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© 2014 AIP Publishing LLC. This work was supported by Spanish MINECO under grants FIS2008-06249 and Consolider CSD2007-00010 and by CAM under grant S2009/MAT-1726., Magnetization reversal processes have been studied in hybrid magnetic/superconducting Nd-Co/Nb bilayers by the comparison of out-of-plane magnetic hysteresis loops and superconducting phase diagrams as a function of magnetic layer thickness and of disorder in the magnetic layer induced by a nanostructured copolymer template. A good correlation is found between the regimes corresponding to percolation effects in the superconductor and to the transition from extended to confined superconductivity with the characteristic fields for reverse domain nucleation and fast domain expansion in the magnetic layer, indicating that superconductivity nucleates on the disordered network imprinted on the superconducting layer by the labyrinth domain structure of the magnetic layer. As disorder increases in the magnetic layer, percolation effects disappear from the superconducting transitions in agreement with a more homogeneous magnetization reversal process., MINECO, Comunidad de Madrid, Depto. de Física de Materiales, Fac. de Ciencias Físicas, TRUE, pub

Very recently magnetic stripe domains, characterized by alternating up and down out-of-plane orientation of the magnetization, have received great interest due to the possibility to use stripe patterns to manipulate spin-wave (SW) propagation as in artificial magnonic crystals. In this work, we demonstrate the control of the SW propagation by using reconfigurable regular stripe-pattern domain structure in the hybrid system. The investigated system consists of 64-nm-thick NdCo layer and 10-nm-thick NiFe layer, coupled through an Al layer of different thicknesses. Magnetic force microscopy (MFM) measurements show that, due to the perpendicular magnetic anisotropy of the NdCo film, the system develops stripe domains aligned with the last in-plane saturation direction. The domain pattern is found to have a period of about 140 nm, which is almost independent on the thickness of the Al layer. The magnetization reversal of the trilayer system was investigated by vibrating sample magnetometer, showing that the hysteresis loop is characterized by a two-step process, due to the different coercivity of the NiFe and NdCo films. Detailed analysis of the hysteresis loops along with micromagnetic simulations indicates that the stray magnetic field coming from the NdCo layer induces a regular domain structure also in the NiFe layer, which is tuned by the thickness of Al spacer. In addition, upon reversing the applied magnetic field, an antiparallel state, characterized by an antiparallel alignment of the magnetization component parallel to the domain axis in the NdCo and NiFe stripes, is formed. Then, Brillouin light scattering spectroscopy has been used to measure the spectra of the SWs propagating in the direction perpendicular to stripe domains for the parallel and the antiparallel state. For both configurations, the dispersion relation shows a strongly nonreciprocal mode. However, in the parallel state SWs propagating with positive and negative wavevector are both characterized

Resumen del trabajo presentado al American Physical Society (APS) March Meeting, celebrado del 4 al 8 de 2019 en Boston, Massachusetts (USA).

In the last decade, magnetic ratchet effect of domain walls has been proposed by several groups as a working principle for spintronic devices like magnetic diodes or shift registers. Indeed, some of the proposed strategies are promising candidates to be seen in the near future in working devices. As an alternative route, we have succeeded in producing and observing magnetic ratchet effects, as well as crossed-ratchet effects, in a ferromagnetic thin film where we have nanopatterned 2D arrays of asymmetric holes in the submicrometric limit for hole sizes. The combination of Kerr microscopy, X-Ray PhotoEmission Electron Microscopy and micromagnetic simulations has allowed a full magnetic characterisation of the domain wall (DW) propagation processes over the whole array and the local DW morphology and pinning at the holes. It is found that the DW dynamics in the submicrometric size limit are governed by the interplay between DW elasticity and half vortex propagation along hole edges: as hole size becomes comparable to DW width, flat DW propagation modes are favoured over kinked DW propagation due to an enhancement of DW stiffness, and pinned DW segments adopt asymmetric configurations related with Nèel DW chirality. Both ratchet and crossed-ratchet effects have been experimentally found, and we propose a new ratchet/inverted-ratchet effect in the submicrometric range driven by magnetic fields and electrical currents.

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 NdCox 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.

Block copolymers offer the fabrication of mesoporous thin films with distinct nanoscale structural features. In this contribution, we present the use of acetic acid (CH3COOH) as a low‐molecular‐weight guest molecule to tune the supramolecular assembly of poly[styrene‐block‐(4‐vinylpyridine)] (PS‐b‐P4VP), offering a versatile and straightforward method to obtain tailored nanostructured films with controlled topography and pore size. Spin‐coating toluene solutions of PS‐b‐P4VP, with a variable amount of CH3COOH, leads to micellar thin films, where the micelles contain the carboxylic acid as a guest molecule. The size can be conveniently modified in these films (from 48 to 75 nm) by varying the amount of organic acid in the starting solutions. Subsequent surface reconstruction of micellar films using ethanol leads to ring‐shaped copolymer nanoporous films with modulated diameter. Controlling the micelle reconstruction process, cylindrical porous films are also obtained. Interestingly, changing the type of aliphatic carboxylic acid leads to a modification of the observed film morphology from micelles to out‐of‐plane P4VP cylinders (or lamellae) in a PS matrix.

