Determination of optimal experimental conditions for accurate 3D reconstruction of the magnetization vector via XMCD-PEEM.

This work presents a detailed analysis of the performance of X-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) as a tool for vector reconstruction of magnetization. For this, 360° domain wall ring structures which form in a synthetic antiferromagnet are chosen as the model to conduct the quantitative analysis. An assessment is made of how the quality of the results is affected depending on the number of projections that are involved in the reconstruction process, as well as their angular distribution. For this a self-consistent error metric is developed which allows an estimation of the optimum azimuthal rotation angular range and number of projections. This work thus proposes XMCD-PEEM as a powerful tool for vector imaging of complex 3D magnetic structures.

A fast magnetic vector characterization method for quasi two-dimensional systems and heterostructures.

The use of magnetic vector tomography/laminography has opened a 3D experimental window to access the magnetization at the nanoscale. These methods exploit the dependence of the magnetic contrast in transmission to recover its 3D configuration. However, hundreds of different angular projections are required leading to large measurement times. Here we present a fast method to dramatically reduce the experiment time specific for quasi two-dimensional magnetic systems. The algorithm uses the Beer-Lambert equation in the framework of X-ray transmission microscopy to obtain the 3D magnetic configuration of the sample. It has been demonstrated in permalloy microstructures, reconstructing the magnetization vector field with a reduced number of angular projections obtaining quantitative results. The throughput of the methodology is × 10-× 100 times faster than conventional magnetic vector tomography, making this characterization method of general interest for the community.

Controlled evolution of three-dimensional magnetic states in strongly coupled cylindrical nanowire pairs.

Cylindrical magnetic nanowires are promising systems for the development of three-dimensional spintronic devices. Here, we simulate the evolution of magnetic states during fabrication of strongly-coupled cylindrical nanowires with varying degrees of overlap. By varying the separation between wires, the relative strength of exchange and magnetostatic coupling can be tuned. Hence, we observe the formation of six fundamental states as a function of both inter-wire separation and wire height. In particular, two complex three-dimensional magnetic states, a 3D Landau Pattern and a Helical domain wall, are observed to emerge for intermediate overlap. These two emergent states show complex spin configurations, including a modulated domain wall with both Néel and Bloch character. The competition of magnetic interactions and the parallel growth scheme we follow (growing both wires at the same time) favours the formation of these anti-parallel metastable states. This works shows how the engineering of strongly coupled 3D nanostructures with competing interactions can be used to create complex spin textures.

Revealing 3D magnetization of thin films with soft X-ray tomography: magnetic singularities and topological charges.

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.

Hydrogen sensing via anomalous optical absorption of palladium-based metamaterials.

A palladium (Pd)-based optical metamaterial has been designed, fabricated and characterized for its application in hydrogen sensing. The metamaterial can replace Pd thin films in optical transmission schemes for sensing with performances far superior to those of conventional sensors. This artificial material consists of a palladium-alumina metamaterial fabricated using inexpensive and industrial-friendly bottom-up techniques. During the exposure to hydrogen, the system exhibits anomalous optical absorption when compared to the well-known response of Pd thin films, this phenomenon being the key factor for the sensor sensitivity. The exposure to hydrogen produces a large variation in the light transmission through the metamembrane (more than 30% with 4% in volume hydrogen-nitrogen gas mixture at room temperature and atmospheric pressure), thus avoiding the need for sophisticated optical detection systems. An optical homogenization model is proposed to explain the metamaterial response. These results contribute to the development of reliable and low-cost hydrogen sensors with potential applications in the hydrogen economy and industrial processes to name a few, and also open the door to optically study the hydrogen diffusion processes in Pd nanostructures.

Nanoscale imaging of buried topological defects with quantitative X-ray magnetic microscopy.

Advances in nanoscale magnetism increasingly require characterization tools providing detailed descriptions of magnetic configurations. Magnetic transmission X-ray microscopy produces element specific magnetic domain images with nanometric lateral resolution in films up to ∼100 nm thick. Here we present an imaging method using the angular dependence of magnetic contrast in a series of high resolution transmission X-ray microscopy images to obtain quantitative descriptions of the magnetization (canting angles relative to surface normal and sense). This method is applied to 55-120 nm thick ferromagnetic NdCo5 layers (canting angles between 65° and 22°), and to a NdCo5 film covered with permalloy. Interestingly, permalloy induces a 43° rotation of Co magnetization towards surface normal. Our method allows identifying complex topological defects (merons or ½ skyrmions) in a NdCo5 film that are only partially replicated by the permalloy overlayer. These results open possibilities for the characterization of deeply buried magnetic topological defects, nanostructures and devices.

Unravelling the tunable exchange bias-like effect in magnetostatically-coupled two dimensional hybrid (hard/soft) composites.

Hybrid 2D hard-soft composites have been fabricated by combining soft (Co73Si27) and hard (NdCo5) magnetic materials with in-plane and out-of-plane magnetic anisotropies, respectively. They have been microstructured in a square lattice of CoSi anti-dots with NdCo dots within the holes. The magnetic properties of the dots allow us to introduce a magnetostatic stray field that can be controlled in direction and sense by their last saturating magnetic field. The magnetostatic interactions between dot and anti-dot layers induce a completely tunable exchange bias-like shift in the system's hysteresis loops. Two different regimes for this shift are present depending on the lattice parameter of the microstructures. For large parameters, dipolar magnetostatic decay is observed, while for the smaller one, the interaction between the adjacent anti-dot's characteristic closure domain structures enhances the exchange bias-like effect as clarified by micromagnetic simulations.

Magnetic behavior of high density arrays of Co bars with strong magnetostatic coupling.

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.

Topological defects and misfit strain in magnetic stripe domains of lateral multilayers with perpendicular magnetic anisotropy.

Stripe domains are studied in perpendicular magnetic anisotropy films nanostructured with a periodic thickness modulation that induces the lateral modulation of both stripe periods and in-plane magnetization. The resulting system is the 2D equivalent of a strained superlattice with properties controlled by interfacial misfit strain within the magnetic stripe structure and shape anisotropy. This allows us to observe, experimentally for the first time, the continuous structural transformation of a grain boundary in this 2D magnetic crystal in the whole angular range. The magnetization reversal process can be tailored through the effect of misfit strain due to the coupling between disclinations in the magnetic stripe pattern and domain walls in the in-plane magnetization configuration.