Vortex circulation patterns in planar microdisk arrays

We report a magnetic X-ray microscopy study of the pattern formation of circulation in arrays of magnetic vortices ordered in a hexagonal and a honeycomb lattice. In the honeycomb lattice, we observe at remanence an ordered phase of alternating circulations, whereas in the hexagonal lattice, small regions of alternating lines form. A variation in the edge-to-edge distance shows that the size of those regions scales with the magnetostatic interaction. Micromagnetic simulations reveal that the patterns result from the formation of flux closure states during the nucleation process

Collective nuclear excitation dynamics in mono-modal x-ray waveguides

Ensembles of identical atoms exhibit peculiar collective properties in their interaction with radiation depending on geometry and environment where they are embedded in. A remarkably clean and versatile platform to study collective effects in resonant light scattering are M\"ossbauer nuclei placed in planar x-ray waveguides. Here we conceive and demonstrate experimentally distinct temporal emission characteristics in these systems, ranging from a tunable accelerated exponential decay all the way to a pronounced oscillatory emission pattern, depending on the waveguide geometry and mode of excitation. The observed temporal and spatial emission characteristics of the collectively excited nuclear state in the waveguide -- the nuclear exciton -- are well reproduced by a unified theoretical model. Our findings pave the way for applications ranging from fundamental studies of cooperative emission at hard x-ray frequencies up to new methods of narrowband x-ray control via the engineering of collective radiation patterns

Phonon engineering in thin tin films for tuning photon-nuclei interaction

When materials are fabricated at the nanoscale, properties such as thermodynamic quantities can drastically deviate from bulk properties. Thereby, the embedding environment has a strong impact on the behavior of the nanostructured material and thus provides a handle to engineer the properties as desired. In this work, the vibrational behavior of embedded thin tin films is probed and engineered in order to tune the photon-nuclei interaction in nuclear resonance scattering.The fabrication of tin films is typically affected by tin clustering. Here, they are prepared by combining magnetron sputter deposition with vacuum quenching resulting in $\beta$-Sn films with roughnesses of below one nanometer. By employing nuclear resonance scattering at the 23.88 keV resonance of $^{119}$Sn, not only a deviating tin hyperfine structure is found at the interface to the embedding material, but also indications for a strong distortion of thermodynamic properties in the interface region. Thus, nuclear inelastic scattering is performed to access the phonon density of states of the tin films. A shift of the phonon modes to higher energy is measured reflecting an increase of the rigidness of the tin atoms. This results in an enhancement by up to a factor of eight of the Lamb-Mössbauer factor, a quantity describing the probability of an elastic nuclear resonant scattering process. In addition to tin films, also embedded thin films of stannic oxide are investigated for nuclear quantum optics. Embedded in magnesium oxide, the stannic oxide films exhibit a nearly ideal two-level quantum system independent of the layer thickness. Collective quantum optics effects such as the collective Lamb shift and superradiance are measured as a function of the tin dioxide layer thickness. The obtained results pave the way for applying thin tin films to probe spin structures in paramagnetic materials and to extend nuclear quantum optics to the resonance energy of $^{119}$Sn by employing stannic oxide films with their nearly unperturbed hyperfine structure

Network properties of mixtures of protonated and deuterated polyethylene

The original version of this article contained an error in the legend to Figure 4. The yellow scale bar should have been defined as '~600 nm', not '~600 µm'. This has now been corrected in both the PDF and HTML versions of the article

Two-body problem of core-region coupled magnetic vortex stacks

The dynamics of all four combinations of possible polarity and circularity states in a stack of two vorticesis investigated by time-resolved scanning transmission x-ray microscopy. The vortex stacks are excited byunidirectional magnetic fields leading to a collective oscillation. Four different modes are observed that dependon the relative polarizations and circularities of the stacks. They are excited to a driven oscillation. We observe arepulsive and attractive interaction of the vortex cores depending on their relative polarizations. The nonlinearityof this core interaction results in different trajectories that describe a two-body problem

Destructive interferences of THz emission by spin emitters

Artificial antiferromagnets have a long tradition in the study of spin transport in magnetic materials. Here we use multilayer spin emitters prepared by oblique incidence deposition (OID) [1] that sets a uniaxial anisotropy in the magnetic materials such that the magnetic static interlayer coupling is not needed to stabilize the antiferromagnetic state. The ferromagnetic and antiferromagnetic states can be prepared without a stabilizing external field. Thus, in single layers OID spin emitters the external magnetic field is obsolete. In multilayers the antiferromagnetic alignment can be set without any magnetic interlayer coupling by tuning the anisotropy in each layer. We study the THz emission in both states and find a strong increase in the antiferromagnetic state

Spin-structured multilayer THz emitters by oblique incidence deposition

State-of-the-art THz spintronic emitters require a constant magnetic field to saturate their magnetization. We demonstrate that depositing the ferromagnetic layers at oblique incidence confines the magnetization to a chosen in-plane easy axis and maintains the saturation in the absence of an external magnetic field. We use this method to build THz emitters structured as spin valves, for which we use an external magnetic field to turn on and off the emission of THz radiation as well as to change its polarization. We are able to reproduce the THz waveforms by modeling the spin current and the THz propagation through the multilayer system

Two-body problem of core-region coupled magnetic vortex stacks

The dynamics of all four combinations of possible polarity and circularity states in a stack of two vorticesis investigated by time-resolved scanning transmission x-ray microscopy. The vortex stacks are excited byunidirectional magnetic fields leading to a collective oscillation. Four different modes are observed that dependon the relative polarizations and circularities of the stacks. They are excited to a driven oscillation. We observe arepulsive and attractive interaction of the vortex cores depending on their relative polarizations. The nonlinearityof this core interaction results in different trajectories that describe a two-body problem