Jörg Raabe

Magnetization pattern of ferromagnetic nanodisks

We explore the magnetization pattern of Co and permalloy disks with diameters between 80 nm and 1 μm by using two complementary experimental techniques: Lorentz microscopy and magnetic force microscopy (MFM). By means of Lorentz microscopy we show that the dominating magnetization pattern of the disks is a vortex structure with closed flux lines in the plane of the disks. Complementary MFM measurements demonstrate that the magnetization in the center of the disks is tilted out of the plane of the disk. The experimental findings closely agree with corresponding micromagnetic calculations.

High-resolution non-destructive three-dimensional imaging of integrated circuits

Modern nanoelectronics has advanced to a point at which it is impossible to image entire devices and their interconnections non-destructively because of their small feature sizes and the complex three-dimensional structures resulting from their integration on a chip. This metrology gap implies a lack of direct feedback between design and manufacturing processes, and hampers quality control during production, shipment and use. Here we demonstrate that X-ray ptychography—a high-resolution coherent diffractive imaging technique—can create three-dimensional images of integrated circuits of known and unknown designs with a lateral resolution in all directions down to 14.6 nanometres. We obtained detailed device geometries and corresponding elemental maps, and show how the devices are integrated with each other to form the chip. Our experiments represent a major advance in chip inspection and reverse engineering over the traditional destructive electron microscopy and ion milling techniques. Foreseeable developments in X-ray sources, optics and detectors, as well as adoption of an instrument geometry optimized for planar rather than cylindrical samples, could lead to a thousand-fold increase in efficiency, with concomitant reductions in scan times and voxel sizes.

Assessment of the 3 D Pore Structure and Individual Components of Preshaped Catalyst Bodies by X-Ray Imaging

Porosity in catalyst particles is essential because it enables reactants to reach the active sites and it enables products to leave the catalyst. The engineering of composite‐particle catalysts through the tuning of pore‐size distribution and connectivity is hampered by the inability to visualize structure and porosity at critical‐length scales. Herein, it is shown that the combination of phase‐contrast X‐ray microtomography and high‐resolution ptychographic X‐ray tomography allows the visualization and characterization of the interparticle pores at micro‐ and nanometer‐length scales. Furthermore, individual components in preshaped catalyst bodies used in fluid catalytic cracking, one of the most used catalysts, could be visualized and identified. The distribution of pore sizes, as well as enclosed pores, which cannot be probed by traditional methods, such as nitrogen physisorption and isotherm analysis, were determined.

Magnetic vortex cores as tunable spin-wave emitters

The use of spin waves as information carriers in spintronic devices can substantially reduce energy losses by eliminating the ohmic heating associated with electron transport. Yet, the excitation of short-wavelength spin waves in nanoscale magnetic systems remains a significant challenge. Here, we propose a method for their coherent generation in a heterostructure composed of antiferromagnetically coupled magnetic layers. The driven dynamics of naturally formed nanosized stacked pairs of magnetic vortex cores is used to achieve this aim. The resulting spin-wave propagation is directly imaged by time-resolved scanning transmission X-ray microscopy. We show that the dipole-exchange spin waves excited in this system have a linear, non-reciprocal dispersion and that their wavelength can be tuned by changing the driving frequency.

Spatially and time-resolved magnetization dynamics driven by spin–orbit torques

Current-induced spin-orbit torques are one of the most effective ways to manipulate the magnetization in spintronic devices, and hold promise for fast switching applications in non-volatile memory and logic units. Here, we report the direct observation of spin-orbit-torque-driven magnetization dynamics in Pt/Co/AlOx dots during current pulse injection. Time-resolved X-ray images with 25 nm spatial and 100 ps temporal resolution reveal that switching is achieved within the duration of a subnanosecond current pulse by the fast nucleation of an inverted domain at the edge of the dot and propagation of a tilted domain wall across the dot. The nucleation point is deterministic and alternates between the four dot quadrants depending on the sign of the magnetization, current and external field. Our measurements reveal how the magnetic symmetry is broken by the concerted action of the damping-like and field-like spin-orbit torques and the Dzyaloshinskii-Moriya interaction, and show that reproducible switching events can be obtained for over 1012 reversal cycles.

