  We have systematically measured resistivity, susceptibility and specific heat
under different magnetic fields (H) in Eu$_{1-x}$La$_x$Fe$_2$As$_2$ single
crystals. It is found that a metamagnetic transition from A-type
antiferromagnetism to ferromagnetism occurs at a critical field for magnetic
sublattice of $Eu^{2+}$. The jump of specific heat is suppressed and shifts to
low temperature with increasing H up to the critical value, then shifts to high
temperature with further increasing H. Such behavior supports the metamagnetic
transition. Detailed H-T phase diagrams for x=0 and 0.15 crystals are given,
and possible magnetic structure is proposed. Magnetoresistance measurements
indicate that there exists a strong coupling between local moment of $Eu^{2+}$
and charge in Fe-As layer. These results are very significant to understand the
underlying physics of FeAs superconductors.
