Charge-Density-Wave Order and Multiple Magnetic Transitions in Divalent Europium Compound EuAl${}_4$

Abstract

Multiple transition phenomena in divalent Eu compound EuAl${}_4$ 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 $T_{\mathrm{CDW}}$ = 140 K, superlattice peaks emerge near nuclear Bragg peaks described by an ordering vector $q_{\mathrm{CDW}}$=(0 0 ${\delta }_c$) with ${\delta }_c\sim$0.19. In contrast, magnetic peaks appear at $q_2=({\delta }_2{\delta }_{2}0)$ with ${\delta }_2$ = 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 $q_1=({\delta }_{1}00)$ with ${\delta }_1$ = 0.17 at 11.5 K and slightly shifts to ${\delta }_1$ = 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 $T_{\mathrm{CDW}}$ with $q_{\mathrm{CDW}}$ is characterized by the absence of the superlattice peak in the (0 0 $l$) axis. As a similar CDW transition was observed in SrAl${}_4$, the structural modulation with $q_{\mathrm{CDW}}$ 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 EuGa${}_4$. This could stem from different electronic structures with the CDW transition between two compounds.

6 pages, 3 figures, accepted for publication in J. Phys. Soc. Jpn