Imaging the Magnetization of Single Magnetite Nanoparticle Clusters via Photothermal Circular Dichroism.

Magnetic imaging is a versatile tool in biological and condensed-matter physics. Existing magnetic imaging techniques either require demanding experimental conditions which restrict the range of their applications or lack the spatial resolution required for single-particle measurements. Here, we combine photothermal (PT) microscopy with magnetic circular dichroism (MCD) to develop a versatile magnetic imaging technique using visible light. Unlike most magnetic imaging techniques, photothermal magnetic circular dichroism (PT MCD) microscopy works particularly well for single nanoparticles immersed in liquids. As a proof of principle, we demonstrate magnetic CD imaging of superparamagnetic magnetite nanoparticulate clusters immersed in microscope immersion oil. The sensitivity of our method allowed us to probe the magnetization curve of single ∼400-nm-diameter magnetite nanoparticulate clusters.

Superconducting Triplet Rim Currents in a Spin-Textured Ferromagnetic Disk.

Since the discovery of the long-range superconducting proximity effect, the interaction between spin-triplet Cooper pairs and magnetic structures such as domain walls and vortices has been the subject of intense theoretical discussions, while the relevant experiments remain scarce. We have developed nanostructured Josephson junctions with highly controllable spin texture, based on a disk-shaped Nb/Co bilayer. Here, the vortex magnetization of Co and the Cooper pairs of Nb conspire to induce long-range triplet (LRT) superconductivity in the ferromagnet. Surprisingly, the LRT correlations emerge in highly localized (sub-80 nm) channels at the rim of the ferromagnet, despite its trivial band structure. We show that these robust rim currents arise from the magnetization texture acting as an effective spin-orbit coupling, which results in spin accumulation at the bilayer-vacuum boundary. Lastly, we demonstrate that by altering the spin texture of a single ferromagnet, both 0 and π channels can be realized in the same device.

Direct-Write Printing of Josephson Junctions in a Scanning Electron Microscope.

Josephson junctions are the building blocks of superconducting electronics, with well-established applications in precision metrology and quantum computing. Fabricating a Josephson junction has been a resource-intensive and multistep procedure, involving lithography and wet-processing, which are not compatible with many applications. Here, we introduce a fully additive direct-write approach, where a scanning electron microscope can print substrate-conformal Josephson devices in a matter of minutes, requiring no additional processing. The junctions are made entirely by electron-beam-induced deposition (EBID) of tungsten carbide. We utilize EBID-tunable material properties to write, in one go, full proximity junctions with superconducting electrodes and metallic weak links and tailor their Josephson coupling. The Josephson behavior of these junctions is established and characterized by their microwave-induced Shapiro response and field-dependent transport. Our efforts provide a versatile and nondestructive alternative to conventional nanofabrication and can be expanded to print three-dimensional superconducting sensor arrays and quantum networks.

Controlling supercurrents and their spatial distribution in ferromagnets.

Spin-triplet Cooper pairs induced in ferromagnets form the centrepiece of the emerging field of superconducting spintronics. Usually the focus is on the spin-polarization of the triplets, potentially enabling low-dissipation magnetization switching. However, the magnetic texture which provides the fundamental mechanism for generating triplets also permits control over the spatial distribution of supercurrent. Here we demonstrate the tailoring of distinct supercurrent pathways in the ferromagnetic barrier of a Josephson junction. We combine micromagnetic simulations with three-dimensional supercurrent calculations to design a disk-shaped structure with a ferromagnetic vortex which induces two transport channels across the junction. By using superconducting quantum interferometry, we show the existence of two channels. Moreover, we show how the supercurrent can be controlled by moving the vortex with a magnetic field. This approach paves the way for supercurrent paths to be dynamically reconfigured in order to switch between different functionalities in the same device.