  We report a direct observation of the magnetic vortex lattice in the model
high-temperature superconductor HgBa2CuO4+x. Using small-angle neutron
scattering on high-quality crystals, we observe two equal domains of
undistorted triangular vortex lattices well-aligned with the tetragonal
crystallographic axes. The signal decreases rapidly with increasing magnetic
field and vanishes above 0.4 Tesla, which we attribute to a crossover from a
three-dimensional to a two-dimensional vortex system, similar to previous
results for the more anisotropic compound Bi2.15Sr1.95CaCu2O8+x. Our result
indicates that a triangular vortex lattice at low magnetic fields is a generic
property to cuprates with critical temperatures above 80 K.
