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Magnetic lateral multilayers have been fabricated on weak perpendicular magnetic anisotropy amorphous Nd-Co films in order to perform a systematic study on the conditions for controlled nucleation of topological defects within their magnetic stripe domain pattern. A lateral thickness modulation of period w is defined on the nanostructured samples that, in turn, induces a lateral modulation of both magnetic stripe domain periods λ and average in-plane magnetization component Min-plane. Depending on lateral multilayer period and in-plane applied field, thin and thick regions switch independently during in-plane magnetization reversal and domain walls are created within the in-plane magnetization configuration coupled to variable angle grain boundaries and disclinations within the magnetic stripe domain patterns. This process is mainly driven by the competition between rotatable anisotropy (that couples the magnetic stripe pattern to in-plane magnetization) and in-plane shape anisotropy induced by the periodic thickness modulation. However, as the structural period w becomes comparable to magnetic stripe period λ, the nucleation of topological defects at the interfaces between thin and thick regions is hindered by a size effect and stripe domains in the different thickness regions become strongly coupled. © 2013 American Physical Society. ; Work was supported by Spanish MICINN under Grant No. FIS2008-06249. R.M. and N.S. acknowledge support from UPV/EHU UFI11/23 and Basque Country Government Grant No. Etorek SE11-304. A.H.-R. acknowledges support from FCT of Portugal grant (Grant No. SFRH/BPD/90471/2012). ; Peer Reviewed

This article belongs to the Special Issue Recent Advances in Nanomagnetism. ; Ferromagnetic resonance is a powerful method for the study of all classes of magnetic materials. The experimental technique has been used for many decades and is based on the excitation of a magnetic spin system via a microwave (or rf) field. While earlier methods were based on the use of a microwave spectrometer, more recent developments have seen the widespread use of the vector network analyzer (VNA), which provides a more versatile measurement system at almost comparable sensitivity. While the former is based on a fixed frequency of excitation, the VNA enables frequency-dependent measurements, allowing more in-depth analysis. We have applied this technique to the study of nanostructured thin films or nanodots and coupled magnetic layer systems comprised of exchange-coupled ferromagnetic layers with in-plane and perpendicular magnetic anisotropies. In the first system, we have investigated the magnetization dynamics in Co/Ag bilayers and nanodots. In the second system, we have studied Permalloy (Ni80Fe20, hereafter Py) thin films coupled via an intervening Al layer of varying thickness to a NdCo film which has perpendicular magnetic anisotropy. ; D.S.S., D.M. and L.M.Á.-P. acknowledge financial support from the Institut de Physique of CNRS for experimental equipment, a post doc position, and guest researcher stays, respectively. A.H.-R. acknowledges European Union's Horizon 2020 framework program for research and innovation under the Marie Skłodowska-Curie Action No. H2020-MSCA-IF-2016-746958. A.H.-R., C.Q., J.D. and L.M.Á.-P. would like to thank the Spanish Ministerio de Ciencia e Innovación (MCI) for financial support under Project PID2019-104604RB/AEI/10.13039/501100011033. ; Peer reviewed

Resumen del trabajo presentado al American Physical Society March Meeting, celebrado on-line del 15 al 19 de marzo de 2021. ; Work supported by Spanish MINECO (Grants FIS2016-76058 (AEI/FEDER, EU) and PID2019-104604RB/AEI/10.13039/501100011033) and European Union's Horizon 2020 Marie Sklodowska-Curie Grant ref. H2020-MSCA-IF-2016-746958. ; Peer reviewed

The knowledge of how magnetization looks inside a ferromagnet is often hindered by the limitations of the available experimental methods which are sensitive only to the surface regions or limited in spatial resolution. Here we report a vector tomographic reconstruction based on soft X-ray transmission microscopy and magnetic dichroism data, which has allowed visualizing the three-dimensional magnetization in a ferromagnetic thin film heterostructure. Different non-trivial topological textures have been resolved and the determination of their topological charge has allowed us to identify a Bloch point and a meron-like texture. Our method relies only on experimental data and might be of wide application and interest in 3D nanomagnetism. ; Alba light source is funded by the Ministry of Research and Innovation of Spain and by the Generalitat de Catalunya and by European FEDER funds. This project has been supported by Spanish MINECO under grant FIS2016-76058 (AEI/FEDER, EU) and grant PID2019-104604RB/AEI/10.13039/501100011033. A.H.-R. and S.MV. acknowledge the support from European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie grant ref. H2020-MSCA-IF-2016-746958. ; Peer reviewed