We have designed, fabricated and tested a robust superconducting ratchet device based on topologically frustrated spin ice nanomagnets. The device is made of a magnetic Co honeycomb array embedded in a superconducting Nb film. This device is based on three simple mechanisms: (i) the topology of the Co honeycomb array frustrates in-plane magnetic configurations in the array yielding a distribution of magnetic charges which can be ordered or disordered with in-plane magnetic fields, following spin ice rules; (ii) the local vertex magnetization, which consists of a magnetic half vortex with two charged magnetic Néel walls; (iii) the interaction between superconducting vortices and the asymmetric potentials provided by the Néel walls. The combination of these elements leads to a superconducting ratchet effect. Thus, superconducting vortices driven by alternating forces and moving on magnetic half vortices generate a unidirectional net vortex flow. This ratchet effect is independent of the distribution of magnetic charges in the array.

Films of four different nanostructures, namely micelles, filled cylinders, ring-shaped porous nanostructures and hollow cylinders, can be prepared easily in a straightforward and simple way by spin coating onto a silicon plate toluene solutions of poly(styrene-block-4-vinylpyridine) copolymer containing formic acid of increasing concentration. Despite the enormous progress in preparation of porous polymers, most of them require multiple steps. In this sense, this method represents an effort to obtain not only porous thin films but also other bidimensional nanostructures in a very simple way. © 2017 Society of Chemical Industry.

The study of ratchet and crossed-ratchet effects in magnetic domain wall motion through 2D arrays of asymmetric holes is extended in this article to the submicrometric limit in hole size (small size regime). Therefore, the gap has been closed between the 2D ratchets in the range of tens-of-micrometers (large size regime) and the small size regime 1D ratchets based on nanowires. The combination of Kerr microscopy, X-ray PhotoEmission Electron Microscopy and micromagnetic simulations has allowed a full magnetic characterisation of both the domain wall (DW) propagation process over the whole array and the local DW morphology and pinning at the holes. It is found that the 2D small size limit is driven by the interplay between DW elasticity and half vortex propagation along hole edges: as hole size becomes comparable to DW width, flat DW propagation modes are favoured over kinked DW propagation due to an enhancement of DW stiffness, and pinned DW segments adopt asymmetric configurations related with Néel DW chirality. Nevertheless, both ratchet and crossed-ratchet effects have been experimentally found, and we propose a new ratchet/inverted-ratchet effect in the submicrometric range driven by magnetic fields and electrical currents respectively.

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.

Hexagonal antidot arrays have been patterned on weak perpendicular magnetic anisotropy NdCo films by e-beam lithography and lift off. Domain structure has been characterized by Magnetic Force Microscopy at remanence. On a local length scale, of the order of stripe pattern period, domain configuration is controlled by edge effects within the stripe pattern: stripe domains meet the hole boundary at either perpendicular or parallel orientation. On a longer length scale, in-plane magnetostatic effects dominate the system: clear superdomains are observed in the patterned film with average in-plane magnetization along the easy directions of the antidot array, correlated over several antidot array cells.

