Spatiotemporal magnetization reversal dynamics in a Ni(80)Fe(20) microstructure is studied using ps time scale scanning Kerr microscopy. Time domain images reveal a striking change in the reversal associated with the reduction in switching time when a transverse bias field is applied. Magnetization oscillations subsequent to reversal are observed at two resonance frequencies, which sensitively depend on the bias field strength. The oscillation at f = 2 GHz is caused by the damped precession of M, while the lower frequency approximately 0.8 GHz mode is interpreted in terms of domain wall oscillation.

The mechanical resonance of laterally grown silicon nanowires measured by an optical interferometric technique is reported. The lengths and diameters of the nanowires ranged from L = 2 to 20 m and D = 39 to 400 nm, respectively. The wires showed resonant frequencies in the f 0 =1-12 MHz range and resonant quality factors Q at low pressure ranging from Q = 5000 to Q = 25 000. The dependence of resonant frequency on the ratio of diameter to length squared, D / L 2 , yielded a ratio of ͱ E / = 9400Ϯ 450 m / s. Assuming a density of = 2330 kg/ m 3 , this experimental result yields an experimental Young modulus of E = 205Ϯ 10 GPa, consistent with that of a bulk silicon. As the wires were cooled from T = 270 K to T = 77 K, a 0.35% increase of resonant frequency was observed. This increase of resonant frequency with cooling resulted from a change in Young's modulus and from the thermal contraction of silicon. The quality factor did not vary significantly from P =10 −4 to 10 2 Torr, suggesting that viscous damping does not dominate the dissipative processes in this pressure range. Although viscous damping became important above P =10 2 Torr, relatively high quality factors of Q = 7000 were still observed at atmospheric pressure.

Oligomeric aggregates are widely suspected as toxic agents in diseases caused by protein aggregation, yet they remain poorly characterized, partly because they are challenging to isolate from a heterogeneous mixture of species. We developed an assay for characterizing structure, stability, and kinetics of individual oligomers at high resolution and sensitivity using single-molecule force spectroscopy, and applied it to observe the formation of transient structured aggregates within single oligomers of α-synuclein, an intrinsically-disordered protein linked to Parkinson’s disease. Measurements of the molecular extension as the proteins unfolded under tension in optical tweezers revealed that even small oligomers could form numerous metastable structures, with a surprisingly broad range of sizes. Comparing the structures formed in monomers, dimers and tetramers, we found that the average mechanical stability increased with oligomer size. Most structures formed within a minute, with size-dependent rates. These results provide a new window onto the complex α-synuclein aggregation landscape, characterizing the microscopic structural heterogeneity and kinetics of different pathways.