Coexistence of positive and negative magnetic entropy changes in CeMn₂(Si_1-xGe_x)₂ compounds

A series of CeMn₂(Si_1-xGe_x)₂ (x=0.2, 0.4, 0.6, 0.8) compounds are prepared by the arc-melting method. All the samples primarily crystallize in the ThCr₂Si₂-type structure. The temperature dependences of zero-field-cooled (ZFC) and FC magnetization measurements show a transition from antiferromagnetic (AFM) state to ferromagnetic (FM) state at room temperature with the increase of the Ge concentration. For x=0.4, the sample exhibits two kinds of phase transitions with increasing temperature: from AFM to FM and from FM to paramagnetic (PM) at around T_N ～ 197 K and T_C ～ 300 K, respectively. The corresponding Arrott curves indicate that the AFM-FM transition is of first-order character and the FM-PM transition is of second-order character. Meanwhile, the coexistence of positive and negative magnetic entropy changes can be observed, which are corresponding to the AFM-FM and FM-PM transitions, respectively.