First flux-penetration fields in ${\mathit{L}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ single crystals

High-resolution measurements of low-field dc magnetization are used to obtain flux-penetration fields ${\mathit{H}}_{\mathit{p}}$(T) along the two main crystallographic directions of several ${\mathit{L}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ (L=Pr,Nd,Sm) single crystals. The departure from the linearity of the virgin M(H) curve and the appearance of remanence and magnetic relaxation are used to determine ${\mathit{H}}_{\mathit{p}}$. It is shown that ${\mathit{H}}_{\mathit{p}}$(T) may not be safely identified with ${\mathit{H}}_{\mathit{c}1}$(T) when H∥c; the implications of these results are discussed in order to estimate the values of the fundamental superconducting quantities as the lower critical field, the penetration depth, the Ginzburg-Landau parameter, and the anisotropy in these electron-doped copper oxide superconductors.