Research lines

We seek for control of magnetic and magnetotransport anisotropy in a variety of planar 1D structures obtained by deposition on rippled /grooved substrates: arrays of nanowires, continuous (but magnetically structured) nanostripes, and NP nanoribbons (thin wires made of NPs deposited using a cluster source). Surface patterns like that of figure (a) serve as templates since the ratio of ripple amplitude to ripple wavelength is sufficiently high. Subsequent oblique deposition in the geometry of figure (b) will produce an array of wires on top of the pattern, viz. ridge wires. If necessary, we can embed such an array of wires under a thin cap layer with normal deposition in an all-in-situ sequential process – see figure (c). Not only will we extend the template method for nanowire fabrication but will also study physical properties of these new materials systems, since they present a variety of open questions. We propose to study the magnetic state of planar arrays of nanowires and seek a breakthrough with the realization of wide-area arrays of nanowires suitable for device integration.

We have explored a similar planar system as a sensing, multifunctional device. In collaboration with Instituto Rocasolano (CSIC, Madrid), who provided laser-irradiated undulated polymer substrates, we succeeded in the preparation of nanoparticle stripes by self-shadowing deposition of gas-aggregated Pd particles. The synthesis procedure, as well as optical, electrical and hydrogen-detection properties, is published in the journal Applied Surface Science.

In last years the group has increasingly got involved in local probe microscopies, especially after the installation of a new cryogenic microscope, the arrival of Gabriel Rodriguez and the PhD thesis of Elena Hernández. A closed-cycle helium gas, high vector field microscopy equipment (Attocube AttoDry1000 MFM-SHPM) has been recently improved with a second cryogenic insert for Hall bar microscopy. With this equipment, thus, two magnetic microscopes run on samples at a temperature down to 4 K in separate cryogenic inserts: (1) a scanning Hall probe microscope (SHPM) – non-invasive probe and quantifiable flux response – and (2) a force microscope (MFM, AFM) with interferometric deflection detection. Analysis of magnetic images can be assisted by micromagnetic modelling (finite differences and/or finite elements). Another two atomic force microscopes are available for room-temperature studies under in-situ magnetic field, for electrical conductance microscopy and for inspection purposes. One Solver MFM (NDT-Europe) microscope is equipped with an electromagnet to provide magnetic field up to 2 kG in the sample-probe area. For inspection purposes a table-top Nanosurf AFM ready for operation in a few modes such as contact or taping AFM, MFM and conductive tip ct-AFM.

 
Left pannel is Hall effect (SHPM) image scanned above an array of 20 nm thick permalloy microellipses after aplication of a saturating field along the ellipse major axis. Each of these ellipses makes a dipole fingerprint of single-domain with easy axis magnetization. Image size 20×20 µm2. Drawn for clarity are yellow lines corresponding to microellipe contours as obtained separately from SEM. Red cross points the position of Hall probe for dynamic study of the easy axis reversal: right pannel plots the switching field measured for different field rates, number of sweeps or temperature (see legend).

Force images of an array of microscale permalloy pseudo-wires measured in dual-pass dynamic mode with a magnetic tip: Left panel is the topography image obtained with the first pass and right panel is the magnetic force image (phase shift) as obtained from the second pass at a fixed distance above the surface defined by the first pass. Image phase spans 8.5 deg. Sample fabricated by Alvaro Muñoz-Noval (UCM) and measured with the Attocube MFM microscope by Gabriel Rodríguez.

 
Magnetic Force Microscopy 6 x 6 mm2 scans of an IBS patterned cobalt film in easy axis magnetization i.e. with field applied along the ripple pattern. (a) Virgin state before the application of any magnetic field (dipole-like elements separated by ~ 0,3 mm). Inset shows the ripple pattern surface of AFM image 2 x 2 mm2 (b) Saturation state with in-situ applied field H = 100 mT – contrast due to magnetization ripple -, and (c) Remanence state after the removal of field. (J. M. Colino et al. J. Phys. D: Appl. Phys. 2016)

 
Left panel is dynamic mode AFM image of a permalloy film surface deposited on a LIPSS polymer foil (LIPSS means Laser Induced Patterning of Surface Structures). Image size is 10 x 10 µm2. This thin permalloy film (thickness up to at least 30 nm) is a nanoscale very ragged, continuous magnetic system whereby the hard axis magnetization perfectly follows the surface contour (undulated state): the right panel shows volume-like magnetic poles in such magnetic profile as calculated with micromagnetic modelling and fitting Moke loops (E. H. Sánchez et al. J. Magn. Magn. Mater. 2020)

Se pretende desarrollar una nueva tecnología disruptiva en el área de redes neuronales, específicamente la computación de reservorio. El concepto de inteligencia artificial propone un sistema físico, el reservorio, para entrenarlo y llevar a cabo tareas de manera autónoma. La computación de reservorio se espera sea un avance significativo en la previsión de series temporales por su habilidad reconociendo patrones. Esta tecnología será la siguiente revolución digital donde conceptos como aprendizaje profundo o aprendizaje automático serán predominantes, en la maquinas del mundo real, sobre los de programación clásica. Proponemos el uso de capas delgadas ferromagnéticas, dada su habilidad natural para su entrenamiento ya que naturalmente recuerdan eventos pasados, y el torque de spin orbita para operar en el sistema físico. Nuestra propuesta desarrollará un dispositivo prueba de concepto que se comparará con dispositivos de referencia en la literatura. El proyecto cubrirá áreas de investigación en nuevas áreas tales como el efecto Hall orbital y superconductividad, que actualmente se están investigando como nueva fuente de corrientes de spin, y sustratos ranurados, que conforman una nueva forma de introducir una nueva rotura de simetría es dispositivos espín-orbitrónicos. El equipo de investigación tiene experiencia en el campo de la espintrónica y la espín-orbitrónica, magnetotransporte, magnetóptica, superconductividad y microscopia de sonda de barrido.

 
Panel a: Reservoir Computing is a new computer paradigm that uses a reservoir previously trained to project the vector input into the outputs. This system was originally developed in water reservoirs as shown in panel b, after reference 4 and are currently implemented in software. Recently several works proposed (panels c and d respectively), the use of ferromagnetic films as “water reservoir” or “liquid” given its innate nonlinearity and its ability to retain information.  Both approaches are based on micromagnetic calculation, and they propose the use of tunneling magnetoresistance elements to readout the local magnetization, i.e., the square drawn on top of both pictures. The write mechanism in the c proposal is the spin orbit torque whereas in d are local magnetic fields.