Radio-frequency magnetic response of vortex lattices undergoing structural transformations in superconducting borocarbide crystals

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 χ with a structural transition in the vortex lattice. χ is sensitive to the Campbell penetration depth λ_C of the rf perturbation into the mixed state. λ_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.