Rafael Morales

This study reports an easy procedure to fabricate high and reproducible-performance SERS platforms (on >2 cm ² scales) from hybrid Au nanodisk–nanohole arrays produced via single-beam laser interference lithography.

Nanostructured magnetic multilayers with perpendicular magnetic anisotropy (PMA) are interesting materials for a plethora of magneto-optical and spintronic applications. The search for PMA in these heterostructures is commonly pursued by employing magnetron sputtering under ultra-high vacuum conditions and stacking a large number of layer repetitions. This work introduces a straightforward approach for Co/Pt ultra-thin film growth using electron beam evaporation under lower demanding vacuum conditions—i.e., base pressures around 10−6 mbar—achieving a high degree of PMA and tunable magnetic properties with only one repetition of the magnetic material. The non-conformal deposition geometry of this technique ensures compatibility with micro- and nanofabrication. A Cu buffer layer promotes strong PMA, as it enhances the (111) texture growth of the Co/Pt stack under high vacuum conditions. The control of the Cu and Pt thicknesses allows to fine-tune coercivities across a wide range of magnetic fields while maintaining a well-defined out-of-plane loop squareness and a sharp transition between the two perpendicular states. The influence of the Cu layer on the magneto-optical signal reveals a trade-off between the increase in coercivity and the reduction of the magneto-optical signal as the Cu buffer layer grows. Adding a second Co/Pt repetition mitigates this limitation, yielding samples with both strong magneto-optical signals and high coercivities, featuring the largest effective anisotropy.

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.

Nanoegiturazko biomaterial magnetikoen aplikazioa ehun-ingeniaritzan produkzioaren informazio-fitxa

Controlling the magnetic ground states at the nanoscale is a longstanding basic research problem and an important issue in magnetic storage technologies. Here, we designed a nanostructured material that exhibits very unusual hysteresis loops due to a transition between vortex and double pole states. Arrays of 700 nm diamond-shaped nanodots consisting of Py(30 nm)/Ru(tRu)/Py(30 nm) (Py, permalloy (Ni80Fe20)) trilayers were fabricated by interference lithography and e-beam evaporation. We show that varying the Ru interlayer spacer thickness (tRu) governs the interaction between the Py layers. We found this interaction mainly mediated by two mechanisms: magnetostatic interaction that favors antiparallel (antiferromagnetic, AFM) alignment of the Py layers and exchange interaction that oscillates between ferromagnetic (FM) and AFM couplings. For a certain range of Ru thicknesses, FM coupling dominates and forms magnetic vortices in the upper and lower Py layers. For Ru thicknesses at which AFM coupling dominates, the magnetic state in remanence is a double pole structure. Our results showed that the interlayer exchange coupling interaction remains finite even at 4 nm Ru thickness. The magnetic states in remanence, observed by magnetic force microscopy (MFM), are in good agreement with corresponding hysteresis loops obtained by the magneto-optic Kerr effect (MOKE) and micromagnetic simulations.

Antidot lattices made of magnetic thin films are good candidates to be employed in future magnetic recording media. In this manuscript we present a study on the effect of shape and field-induced magnetic anisotropies on the magnetization reversal of 10 nm and 50 nm thick permalloy antidot lattices. Rounded antidot square lattices were fabricated using a combination of electron beam evaporation and laser interference lithography, covering surfaces of a few cm². We demonstrate that a magnetic anisotropy induced in the samples, as a consequence of an applied magnetic field during growth, competes with the shape anisotropy that dominates the response of the patterned thin films, and that the effect of the field-induced magnetic anisotropy scales with the thickness of the antidot thin films. Finally, we have quantified the anisotropy constant attributable to the uniaxial field-induced magnetic anisotropy in our antidot lattices. These findings are supported by micromagnetic simulations performed using MuMax3.

Remote manipulation of superhydrophobic surfaces provides fascinating features in water interface-related applications. A superhydrophobic magnetic nanoparticle colloid layer is able to float on the water–air interface and form a stable water–solid–air interface due to its inherent water repulsion, buoyancy, and lateral capillarity properties. Moreover, it easily bends downward under an externally applied gradient magnetic field. Thanks to that, the layer creates a stable twister-like structure with a flipped conical shape, under controlled water levels, behaving as a soft and elastic material that proportionally deforms with the applied magnetic field and then goes back to its initial state in the absence of an external force. When the tip of the twister structure touches the bottom of the water container, it provides a stable magneto movable system, which has many applications in the microfluidic field. We introduce, as a proof-of-principle, three possible implementations of this structure in real scenarios, the cargo and transport of water droplets in aqueous media, the generation of magneto controllable plugs in open surface channels, and the removal of floating microplastics from the air–water interface.

Noninvasive manipulation of cell signaling is critical in basic neuroscience research and in developing therapies for neurological disorders and psychiatric conditions. Here, the wireless force‐induced stimulation of primary neuronal circuits through mechanotransduction mediated by magnetic microdiscs (MMDs) under applied low‐intensity and low‐frequency alternating magnetic fields (AMFs), is described. MMDs are fabricated by top‐down lithography techniques that allow for cost‐effective mass production of biocompatible MMDs with high saturation and zero magnetic magnetic moment at remanence. MMDs are utilized as transducers of AMFs into mechanical forces. When MMDs are exposed to primary rat neuronal circuits, their magneto‐mechanical actuation triggers the response of specific mechanosensitive ion channels expressed on the cell membranes activating ≈50% of hippocampal and ≈90% of cortical neurons subjected to the treatment. Mechanotransduction is confirmed by the inhibition of mechanosensitive transmembrane channels with Gd³⁺. Mechanotransduction mediated by MMDs cause no cytotoxic effect to neuronal cultures. This technology fulfills the requirements of cell‐type specificity and weak magnetic fields, two limiting factors in the development of noninvasive neuromodulation therapies and clinical equipment design. Moreover, high efficiency and long‐lasting stimulations are successfully achieved. This research represents a fundamental step forward for magneto‐mechanical control of neural activity using disc‐shaped micromaterials with tailored magnetic properties.

An enhanced exchange bias was found in FeF₂/Ni interfaces by inserting dusting of Pd and Cu atoms. We have used an ultrathin wedge to investigate systematically the effect of a discontinuous nonmagnetic spacer between the antiferromagnetic and ferromagnetic layers. Negative and positive exchange biases are symmetric and maximized for less than two angstroms of the nonmagnetic layer. Moreover, the dusting reduces the field cooling threshold to switch between the negative and the positive exchange bias. This finding demonstrates that nonmagnetic dusted layers can improve the interfacial exchange energy density in antiferromagnetic/ferromagnetic heterostructures and modify the bulk antiferromagnetic domain structure. Other dusting materials such as Ag, Ti, V, and SiO₂ showed no enhancement in FeF₂/Ni bilayers, which indicates the atomic sensitivity of this effect.