Three-dimensional magnetization structures revealed with X-ray vector nanotomography

In soft ferromagnetic materials, the smoothly varying magnetization leads to the formation of fundamental patterns such as domains, vortices and domain walls. These have been studied extensively in thin films of thicknesses up to around 200 nanometres, in which the magnetization is accessible with current transmission imaging methods that make use of electrons or soft X-rays. In thicker samples, however, in which the magnetization structure varies throughout the thickness and is intrinsically three dimensional, determining the complex magnetic structure directly still represents a challenge. We have developed hard-X-ray vector nanotomography with which to determine the three-dimensional magnetic configuration at the nanoscale within micrometre-sized samples. We imaged the structure of the magnetization within a soft magnetic pillar of diameter 5 micrometres with a spatial resolution of 100 nanometres and, within the bulk, observed a complex magnetic configuration that consists of vortices and antivortices that form cross-tie walls and vortex walls along intersecting planes. At the intersections of these structures, magnetic singularities—Bloch points—occur. These were predicted more than fifty years ago but have so far not been directly observed. Here we image the three-dimensional magnetic structure in the vicinity of the Bloch points, which until now has been accessible only through micromagnetic simulations, and identify two possible magnetization configurations: a circulating magnetization structure and a twisted state that appears to correspond to an ‘anti-Bloch point’. Our imaging method enables the nanoscale study of topological magnetic structures in systems with sizes of the order of tens of micrometres. Knowledge of internal nanomagnetic textures is critical for understanding macroscopic magnetic properties and for designing bulk magnets for technological applications.

Electrodeposition of NiFe and Fe nanopillars

Electron beam lithography combined with electrodeposition was used to fabricate arrays of NiFe and Fe nanomagnets with high aspect ratios (height/diameter). MFM-measurements show the single domain behavior of the NiFe-pillars, which have an easy axis parallel to their long axis. AGM-measurements carried out on large NiFe-arrays clearly show the increasing importance of interaction effects with decreasing periodicity. On top of overgrown Fe-pillars a nanocrystalline structure can be observed.

Mass Density and Water Content of Saturated Never-Dried Calcium Silicate Hydrates

Calcium silicate hydrates (C-S-H) are the most abundant hydration products in ordinary Portland cement paste. Yet, despite the critical role they play in determining mechanical and transport properties, there is still a debate about their density and exact composition. Here, the site-specific mass density and composition of C-S-H in hydrated cement paste are determined with nanoscale resolution in a nondestructive approach. We used ptychographic X-ray computed tomography in order to determine spatially resolved mass density and water content of the C-S-H within the microstructure of the cement paste. Our findings indicate that the C-S-H at the border of hydrated alite particles possibly have a higher density than the apparent inner-product C-S-H, which is contrary to the common expectations from previous works on hydrated cement paste.

Nanoscale switch for vortex polarization mediated by Bloch core formation in magnetic hybrid systems

Vortices are fundamental magnetic topological structures characterized by a curling magnetization around a highly stable nanometric core. The control of the polarization of this core and its gyration is key to the utilization of vortices in technological applications. So far polarization control has been achieved in single-material structures using magnetic fields, spin-polarized currents or spin waves. Here we demonstrate local control of the vortex core orientation in hybrid structures where the vortex in an in-plane Permalloy film coexists with out-of-plane maze domains in a Co/Pd multilayer. The vortex core reverses its polarization on crossing a maze domain boundary. This reversal is mediated by a pair of magnetic singularities, known as Bloch points, and leads to the transient formation of a three-dimensional magnetization structure: a Bloch core. The interaction between vortex and domain wall thus acts as a nanoscale switch for the vortex core polarization.

Luminescence-based magnetic imaging with scanning x-ray transmission microscopy

We demonstrate the imaging of the magnetic domain configuration of cobalt structures fabricated on MgO(001) using x-ray induced optical luminescence in a scanning transmission microscope. The technique relies on the measurement of the magnetization-dependent x-ray absorption probed by the optical luminescence radiated from the MgO substrate and induced by the x-rays transmitted through the magnetic layer. This method enables the measurement of the electronic and magnetic spectroscopic properties of single crystalline layers and buried heterostructures with nanometer lateral resolution and elemental sensitivity and opens scanning transmission x-ray microscopy to materials which cannot be grown on membranes or as freestanding thin films.