings clarify the role of the surface state on the Kondo effect and pave a pathway to tailor the Kondo effect via quantum confinement of the surface state.

We present a micromagnetic study on the eigen excitations of an artificial skyrmion crystal, which has been experimentally confirmed to be stable at room temperature without the need of any Dzyaloshinsky-Moriya interaction (DMI). Three in-plane rotational modes and one breathing-type mode are identified. We find the intrinsic origin of the dynamics of skyrmion crystal is the nontrivial magnetic texture instead of DMI. And the rotational direction of a skyrmion is solely determined by the sign of the skyrmion number, irrespective of its circulation sense, evidencing the topological nature of the magnetic skyrmion.