Los sistemas con anisotropía magnética perpendicular (PMA) presentan un gran interés en áreas como los medios de almacenamiento de la información, o la espintrónica. El control de sus propiedades ha promovido su nanoestructuración, centrándose inicialmente en materiales con una fuerte componente perpendicular. Cuando esta componente es de menor magnitud se habla de PMA débil: la componente perpendicular únicamente se manifiesta por encima de un espesor crítico. A partir de éste la imanación pasa a configurar un sistema de bandas con imanación constante, cuya componente en el plano es idéntica, pero con la componente perpendicular alterna en sentido. Las aleaciones de tipo tierra rara- metal de transición (TR-MT) son un tipo de materiales que presentan PMA. La fenomenología de la configuración de las bandas magnéticas ha atraído interés desde un punto de vista topológico. En esta tesis se han fabricado, mediante pulverización catódica. láminas delgadas de un amorfo de este tipo de aleaciones con PMA débil, NdCo 5 . Se ha estudiado como la modificación a escala nanométrica realizada sobre las mismas afecta a su comportamiento magnético. La nanoestructuración ha sido llevada a cabo, en primer lugar, mediante la técnica autoorganizada de los copolímeros en bloque. Este tipo de materiales están compuestos por dos especies de polímeros diferentes unidos por enlace covalente, que presentan una repulsión química entre sí,. Por tanto, el sistema minimiza su energía segregándose espacialmente, y de forma espontánea, en estructuras unidad básicas. En la tesis se usa el copolímero poliestireno-poli- 4-vinilpiridina (PS-P4VP), estudiándose las distintas estructuras morfológicas a que da lugar alterando parámetros como la velocidad de depósito o la concentración. Este copolímero presenta una reactividad química adicional, que permite la unión de partículas, por enlace de hidrógeno o iónico a la P4VP. En nuestro caso se ha ligado una partícula ácida para aumenta el rango de dimens, para materiales con la imanación en el plano. En esta tesis se ha comparado el comportamiento de este tipo de redes para casos con PMA al diseñarse mediante Litografía Electrónica (EBL) varias redes para los casos de Co y NdCo 5 . La comparación se llevado a cabo con MFM. Ambas redes han manifestado fenomenología de tipo Spin Ice dentro de unos valores dimensionales dependientes del material. Se han registrado y analizado estructuras magnéticas multidominio en el caso del Co. Se forman para evitar la formación de cargas magnéticas en los bordes. En el NdCo5 no existe esta condición, lo que evita su formación en los estados saturados, pero pudiendo ser alcanzada en los estados desordenados.

Magnetic transmission X-ray microscopy (TXM) is a powerful imaging technique that can produce element specific images of magnetic domains with nanometric lateral resolution. Here we present a novel imaging method in which the angular dependence of the magnetic contrast in a series of high resolution TXM images is used to obtain quantitative descriptions of the magnetization (canting angles and sense). This has been applied first to analyze parallel stripe domains in weak perpendicular anisotropy ferromagnetic NdCo5 layers of different thickness, and in NdCo5/Permalloy bilayers. Then, our method has been used to identify complex topological defects (merons or 1/2 skyrmions) in a NdCo5 film that are only partially replicated by the Permalloy overlayer. Meron propagation in trilayers (across the thickness) and in hexagonal networks (across bifurcations) will be discussed in terms of their topological characteristics (chirality and polarity).

In the present study, we have determined the intrinsic refractive index of anodic aluminium oxide, which is originated by the formation of nanoporous alumina templates. Different templates have been fabricated by the conventional two-step anodization procedure in oxalic acid. Their porosities were modified by chemical wet etching allowing the tuning of their effective refractive indexes (air-filled nanopores + anodic aluminium oxide). By standard spectroscopic light transmission measurements, the effective refractive index for each different template was extracted in the VIS-NIR region. The determination of the intrinsic anodic aluminium oxide refractive index was performed by using the Maxwell-Garnett homogenization theory. The results are coincident for all the fabricated samples. The obtained refractive index (∼1.55) is quite lower (∼22%) than the commonly used AlO handbook value (∼1.75), showing that the amorphous nature of the anodic oxide structure strongly conditions its optical properties. This difference is critical for the correct design and modeling of optical plasmonic metamaterials based on anodic aluminium oxide nanoporous templates.

