  Magnetization and ac-susceptibility measurements are used to characterize the
mixed phase of the high-temperature cuprate superconductor
La$_{2-x}$Sr$_{x}$CuO$_{4}$ over a large range of doping (0.075 $\leq x\leq$
0.20). The first order vortex lattice phase transition line $H_{FOT}(T)$, the
upper critical field $H_{c2}(T)$ and the second peak $H_{sp}(T)$ have been
investigated up to high magnetic fields (8 Tesla applied perpendicular to the
$CuO_2$ planes). Our results reveal a strong doping dependence of the magnetic
phase diagram, which can mainly be explained by the increasing anisotropy with
underdoping. Within our interpretation, the first order vortex lattice phase
transition is due to the sublimation (rather than melting) of the vortex
lattice into a gas of pancake vortices, whereas the second peak is related to
the transition to a more disordered vortex glass state.
