Aleix Barrera

Spin transport in superconductors offers a compelling platform to merge the dissipationless nature of superconductivity with the functional promise of spin‐based electronics. A significant challenge in achieving spin polarization in conventional superconductors stems from the singlet state of Cooper pairs, which exhibit no net spin. The generation of spin‐polarized carriers, quasiparticles, or triplet pairs in superconductors has predominantly been realized in hybrid superconductor/ferromagnet systems through proximity‐induced spin polarization. Historically, cuprate superconductors have been characterized by strong electronic correlations but negligible spin–orbit coupling. In this study, we observe a large in‐plane angle‐dependent magnetoresistance and a pronounced planar Hall effect arising near the superconducting phase transition in the prototypical high‐temperature cuprate superconductor without using a proximity ferromagnet. These effects ‐ unusual in centrosymmetric cuprates ‐ may arise from spin‐polarized quasiparticle transport potentially mediated by strong spin–orbit coupling. By systematically tuning magnetic field strength, orientation, temperature, and doping, we identify transport signatures that are consistent with spin–orbit‐driven phenomena. Our findings suggest the presence of a previously underappreciated spin–orbit landscape in cuprates, which may provide the basis for exploring spintronic functionalities in high‐temperature superconductors.

Many nanoscale magnetic imaging techniques are constrained by the maximum magnetic field that can be applied during measurements, due to geometrical limitations or interactions with the probe or the detected signal (e.g., electrons). Here, it is demonstrated that sample‐integrated metamaterial‐inspired magnetic flux concentrators (MFCs) locally amplify magnetic fields, allowing observation of magnetization processes beyond instrumental limits. Micrometer‐sized MFCs fabricated directly on the samples are tested in photoemission electron microscopy experiments employing X‐ray magnetic circular dichroism as magnetic contrast mechanism. At low applied fields, substantial amplification factors enable observation of magnetization reversal in a chain of magnetite nanoparticles synthesized by magnetotactic bacteria at an applied field of 8 mT, substantially smaller than the ∼50 mT predicted by simulations in the absence of MFCs. At higher fields, the field enhancement extends the accessible field range by a factor of five, enabling for the first time, imaging of the field‐dependent magnetic domain structure evolution of an isolated giant magnetofossil. Finally, we show how MFC geometry and material parameters can be tuned to optimize performance considering sample and experimental constraints, providing a tunable and broadly applicable strategy for extending the accessible field range in a wide variety of nanoscale magnetic imaging techniques.

Rapid progress in information technologies has spurred the need for innovative memory concepts, for which advanced data-processing methods and tailor-made materials are required. Here we introduce a previously unexplored nanoscale magnetic object: an analog magnetic vortex controlled by electric-field-induced ion motion, termed magneto-ionic vortex or “vortion”. This state arises from paramagnetic FeCoN through voltage gating and gradual N^3– ion extraction within patterned nanodots. Unlike traditional vortex states, vortions offer comprehensive analog adjustment of key properties such as magnetization amplitude, nucleation/annihilation fields, or coercivity using voltage as an energy-efficient tuning knob. This manipulation occurs post-synthesis, obviating the need for energy-demanding methods like laser pulses or spin-torque currents. By leveraging an overlooked aspect of N^3– magneto-ionics—planar ion migration within nanodots—precise control of the magnetic layer’s thickness is achieved, which enables reversible transitions among paramagnetic, single-domain, and vortion states, offering future prospects for analog computing, multi-state data storage, or brain-inspired devices.

Transformation optics applied to low frequency magnetic systems have been recently implemented to design magnetic field concentrators and cloaks with superior performance. Although this achievement has been amply demonstrated theoretically and experimentally in bulk 3D macrostructures, the performance of these devices at low dimensions remains an open question. In this work, we numerically investigate the non-monotonic evolution of the gain of a magnetic metamaterial field concentrator as the axial dimension is progressively shrunk. In particular, we show that in planar structures, the role played by the diamagnetic components becomes negligible, whereas the paramagnetic elements increase their magnetic field channeling efficiency. This is further demonstrated experimentally by tracking the gain of superconductor-ferromagnet concentrators through the superconducting transition. Interestingly, for thicknesses where the diamagnetic petals play an important role in the concentration gain, they also help to reduce the stray field of the concentrator, thus limiting the perturbation of the external field (invisibility). Our findings establish a roadmap and set clear geometrical limits for designing low dimensional magnetic field concentrators.

Magnetic sensors are used in many technologies and industries, such as medicine, telecommunications, robotics, the Internet of Things, etc. The sensitivity of these magnetic sensors is a key aspect, as it determines their precision. In this article, we investigate how a thin windmill-like ferromagnetic system can hugely concentrate a magnetic field at its core. A magnetic sensor combined with such a device enhances its sensitivity by a large factor. We describe the different effects that provide this enhancement: the thickness of the device and its unique windmill-like geometry. An expression for the magnetic field in its core is introduced and verified using finite-element calculations. The results show that a high magnetic field concentration is achieved for a low thickness-diameter ratio of the device. Proof-of-concept experiments further demonstrate the significant concentration of the magnetic field when the thickness-diameter ratio is low, reaching levels up to 150 times stronger than the applied field.