Anomalous peak effect inCeRu2and2H−NbSe2:Fracturing of a flux line lattice

${\mathrm{CeRu}}_2$ and $2H-{\mathrm{NbSe}}_2$ display remarkable similarities in their magnetic response, reflecting the manner in which the weakly pinned flux line lattice (FLL) loses spatial order in the peak-effect (PE) regime. We present evidence for discontinuous changes in the screening response near the onset of the PE in these systems, and demonstrate history-dependent effects. We attribute these features to a disorder-induced fracturing and entanglement of the FLL, as an alternative to the appearance of a spatially modulated ground state for ${\mathrm{CeRu}}_2.$