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 TN1 = 15.4 K, with three successive antiferromagnetic transitions at TN2 = 13.2 K, TN3 = 12.2 K, and TN4 = 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 TCDW = 140 K. We also studied an effect of pressure on the electronic state by measuring the electrical resistivity and thermoelectric power. TCDW is found to decrease with increasing pressure at a rate of dTCDW/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 TCDW 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.
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