First-Order Transition between Weak and Strong Pinning in Clean Superconductors with Enhanced Spin Susceptibility

Dilatometric and magnetic experiments on high-quality single crystals of the superconducting compounds UPd₂Al₃ and CeRu₂, which have enhanced spin susceptibilities, demonstrate a first-order transition between weak and strong pinning at T<0.9Tc ( $H>10$ kOe). This is compatible with a staggered order parameter due to the formation of a ``Fulde-Ferrell-Larkin-Ovchinnikov'' state.