  The linear ac response of the vortex system is measured locally in Bi-2212
single crystals at various doping, using a miniature two-coil mutual-inductance
technique. It was found that a step-like change in the local ac response takes
place exactly at the first-order transition (FOT) temperature T_{FOT}(H)
determined by a global dc magnetization measurement. The T_{FOT}(H) line in the
H-T phase diagram becomes steeper with increasing doping. In the higher-field
region where the FOT is not observed, the local ac response still shows a
broadened but distinct feature, which can be interpreted to mark the growth of
a short-range order in the vortex system.