The magnetization reversal of each individual layer in magnetic trilayers (permalloy/NdCo/GdCo) is investigated in detail with x-ray microscopy and micromagnetic calculations. Two sequential inversion mechanisms are identified. First, magnetic vortex-antivortex pairs move along the field direction while inverting the magnetization of magnetic stripes until they are pinned by defects. The vortex-antivortex displacements are reversible within a field interval which allows their controlled motion. Second, as the reversed magnetic field increases, cycloidal domains appear in the permalloy layer as a consequence of the dissociation of vortex-antivortex pairs due to pinning. The field range where magnetic vortices and antivortices are effectively guided by the stripe pattern is of the order of tens of mT for the NiFe layer, as estimated from the stability of cycloid domains in the sample. ; Work supported by Spanish MINECO [Grants No. FIS 2013-45469 and No. FIS2016-76058 (AEI/FEDER,EU)] and by FICYT-Asturias (Grant No. FC-GRUPIN14-040). We thank J. Avila (Alba staff) for assistance in the pulsed magnetic field set up. We thank Unidad de Medidas magnéticas y RMN de Sólidos de los Servicios Cientifico Técnicos of Universidad de Oviedo for the VSM characterization. A.H.-R. acknowledges the support from European Union's Horizon 2020 research and innovation programme under Marie Skłodowska-Curie Grant No. H2020-MSCAIF-2016-746958. ; Peer Reviewed

This article belongs to the Special Issue 2D Materials and Their Heterostructures and Superlattices. ; 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. ; P.A.-M. acknowledges financial support through JAE Intro program from the Superior Council of Scientific Investigations and the Spanish Ministry of Science and Innovation (grant number JAEINT_20_00589). G.Á.-P. and J.T.-G. acknowledge financial support through the Severo Ochoa Program from the Government of the Principality of Asturias (grant numbers PA-20-PF-BP19-053 and PA-18-PF-BP17-126, respectively). J.M.-S. acknowledges financial support from the Ramón y Cajal Program of the Government of Spain (RYC2018-026196-I) and the Spanish Ministry of Science and Innovation (State Plan for Scientific and Technical Research and Innovation grant number PID2019-110308GA-I00). P.A.-G. acknowledges support from the European Research Council under starting grant no. 715496, 2DNANOPTICA and the Spanish Ministry of Science and Innovation (State Plan for Scientific and Technical Research and Innovation grant number PID2019-111156GB-I00). ; Peer reviewed

The development of magnetic nanostructures for applications in spintronics requires methods capable of visualizing their magnetization. Soft X-ray magnetic imaging combined with circular magnetic dichroism allows nanostructures up to 100–300 nm in thickness to be probed with resolutions of 20–40 nm. Here a new iterative tomographic reconstruction method to extract the three-dimensional magnetization configuration from tomographic projections is presented. The vector field is reconstructed by using a modified algebraic reconstruction approach based on solving a set of linear equations in an iterative manner. The application of this method is illustrated with two examples (magnetic nano-disc and micro-square heterostructure) along with comparison of error in reconstructions, and convergence of the algorithm. ; The following funding is acknowledged: Spanish MINECO (grant No. FIS2013-45469; grant No. FIS2016-76058 (AEI/FEDER, EU); contract No. FIS2016-76058 (AEI/FEDER, EU) to AHR); FICYT-Asturias (grant No. FC-GRUPIN14-040); US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory (contract No. DE-AC02-06CH11357 to DG); US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division (contract to CP). AHR acknowledges support from European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Action (reference H2020-MSCA-IF-2016-746958). ; Peer Reviewed

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. ; The financial support of the Spanish MINECO MAT2014-53921-R, FIS2013-45469 and FIS2016-76058 (AEI) and the Aragonese DGA IMANAE34 and CAMRADSE69, all of them cofunded by Fondo Social Europeo and European Union FEDER funds, is acknowledged. A. H-R acknowledges the support from FCT of Portugal (Grant SFRH/BPD/90471/2012). ; Peer Reviewed