Charge-density-wave order and multiple magnetic transitions in divalent Europium compound EuAl₄

Multiple transition phenomena in divalent Eu compound EuAl₄ with the tetragonal structure were investigated via the single-crystal time-of-flight neutron Laue technique. At 30.0 K below a charge-density-wave (CDW) transition temperature of T_CDW = 140 K, superlattice peaks emerge near nuclear Bragg peaks described by an ordering vector q_CDW ¼ ð0; 0; δ_cÞ with δ_c ∼ 0.19. In contrast, magnetic peaks appear at q₂ ¼ ðδ₂; δ₂; 0Þ with δ₂ = 0.085 in a magnetic-ordered phase at 13.5 K below T_N1 = 15.4 K. By further cooling to below T_N3 = 12.2 K, the magnetic ordering vector changes into q₁ ¼ ðδ₁; 0; 0Þ with δ₁ = 0.17 at 11.5 K and slightly shifts to δ₁ = 0.194 at 4.3 K. No distinct change in the magnetic Bragg peak was detected at T_N2 = 13.2 K and T_N4 = 10.0 K. The structural modulation below T_CDW with qCDW is characterized by the absence of the superlattice peak in the (0 0 l) axis. As a similar CDW transition was observed in SrAl₄, the structural modulation with q_CDW could be mainly ascribed to the displacement of Al ions within the tetragonal ab-plane. Complex magnetic transitions are in stark contrast to a simple collinear magnetic structure in isovalent EuGa₄. This could stem from different electronic structures with the CDW transition between two compounds.