Sergio A. Montoya

We demonstrate an experimental approach to determine magneto-optical effects that combines x-ray magnetic circular dichroism (XMCD) with x-ray interferometry, based on the concepts of Young's canonical double slit. By covering one of two slits with a magnetic thin film and employing XMCD, we show that it is possible to determine both the real and the imaginary parts of the complex refractive index by measuring the fringe shifts that occur due to a change in the sample magnetization. Our hybrid spectroscopic-interferometric methodology provides a means to probe changes in the magnetic refractive index in terms of the electron spin moment.

The Fe/Gd multilayer system hosts a number of magnetic phases, such as stripe, mixed stripe and skyrmion, skyrmion lattice, and isolated skyrmions for a wide range of temperature and magnetic field. We report different Hall transport signals in a Fe/Gd system through multimodal correlative resonant soft x-ray scattering (RSXS), Hall effect, magneto-optic Kerr effect, and transmission x-ray microscopy measurements. The simultaneous nature of the RSXS and Hall transport measurements allowed us to accurately connect various features in the transport data with the specific magnetic phases. We found that the topological Hall effect (THE) shows peaks with opposite signs, which we attribute to two different mechanisms. Our multimodal correlative study indicates that the sign reversal in THE occurs when the system transforms to and from a skyrmion lattice and low density isolated skyrmion phases. We propose that the skyrmion lattice contributes to the THE through a Berry phase induced emergent magnetic field mechanism in one case, and a skew scattering mechanism corresponding to the isolated low density skyrmion state.

Magnetic skyrmions exhibit unique, technologically relevant pseudo‐particle behaviors which arise from their topological protection, including well‐defined, 3D dynamic modes that occur at microwave frequencies. During dynamic excitation, spin waves are ejected into the interstitial regions between skyrmions, creating the magnetic equivalent of a turbulent sea. However, since the spin waves in these systems have a well‐defined length scale, and the skyrmions are on an ordered lattice, ordered structures from spin‐wave interference can precipitate from the chaos. This work uses small‐angle neutron scattering (SANS) to capture the dynamics in hybrid skyrmions and investigate the spin‐wave structure. Performing simultaneous ferromagnetic resonance and SANS, the diffraction pattern shows a large increase in low‐angle scattering intensity, which is present only in the resonance condition. This scattering pattern is best fit using a mass fractal model, which suggests the spin waves form a long‐range fractal network. The fractal structure is constructed of fundamental units with a size that encodes the spin‐wave emissions and are constrained by the skyrmion lattice. These results offer critical insights into the nanoscale dynamics of skyrmions, identify a new dynamic spin‐wave fractal structure, and demonstrate SANS as a unique tool to probe high‐speed dynamics.

Chiral magnets are known to possess interesting electromagnetic properties that result from the coupling of electrons with nontrivial magnetic phases, such as particle-like magnetic spin textures termed skyrmions. So far, it is unclear how the local and global chirality of magnetic spin textures contributes to the electromagnetic transport responses that have so far been observed. In this work, we focus on unraveling the contributions in the field-dependent longitudinal resistivity response that arises from magnetic spin textures in a centrosymmetric Fe/Gd multilayer that exhibits an array of magnetic phases ranging from stripe, mixed stripe-skyrmion, skyrmion lattice, and disordered skyrmion. Using a combination of transport measurements and micromagnetic simulations, we demonstrate a domain wall chirality reconfiguration occurs as the domain morphology transitions from disordered stripe to skyrmion lattice phase under applied fields that is responsible for the interesting transport responses noted in the field-dependent longitudinal resistivity.

We demonstrate light-emitting hyperbolic metasurfaces in the 1200-1600nm spectral range. The multilayer configuration, ideal for planar integration, enables characterization of hyperbolic dispersion by polarization anisotropy of photoluminescence.