Professor Thorsten Hesjedal FInstP

Abstract:

Physical vapour deposition (PVD) is used to grow millimetre-long Bi2 Te3 sub-micron belts catalysed by TiO2 nanoparticles. The catalytic efficiency of TiO2 nanoparticles for the nanostructure growth is compared with the catalyst-free growth employing scanning electron microscopy. The catalyst-coated and catalyst-free substrates are arranged side-by-side, and overgrown at the same time, to assure identical growth conditions in the PVD furnace. It is found that the catalyst enhances the yield of the belts. Very long belts were achieved with a growth rate of 28 nm/min. A ∼1-mm-long belt with a rectangular cross-section was obtained after 8 h of growth. The thickness and width were determined by atomic force microscopy, and their ratio is ∼1:10. The chemical composition was determined to be stoichiometric Bi2Te3 using energy-dispersive X-ray spectroscopy. Temperature-dependent conductivity measurements show a characteristic increase of the conductivity at low temperatures. The room temperature conductivity of 0.20×1^5 S⋅m^−1 indicates an excellent sample quality.

Abstract:

We report the study of the skyrmion state near the surface of Cu₂OSeO₃ using soft resonant elastic x-ray scattering (REXS) at the Cu L₃ edge. Within the lateral sampling area of 200 × 200 µm², we found a long-range-ordered skyrmion lattice phase as well as the formation of skyrmion domains via the multiple splitting of the diffraction spots. In a recent REXS study of the skyrmion phase of Cu₂OSeO₃ [Phys. Rev. Lett. 112, 167202 (2014)], Langner et al. reported a double-splitting which they interpret as arising from the moiré pattern of two superposed skyrmion sublattices, originating from the two inequivalent Cu sites. However, we find no energy splitting of the Cu peak in xray absorption measurements, which is to be expected considering the system in more detail. We show that the experimental data reported by Langner et al. does not support their interpretation and discuss alternative origins of the peak splitting. In particular, we find that for magnetic field directions deviating from the major cubic axes, a multidomain skyrmion lattice state is obtained, which consistently explains the splitting of the magnetic spots into two—and more—peaks.

Abstract:

The extraordinary Hall balance (EHB) is a general device concept that harnesses the net extraordinary Hall effect (EHE) arising from two independent magnetic layers, which are electrically in parallel. Different EHB behavior can be achieved by tuning the strength and type of interlayer coupling, i.e., ferromagnetic or antiferromagnetic of varying strength, allowing for logic and memory applications. The physics of the EHE in such a multilayered systems, especially the interface-induced effect, will be discussed. A discrepancy between the magnetization and the Hall effect, called the magneto-Hall difference (MHD) is found, which is not expected in conventional EHE systems. By taking advantage of the MHD effect, and by optimizing the materials structure, magnetoresistance ratios in excess of 40,000% can be achieved at room-temperature. We present a new design, the planar EHB, which has the potential to achieve significantly larger magnetoresistance ratios.