[Introducción]: Las láminas delgadas de aleaciones amorfas de tierra rara y metal de transición han sido ampliamente estudiadas por la posibilidad de controlar sus propiedades magnéticas mediante una elección adecuada de su composición y grosor. En este trabajo analizamos el comportamiento magnético de dos aleaciones amorfas de DyCo5, una preparada en forma de lámina continua y otra en forma de multicapa de periodo ultrafino, mediante el análisis combinado de ciclos de histéresis globales, obtenidos mediante técnicas de magnetometría, y de ciclos específicos al Dy y al Co, obtenidos mediante dicroísmo magnético circular de rayos X (XMCD). Además, los resultados obtenidos se comparan con los de láminas de referencia de YCo5, preparadas con el fin de extraer el comportamiento magnético de los átomos de Co en un entorno químico similar al de las aleaciones de DyCo5. Nuestros resultados complementan los publicados recientemente para aleaciones de DyCo3, aunque en nuestro caso hemos realizado una caracterización a campos magnéticos más elevados y hemos aplicado una interpretación compatible con los modelos esperimagnéticos característicos de este sistema. [Experimental]: Las muestras, de 35 nm de grosor, han sido preparadas por pulverización catódica de blancos de Dy (pureza 99.9 % atómico), Y (99.9 % atómico) y Co (99.99 % atómico) a una presión de Ar+ de 2.9 x 10-3 mbar. Todas las muestras tienen una capa protectora de 3.5 nm de grosor y una capa de transición, directamente sobre el sustrato, de 5 nm de grosor, ambas de Al. Su composición ha sido estimada mediante medidas de fluorescencia de rayos X obtenidas en una microsonda electrónica (EPMA, modelo CAMECA SX-100) de los Servicios Científico-Técnicos de la Universidad de Oviedo. Las propiedades magnéticas se han determinado utilizando dos métodos complementarios. En primer lugar, medidas de magnetometría por muestra vibratoria (VSM) con campos magnéticos perpendiculares a la superficie (hasta 9 T) y mediante un disp, [Resultados]: Las medidas de magnetometría realizadas con campos aplicados en el plano y fuera del plano de la lámina indican que las muestras de DyCo5 tienen anisotropía perpendicular, mientras que las de YCo5 carecen de ella. Teniendo en cuenta que el YCo5 y el DyCo5 tienen una estructura atómica y un entorno químico similares, con la salvedad de que el Y es no magnético y el Dy sí lo es, se puede concluir que el Co no es el causante de la anisotropía perpendicular observada en las muestras de DyCo5. Además, se observa que los campos coercitivos perpendiculares aumentan al enfriar, en buen acuerdo con lo esperado en modelos de anisotropía aleatoria y canje. Se confirma el comportamiento ferrimagnético de las aleaciones con temperaturas de compensación de 120 K para la muestra DyCoS y 90 K para la lámina DyCoA. La diferencia en el valor de temperatura de compensación puede deberse a la ligera modificación de la composición mencionada anteriormente, puesto que en este tipo de materiales se ha observado que variaciones muy leves de composición, en torno al 1 % atómico, generan cambios de la temperatura de compensación de varias decenas de K. Por otro lado, los ciclos XMCD selectivos al elemento tienen sentidos opuestos para el caso del Dy y del Co. Además, se observan pendientes en los ciclos compatibles con la presencia de una distribución de momentos magnéticos en forma de conos, de acuerdo con lo esperado en aleaciones esperimagnéticas. La evolución de estos ciclos en función de la temperatura confirma la inversión de la orientación de los momentos magnéticos del Dy y del Co a ambos lados de la temperatura de compensación y sugiere cambios en la estructura de los conos observados a medida que se enfría el sistema.

Strips containing arrays of submicrometric triangular antidots with a 2D square periodicity have been fabricated by electron beam lithography. A clear ratchet effect of 180° domain wall motion under a varying applied field parallel to the walls has been observed. The direction is determined by the direction of the triangle vertices. In contrast, no ratchet effect is observed when the antidot array is constituted by symmetric rhomb-shaped antidots.

Magnetization reversal processes have been studied in hybrid magnetic/superconducting Nd-Co/Nb bilayers by the comparison of out-of-plane magnetic hysteresis loops and superconducting phase diagrams as a function of magnetic layer thickness and of disorder in the magnetic layer induced by a nanostructured copolymer template. A good correlation is found between the regimes corresponding to percolation effects in the superconductor and to the transition from extended to confined superconductivity with the characteristic fields for reverse domain nucleation and fast domain expansion in the magnetic layer, indicating that superconductivity nucleates on the disordered network imprinted on the superconducting layer by the labyrinth domain structure of the magnetic layer. As disorder increases in the magnetic layer, percolation effects disappear from the superconducting transitions in agreement with a more homogeneous magnetization reversal process. © 2014 AIP Publishing LLC.

Perpendicular magnetic anisotropy ferromagnetic/superconducting (FM/SC) bilayers with a labyrinth domain structure are used to study nucleation of superconductivity on a fractal network, tunable through magnetic history. As clusters of reversed domains appear in the FM layer, the SC film shows a percolative behavior that depends on two independent processes: the arrangement of initial reversed domains and the fractal geometry of expanding clusters. For a full labyrinth structure, the behavior of the upper critical field is typical of confined superconductivity on a fractal network.

Magnetization reversal processes have been analyzed by Magnetic Force Microscopy in dense arrays of Co bars with well defined shape anisotropy and strong magnetostatic interactions. Two different geometries have been used: rectangular and rhombic so that the sign of dipolar interactions between adjacent chains of bars is changed from antiferromagnetic (rectangular array) to ferromagnetic (rhombic array), having a profound influence on the shape of a nucleus of inversion at the magnetization reversal.

Nd-Co amorphous magnetic films with perpendicular magnetic anisotropy have been grown on nanostructured templates prepared with self-organized di-block poly(styrene)-block-poly(4-vinylpyridine) copolymer layers with a periodic structure of 60 nm spaced pores. These templates modify both the magnetic film topography and mechanical strain on a local scale. The effect of these structural changes is particularly noticeable in the low thickness range of the magnetic films where the transition from in-plane to out-of plane magnetization takes place. The Nd-Co films grown on the copolymer template present lower perpendicular magnetic anisotropy and, also, stronger stripe domain pinning effects in comparison with reference films grown on flat Si substrates. © 2012 American Institute of Physics.