Transport and magnetic properties of EuAl₄ and EuGa₄

We succeeded in growing a single crystal of the Eu-divalent compound EuAl4 with the BaAl₄-type tetragonal structure by the Al self-flux method and measured the electrical resistivity, magnetic susceptibility, magnetization, specific heat, and thermoelectric power. EuAl₄ orders antiferromagnetically below T_N1 = 15.4 K, with three successive antiferromagnetic transitions at T_N2 = 13.2 K, T_N3 = 12.2 K, and T_N4 = 10.0 K. The latter two transitions are of the first-order. The corresponding magnetization curve indicates three successive metamagnetic transitions with hystereses and saturates above 16 kOe. We observed a plausible characteristic feature of the charge density wave (CDW) below T_CDW = 140 K. We also studied an effect of pressure on the electronic state by measuring the electrical resistivity and thermoelectric power. T_CDW is found to decrease with increasing pressure at a rate of dT_CDW/dP = -54.7K/GPa and becomes zero at about 2.5 GPa. The present antiferromagnetic ordering is, however, found to be stable at higher pressures up to 7 GPa in EuAl₄. On the other hand, the different characteristic CDW was observed in EuGa₄, not at ambient pressure but above 1 GPa, and T_CDW increases with increasing pressure. Above 6 GPa, we found that the antiferromagnetic ordering is changed into another first-order-like phase transition. Its characteristic feature is similar to that of the valence transition, and the heavy fermion state is realized at low temperatures.