Magnetic phase diagram of Eu1-xLaxFe2As2 single crystals

We have systematically measured resistivity, susceptibility and specific heat under different magnetic fields (H) in Eu1-xLaxFe2As2 single crystals. It is found that a metamagnetic transition from A-type antiferromagnetism to ferromagnetism occurs at a critical field for magnetic sublattice of Eu2+ ions. 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. Further, a twofold in-plane magnetization is found in antiferromagnetic state which is similar to spin density wave (SDW) order. In contrast, isotropic in-plane magnetization is presented in both ferromagnetic and paramagnetic states. Moreover, ferromagnetic state becomes more easily established under external field with suppressing SDW order. An anisotropic exchange model is proposed to understand our findings. Our results suggest that metamagnetism of Eu2+ ions is somewhat related to SDW order. Finally, detailed H-T phase diagrams for x=0 and 0.15 crystals are given, and possible magnetic structure is proposed.