Evolution of the vortex lattice, the rhombus-square transition included, for LuNi2B2C in the field along the c crystal axis, is tracked by the decoration technique pushed up to the record high (for this method) field of 1480 Oe. Decoration images are analyzed with the help of the Fourier transform, which indicates disordered structures in small fields of a few Oe. In fields H < 200 Oe the coexisting domains of different structures are observed. A technique based on the Fourier transform is employed to see the domains separately. The transition to the square lattice is recorded near 900 Oe. The results are in agreement with predictions of the nonlocal London theory.

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Results of direct imaging of Abrikosov vortices in an organic superconductor by means of the decoration technique are presented. The vortices have been observed in the title compound at temperature of Ӎ0.5T c at the magnetic fields 3.3-9.4 Oe directed perpendicular to the two-dimensional plane of the crystal. The decoration patterns reveal the sixfold coordination short-range order or slightly distorted triangular lattice of the vortices with the magnetic flux per vortex equal to the flux quantum, hc/2e. The slight deviation of the vortex lattice from the hexagonal symmetry is attributed to the in-plane anisotropy of the penetration depth. The average size of the individual vortex images on the sample surface yields the upper limit estimation of the London penetration depth as 0.4 m at the given temperature. Pronounced effects of pinning on the crystal defects and thermally activated mobility of the vortices have been observed.

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