Crystal order is not restricted to the periodic atomic array, but can also be found in electronic systems such as the Wigner crystal or in the form of orbital order, stripe order and magnetic order. In the case of magnetic order, spins align parallel to each other in ferromagnets and antiparallel in antiferromagnets. In other, less conventional, cases, spins can sometimes form highly nontrivial structures called spin textures. Among them is the unusual, topologically stable skyrmion spin texture, in which the spins point in all the directions wrapping a sphere. The skyrmion configuration in a magnetic solid is anticipated to produce unconventional spin-electronic phenomena such as the topological Hall effect. The crystallization of skyrmions as driven by thermal fluctuations has recently been confirmed in a narrow region of the temperature/magnetic field (T-B) phase diagram in neutron scattering studies of the three-dimensional helical magnets MnSi (ref. 17) and Fe(1-x)Co(x)Si (ref. 22). Here we report real-space imaging of a two-dimensional skyrmion lattice in a thin film of Fe(0.5)Co(0.5)Si using Lorentz transmission electron microscopy. With a magnetic field of 50-70 mT applied normal to the film, we observe skyrmions in the form of a hexagonal arrangement of swirling spin textures, with a lattice spacing of 90 nm. The related T-B phase diagram is found to be in good agreement with Monte Carlo simulations. In this two-dimensional case, the skyrmion crystal seems very stable and appears over a wide range of the phase diagram, including near zero temperature. Such a controlled nanometre-scale spin topology in a thin film may be useful in observing unconventional magneto-transport effects.

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Dry etching of GaAs in asymmetric bipolar pulsed dc BCl3 plasmas using only mechanical pumping is reported. The mean pulsed dc voltage on the cathode was used as a process variable in the experiment. When the mean pulsed dc voltage changed from −350 to −550 V at the cathode electrode, the dependent mean current and power were increased from 0.22 to 0.33 A and from 100 to 220 W, respectively. The etch rate of GaAs increased from 0.01 to 0.55 μm/min, and etch selectivity over photoresist was also strongly increased from 0.2:1 to 2.2:1 with this voltage change. The threshold mean voltage for plasma ignition pulsed dc plasma was −350 V. Scanning electron microscopy micrographs showed that dry etching at −500 V provided excellent surface smoothness (rms roughness of ∼1 nm) and sidewall passivation. At a lower mean voltage of −400 V, there was a rough surface and some undercutting of GaAs after etching at −400 V. Degradation of the photoresist surface was negligible in both cases.