We report a clear anomaly in the rf response of tunnel-diode resonator measured in rare earth nickel borocarbide crystals with dc applied magnetic field along the crystallographic c axis. We associate the anomaly in the dynamic magnetic susceptibility chi with a structural transition in the vortex lattice. chi is sensitive to the Campbell penetration depth lambda(C) of the rf perturbation into the mixed state. lambda(C) depends on the vortex lattice elastic moduli, which in turn depend on the vortex lattice structure. Using this technique, we map the high-field transition line for the square-to-rhombic transformation of the vortex lattice and provide a qualitative argument in favor of thermal vortex fluctuations as a major cause driving the transition.
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