Alvaro Sanchez

Control of magnetization reversal processes is a key issue for the implementation of magnetic materials in technological applications. The modulation of shape magnetic anisotropy in nanowire structures with a high aspect ratio is an efficient way to tune sharp in‐plane magnetic switching. However, control of fast magnetization reversal processes induced by perpendicular magnetic fields is much more challenging. Here, tunable sharp magnetoresistance changes, triggered by out‐of‐plane magnetic fields, are demonstrated in thin permalloy strips grown on LaAlO₃ single crystal substrates. Micromagnetic simulations are used to evaluate the resistance changes of the strips at different applied field values and directions and correlate them with the magnetic domain distribution. The experimentally observed sharp magnetic switching, tailored by the shape anisotropy of the strips, is properly accounted for by numerical simulations when considering a substrate‐induced uniaxial magnetic anisotropy. These results are promising for the design of magnetic sensors and other advanced magnetoresistive devices working with perpendicular magnetic fields by using simple structures.

The fusion created by magnetically confined plasma is a promising clean and essentially unlimited future energy source. However, net energy generation has not been yet demonstrated in fusion experiments. Some of the main problems hindering controlled fusion are the imperfect magnetic confinement and the associated plasma instabilities. Here, we theoretically demonstrate how to create a fully confined magnetic field with the precise three-dimensional shape required by fusion theory, using a bulk superconducting toroid with a toroidal cavity. The combination of the properties of superconductors with the toroidal topology makes the vacuum field in the cavity volume consisting of nested flux surfaces, a condition for optimum plasma confinement. The coils creating the field, embedded in the superconducting bulk, can be chosen with very simple shapes, in contrast with the cumbersome arrangements in current experiments, and can be spared from the large magnetic forces between them. Because the field shape in the cavity is given by the boundary conditions in the superconductor surface, the system will always tend to maintain the optimum field distribution in response to instabilities or turbulence in the plasma. This field shape is preserved even when holes are drilled in the superconductor to access the plasma region from the exterior. We demonstrate how a fully-confined magnetic field with the three-dimensional spatial distribution required in two of the most advanced stellarators, Large Helical Device and Wendelstein 7-X, can be exactly generated, using simple round coils as magnetic sources. We argue that state-of-the-art high-temperature superconductors already have the necessary properties to be employed to construct the bulk superconducting toroid. The present strategy can lead to optimized robust magnetic confinement and largely simplified configurations in future fusion experiments.

Magnetic properties of materials are typically studied by applying an external magnetic field and measuring the magnetic response of the material. The technique of AC susceptibility is a particularly useful example because of the combination of simplicity and powerfulness of the results. Magnetic materials can be broadly classified as having paramagnetic or diamagnetic response depending on the sign of the induced magnetic response. The former occurs when the magnetic moment resulting from the application of an external field has the same sign as the applied field and the latter when the sign is opposite. Superconductors are a particular case of diamagnetic materials, actually being the materials providing the most extreme diamagnetic response in a wide range of temperatures and applied magnetic field strengths. An ideal superconductor is a perfectly diamagnetic linear material, with a zero magnetic field induction B in all its interior. In this ideal case, since B = µ₀(H + M), (H is the magnetic field and M is the magnetization, µ₀ is the vacuum permeability), one has M = −H, so that internal DC magnetic susceptibility χ^DC, defined through M = χ^DCH, is −1. In the International System of units used in this chapter, the susceptibility is dimensionless. If the applied field is harmonically varying in time, then a complex AC susceptibility can be defined. For long samples, the ideal superconductor would simply follow −1 and 0 for the real and imaginary parts of the AC susceptibility, respectively.

Some of the most important technological challenges of today’s society, such as fusion reactors for future clean unlimited energy or the next generation of medical imaging techniques, require precise spatial shapes of strong magnetic fields. Achieving these high fields is currently hindered by limitations such as large forces damaging the wires in coils or the saturation of ferromagnets at high fields. Here, we demonstrate a novel paradigm for creating magnetic landscapes. By enclosing magnetic sources within zero-magnetic-permeability (ZMP) media, a set of novel properties is unveiled. The magnetic field shape directly results from the contour of the outer surface of the ZMP enclosure, which allows the realization of basically any imaginable field landscape. Also, currents embedded in ZMP media can be fully magnetically isolated, which eliminates the forces in the wires, one of the main factors that currently impedes achieving very high magnetic fields. We confirm these properties, rooted in fundamental laws of electromagnetism, by numerical simulations and by proof-of-principle experiments using conventional high-temperature superconductors as ZMP materials, which showcase the practical applicability of our ideas. The freedom in the design of magnetic fields provided by ZMP media enables us to concentrate and homogenize magnetic fields with unprecedented precision, as needed in medical imaging techniques and particle-physics experiments, and to realize devices like perfect electromagnetic absorbers of mechanical vibrations.

