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Future perspectives in Spintronics and 3D Nanomagnetism are related to the exploration and comprehension of topologically protected magnetization configurations which can be key for applications as well as bring novel interesting phenomena. However, these topologically non-trivial textures are fully three-dimensional in nature, which requires for their study adequate experimental methods. In this framework, X-ray tomography-based approaches with high lateral resolution and large penetration depth are an excellent choice. Here we have experimentally demonstrated the capabilities of magnetic soft X-ray transmission tomography by reconstructing the complex magnetization of a Ni80Fe20/NdCo5/Ni80Fe20 heterostructure. The system presents weak perpendicular magnetic anisotropy allowing for the formation of stripe domains and supporting magnetic non-trivial topological configurations. After reconstruction, a Bloch point and a Meron-like texture have been identified, and the experimental volumeresolved topological charge map has been computed allowing to get further insight on the formed singularities. The results show the potential of the technique as a unique tool for experimental magnetic 3D characterization of arbitrary systems and heterostructures which could be of great interest for Spintronics, 3D Nanomagnetism and the broader magnetism community.

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.

The development of multilayered materials with engineered magnetic properties compels a deep knowledge of physical properties at the atomic scale. The magnetic anisotropy is a key property in these materials. This work accounts for the magnetic anisotropy energy and its correlation with atomic properties of Co/Pd multilayers with the number of Co/Pd repetitions. Magnetometry measurements confirm stronger perpendicular magnetic anisotropy energies as the number of repetitions increases up to 40. However, the intrinsic anisotropy, related to the Co–Pd orbital hybridization and spin–orbit coupling, saturates at 15 repetitions. This finding is supported by x-ray magnetic circular dichroism analysis that reveals a direct correlation of the atomic Co and Pd orbital magnetic moments and the effective anisotropy of the system. The proximity effect that accounts for the Pd induced magnetization, along with the increasing Co moment, provides a suitable mechanism for the observed anisotropy energy layer dependence.

Magnetization reversal in multilayers based in weak perpendicular magnetic anisotropy (wPMA) materials is controlled by the nucleation and propagation of 3D magnetic textures such as Bloch points, merons, vortices and antivortices. In these samples the basic domain structure is made of parallel stripes with alternating up/down magnetization in the central layer surrounded by in-plane magnetized closure domains at top/bottom surfaces. Magnetic transmission X-ray microscopy has allowed us to resolve the details of the magnetization reversal process at different sample depths in GdCo/NdCo/Permalloy trilayers measuring dichroic images at the different element absorption edges. Defects in the stripe pattern (bifurcations and end-points) are the preferred locations for the nucleation of merons (i.e. half skyrmions) at the first stages of in-plane magnetization reversal. This process is controlled by a set of well defined topological rules, related with the symmetry breaking between top/bottom surfaces by the sense of stray field circulation. Then, magnetization reversal in each layer proceeds by the propagation of magnetic vortices, antivortices and Bloch points guided by the parallel stripe pattern, creating various magnetization configurations such as the cycloidal domains in Permalloy shown in Figs. 1 and 2. Preliminary results of the characterization of these 3D textures across the sample thickness will also be presented using our recently developed tomographic method.

Co/Pd multilayers can exhibit perpendicular magnetic anisotropy (PMA) for ultrathin thicknesses of Co and Pd layers. The PMA magnitude depends on several parameters, as the thickness of each constituent layer, the quality of the Co-Pd interface, the crystallinity of the film, or the total number of layers. In this work we present the PMA dependence with the number of Co/Pd repetitions. Such dependence was investigated by magnetometry and synchrotron light techniques. Magnetization curves obtained by SQUID magnetometer provide macroscopic parameters, as the interfacial and effective anisotropy of the multilayers, or the net magnetization of the sample. In addition, x-ray magnetic circular dichroism (XMCD) reveal microscopic details, as the anisotropy of the atomic angular momentum, the magnitude of the Co spin moment, or the hybridization of Co-Pd orbitals at the interface. Both kinds of results are combined to show a clear picture of the PMA evolution with the number of repetitions. A correlation between macroscopic and microscopic parameters is established in the whole range of thicknesses, from 2 to 40 Co/Pd repetitions.

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.

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.

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.

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.

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.

Amorphous Nd-Co thin films exhibit stripe shaped periodic magnetic domains with local out-of-plane magnetization components due to their perpendicular magnetic anisotropy. This anisotropy has been quantified in a fairly simple way by reproducing the experimental magnetization curves by means of micromagnetic numerical simulations. The simulations show that the first (K1) and second (K2) anisotropy constants must be used to properly describe the variation of the stripe domains with the in plane applied magnetic field. A strong temperature dependence of both K1 and K2 has been obtained between 10 K and room temperature. This anisotropy behavior is characteristic of two magnetically coupled 3d-4f sublattices with competing anisotropies.