Luis Manuel Alvarez-Prado

We show that, in the x-ray magnetic resonant scattering (XRMS) of a two-dimensional array of submicron magnets, the collected intensity at each Bragg reflection is correlated to the reflected light from locations of the magnets that have the same angle of curvature. This converts XRMS in a kind of magnetic microscope capable of spatially resolving the magnetization of the small-size magnets, averaged over the magnets illuminated by the x rays. This result is used to study the magnetization of trilayer submicron disk-shaped magnets consisting of two magnetostatically coupled ferromagnetic layers, about 15 nm thick, separated by a nonmagnetic spacer. These kinds of systems are less known than the single-layer ones, despite having potentially more interesting functionalities for device applications, mainly due to the difficulty to distinguish the magnetization of each of the layers within the magnets. This problem is overcome by XRMS thanks to its chemical sensitivity and its relatively large depth probe. XRMS is also a photon-in photon-out technique that allows measuring under external magnetic fields. This permits the extraction of the local hysteresis loops at different locations of the disks. The technique demonstrates to be very sensitive to the magnetization distribution across each of the layers at any field intensity, with an estimated lateral resolution below 200 nm. This serves to detect, and also explain, chiral asymmetries in the magnetic circulation of the vortex in each of the layers.

We have recently shown that a hybrid magnetic thin film with orthogonal anisotropies presenting weak stripe domains can achieve a high degree of controllability of its ferromagnetic resonance. This work explores the origin of the reconfigurability through micromagnetic simulations. The static domain structures which control the thin film resonance can be found under a deterministic applied field protocol. In contrast to similar systems reported, our effect can be obtained under low magnetic fields. We have also found through simulations that the spin wave propagation in the hybrid is nonreciprocal: two adjacent regions emit antiparallel spin waves along the stripe domains. Both properties convert the hybrid in a candidate for future magnonic devices at the nanoscale.

Layered materials in which individual atomic layers are bonded by weak van der Waals forces (vdW materials) constitute one of the most prominent platforms for materials research. Particularly, polar vdW crystals, such as hexagonal boron nitride (h-BN), alpha-molybdenum trioxide (α-MoO₃) or alpha-vanadium pentoxide (α-V₂O₅), have received significant attention in nano-optics, since they support phonon polaritons (PhPs)―light coupled to lattice vibrations― with strong electromagnetic confinement and low optical losses. Recently, correlative far- and near-field studies of α-MoO₃ have been demonstrated as an effective strategy to accurately extract the permittivity of this material. Here, we use this accurately characterized and low-loss polaritonic material to sense its local dielectric environment, namely silica (SiO₂), one of the most widespread substrates in nanotechnology. By studying the propagation of PhPs on α-MoO₃ flakes with different thicknesses laying on SiO₂ substrates via near-field microscopy (s-SNOM), we extract locally the infrared permittivity of SiO₂. Our work reveals PhPs nanoimaging as a versatile method for the quantitative characterization of the local optical properties of dielectric substrates, crucial for understanding and predicting the response of nanomaterials and for the future scalability of integrated nanophotonic devices.

The sensitivity of circularly polarized x-ray resonant magnetic scattering (CXRMS) to chiral asymmetry has been demonstrated. The study was performed on a 2D array of Permalloy (Py) square nanomagnets of 700 nm lateral size arranged in a chess pattern, in a square lattice of 1000 nm lattice parameter. Previous x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) images on this sample showed the formation of vortices at remanence and a preference in their chiral state. The magnetic hysteresis loops of the array along the diagonal axis of the squares indicate a non-negligible and anisotropic interaction between vortices. The intensity of the magnetic scattering using circularly polarized light along one of the diagonal axes of the square magnets becomes asymmetric in intensity in the direction transversal to the incident plane at fields where the vortex states are formed. The asymmetry sign is inverted when the direction of the applied magnetic field is inverted. The result is the expected in the presence of an unbalanced chiral distribution. The effect is observed by CXRMS due to the interference between the charge scattering and the magnetic scattering.

An original approach to tune the ferromagnetic resonance frequency of a soft magnetic Ni₈₀Fe₂₀ (Permalloy = Py) film with in-plane magnetic anisotropy (IMA) based on the controlled coupling to a hard magnetic NdCo_x film with perpendicular magnetic anisotropy (PMA) through a nonmagnetic Al spacer is studied. Using the transverse magneto-optical Kerr effect (T-MOKE), alternating gradient magnetometry (AGM), and vector network analyzer ferromagnetic resonance (VNA-FMR) spectroscopy, the influence of both the Co concentration and the Al spacer thickness on the static and dynamic magnetic properties of the coupled IMA/PMA system is investigated. Compared to a single Py film, two striking effects of the coupling between IMA and PMA layers can be observed in their FMR spectra. First, there is a significant increase in the zero-field resonance frequency from 2.0 GHz up to 6.4 GHz, and second, an additional frequency hysteresis occurs at low magnetic fields applied along the hard axis. The maximum frequency difference between the frequency branches for increasing and decreasing magnetic fields is as high as 1 GHz, corresponding to a tunability of about 20% at external fields of typically less than ±70 mT. The origin of the observed features in the FMR spectra is discussed by means of magnetization reversal curves.

Domain configuration has been studied by magnetic force microscopy and micromagnetic simulations in NdCo honeycomb lattices in comparison with similar patterned structures made of polycrystalline Co. The change in material anisotropy from in-plane to weak perpendicular magnetic anisotropy (wPMA) modifies the basic domain structure and relevant topological defects in the magnetization in each case: from in-plane domains, vortices, antivortices and half vortices in Co lattices to parallel stripe patterns with dislocations in NdCo samples with large enough thickness. A characteristic feature of wPMA materials is the possibility to drive the system from in-plane to stripe pattern configuration playing with sample thickness (during growth) or with magnetic anisotropy (as a function of temperature). It has allowed us to observe complex magnetic textures within the stripe pattern of NdCo samples imprinted from previous magnetic vortex and antivortex states that nucleate within the honeycomb lattice during the early stages of deposition and, then, become frozen by local rotatable anisotropy as sample thickness increases.

We have studied two nanomagnet systems with strong (Co/Pd multilayers) and weak (NdCo alloy films) stray magnetic fields by probing the out-of-plane magnetic states with superconducting vortices. The hybrid samples are made of array of nanomagnets embedded in superconducting Nb thin films. The vortex motion detects relevant magnetic state features, since superconducting vortices are able to discriminate between different magnetic stray field strengths and directions. The usual matching effect between the superconducting vortex lattice and the periodic pinning array can be quenched by means of disorder magnetic potentials with strong stray fields at random. Ordered stray fields retrieve the matching effect and yield asymmetry and shift in the vortex dissipation signal. Furthermore vortices can discriminate the sizes of the nanomagnet magnetic domains, detecting magnetic domain sizes as small as 70 nm. In addition, we observe that the vortex cores play the crucial role instead of the supercurrents around the vortex.