Levitation of superconductors is becoming an important building block in quantum technologies, particularly, in the rising field of magnetomechanics. In most of the theoretical proposals and experiments, solid geometries, such as spheres are considered for the levitator. Here we demonstrate that replacing them by superconducting rings brings two important advantages: First, the forces acting on the ring remain comparable to those expected for solid objects, whereas the mass of the superconductor is greatly reduced. In turn, this reduction increases the achievable trap frequency. Second, the flux trapped in the ring by in-field cooling yields an additional degree of control for the system. We construct a general theoretical framework with which we obtain analytical formulations for a superconducting ring levitating in an anti-Helmholtz quadrupole field and a dipole field for both zero-field and field cooling. The positions and the trapping frequencies of the levitated rings are analytically found as a function of the parameters of the system and the field applied during the cooling process. Unlike what is commonly observed in bulk superconductors, lateral and rotational stabilities are not granted for this idealized geometry. We, therefore, discuss the requirements for simple superconducting structures to achieve stability in all degrees of freedom.

The performance of skyrmion-based spintronic devices can be greatly affected by the presence of defects in the materials. Here we develop an analytic model for describing the interaction between skyrmions and defects in ultrathin films focusing on the case of extended defects. The dynamics of skyrmions under the driving of in-plane polarized current is studied, considering several types of torques. The system is modeled with Thiele's equation in the complex plane and described with a reduced number of parameters. We start by considering a Gaussian-like interaction with point defects and extend the treatment to segment, line, and grid defects. Conditions for dynamic regimes of pinning, guiding, accelerating, or arranging skyrmions by defects are established and discussed. In particular, expressions for the threshold driven current density to pin or depin skyrmions in such defects, the position of the critical points, as well as the guiding conditions along long defects are found, analytically in some cases. This enables a deeper understanding of the parameter dependence of the skyrmion dynamics under the influence of defects. Micromagnetic simulations show qualitative agreement with the analytical treatment.

Magnetic skyrmions can be guided along rails made by line defects while their speed increased by a factor of ten.

Superconductors are essential in many present and future technologies, from large-scale devices for medical imaging, accelerators, or fusion experiments to ultralow-power superconducting electronics. [1] However, their potential applicability, and particularly that of high-temperature superconductors (HTS), is severely affected by limited performances at large magnetic fields and high temperatures, where their use is most needed. One of the main reasons for these limitations are the presence of quantized vortices, whose movements result in losses, internal noise, and reduced properties. [2,3] The conventional strategy to overcome flow of vortices is to pin them along artificial defects.[3–7] Here we theoretically and experimentally demonstrate that critical-current density in high-temperature superconductors can reach unprecedentedly high values at high fields and temperatures, by preventing vortex entry. By tailoring the geometry, i.e. reducing the width, W, of nanowire-patterned HTS films, the range of the Meissner state, for which no vortices are present, is extended up to very large applied field values, on the order of ∼ 1T. Current densities of the order of the depairing current can be sustained under high fields, for a wide range of temperatures. Results may be relevant both for devising new conductors carrying depairing-current values at high temperatures and large magnetic fields and for reducing flux noise in sensors and quantum systems. (1) S. Nishijima et al. Superconductivity and the environment: a Roadmap, Supercond. Sci. Technol. 26, 113001 (2013). (2) D. Larbalestier et al. Superconducting materials for electric power applications. Nature 414, 368 (2001). (3) R. Wordenweber, Engineering of superconductors and superconducting devices using artificial pinning sites, Physical Sciences Reviews, De Gruyter, Chap. 2, 20178000 (2017). (4) A. Palau et al. Vortex dynamics in nanofabricated chemical solution deposition high-temperature superconducting films, Super- conductors at the Nanoscale, De Gruyter, Chap. 6, 195 (2017). (5) X. Obradors et al. Coated conductors for power applications: materials challenges. Supercond. Sci. Technol. 27, 044003 (2014). (6) W. K. Kwok et al. Vortices in high-performance high-temperature superconductors. Reports Prog. Phys. 79, 116501 (2016). (7) J. P. F. Feighan et al. Materials design for artificial pinning centres in superconductor PLD coated conductors. Supercond. Sci. Technol. 30, 123001 (2017)