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.

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.

Asymmetric grazing incidence small angle x-ray scattering and anisotropic domain wall motion in obliquely grown nanocrystalline Co films.

Strong asymmetries have been observed in grazing incidence small angle x-ray scattering (GISAXS) in situ patterns obtained from 30 nm-thick nanocrystalline Co films prepared by oblique sputtering (15°-75° off-sample normal). These asymmetries have been qualitatively simulated by a simple model consisting of an ensemble of 8 nm-wide inclined Co nanocolumns. It is found that narrow inclined features appear in the diffuse background resembling those characteristic of faceted systems, which can be used to obtain straightforward non-destructive estimations of buried nanocolumnar grains inclination, even for oblique angles below 45°, when the stronger and broader asymmetric features of the pattern are not yet fully formed. Furthermore, using magneto-optical microscopy, a marked change in the magnetic domain's nucleation and growth process has been observed in the sample prepared at 75°, with the stronger GISAXS asymmetries. Easy axis magnetization reversal starts by a random and homogeneous nucleation of small (∼µm) elongated domains aligned with the nanocolumn's long axis and proceeds through the preferred propagation of head-to-head domain walls (DWs) along the applied field direction. This peculiar magnetic behavior indicates that the strongly anisotropic nanostructuring created by the oblique growth process is equivalent, from a magnetic point of view, to an array of self-assembled buried nanowires. These results show how GISAXS and magneto-optical microscopy can be combined as a powerful tool for correlating the morphology and magnetism of thin nanostructured systems.

Large negative thermal expansion of the Co subnetwork measured by EXAFS in highly disordered Nd1-xCox thin films with perpendicular magnetic anisotropy.

We have measured a negative thermal expansion (NTE) of the Co subnetwork in amorphous Nd1-xCox (0.78 < x < 0.84) thin films of the order of 1% in volume using linearly polarized EXAFS spectroscopy at RT and 10 K. The expansion, which is anisotropic, is uncorrelated with the perpendicular magnetic anisotropy (PMA) observed in all the films, but correlated with the method used to deposit them. The atomic environments of the Nd atoms resulted in such a strong disorder that Nd-Nd and Nd-Co environments were invisible to EXAFS, and only Co-Co atomic environments were detected. The information on the Nd subnetwork was obtained through its magnetic moment measured by XMCD. These measurements demonstrate an increasing interaction of neodymium atoms with their particular local crystal field as the temperature decreased, suggesting possible structural modifications at their sites. Since the magnetic moment of the cobalt subnetwork remains essentially constant with the temperature, it is proposed that its detected NTE may be caused by the mechanical response of the amorphous network to structural transformations at the Nd sites. These results support that the PMA in RE-TM alloys is localized at the RE sites. The complete absence of EXAFS oscillations in the Nd L3 EXAFS spectra is remarkable: it means that the coherence length of the photoemitted electrons in disordered matter can be strongly reduced from that expected by atomic calculations to the point of being less than first neighbor distances, which is contrary to the common belief that first neighbors are always visible by EXAFS.

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.

Role of the antiferromagnetic bulk spin structure on exchange bias.

The cooling field dependence of the exchange bias field in ferromagnet/antiferromagnet (FM/AF) multilayers demonstrates that the bulk AF spin structure plays a crucial role on the origin of exchange bias. FM/AF/FM trilayers were designed to eliminate any interlayer exchange coupling between the FM slabs. By choosing the magnetic cooling field, the AF is ordered below its Néel temperature with the FM layers fully saturated either parallel or antiparallel to each other. The significant difference in the exchange bias field between these two cooling configurations confirms that exchange bias cannot be a purely interfacial effect and that the bulk AF moments play a significant role in pinning the uncompensated spins at the AF/FM interface. This experiment also demonstrates that the mechanism responsible for coercivity enhancement has a different origin and is independent of the process that gives rise to exchange bias.

Enhancement of antiferromagnetic coupling in magnetic multilayers by low energy ion beam substrate nanopatterning.

Ion beam irradiation has been shown to be an interesting tool for tailoring the magnetic properties of thin films and multilayers. The modified properties include magnetic anisotropy, interlayer exchange coupling, exchange bias, magnetic domain structure and magnetization reversal. In this work, new results are shown concerning the enhancement, by one order of magnitude, of the antiferromagnetic coupling strength in amorphous CoSi/Si multilayers by irradiating Si(100) substrates with 1 keV Ar(+) ions. The ion beam exposure induces an increase of the substrate roughness, from 0.07 to 0.88 nm, which enhances antiferromagnetic coupling in the magnetic multilayers grown on top. One possible mechanism governing this enhancement is discussed, related to the formation of magnetic/non-magnetic regions where dipolar interactions could stabilize the antiferromagnetic alignment. The presence of non-magnetic regions is suggested by the observed trend to superparamagnetism, and is expected since the Curie temperature of the amorphous CoSi alloy used is slightly above but very close to room temperature. Accordingly, small fluctuations in the local composition, leading to an enrichment of Si, would produce non-magnetic regions enabling dipolar interactions to take place. Furthermore, the ion beam induced increase of roughness makes surface diffusion of the atoms arriving at the sample difficult, favoring the formation of local non-magnetic inhomogeneities. Finally, the role of other possible mechanisms to enhance antiferromagnetic coupling is also briefly discussed.

Crossed-ratchet effects for magnetic domain wall motion.

We study both experimentally and theoretically the driven motion of domain walls in extended amorphous magnetic films patterned with a periodic array of asymmetric holes. We find two crossed-ratchet effects of opposite sign that change the preferred sense for domain wall propagation, depending on whether a flat or a kinked wall is moving. By solving numerically a simple phi(4) model we show that the essential physical ingredients for this effect are quite generic and could be realized in other experimental systems involving elastic interfaces moving in multidimensional ratchet potentials.