Magnetic Order and Phase Transition in a Centrosymmetric Rare Earth Topological System

The topological properties of magnetic materials with a centrosymmetric crystal structure are of current interest, particularly in the class of materials such as EuGa₄, EuGa₂Al₂ and EuAl₄. We have been investigating the magnetic structures, phase transitions, and topological behavior for the rare earth ions in this class on centrosymmetric systems with the orthorhombic Immm space group. In the particular new compound we have been investigating via neutron diffraction, the rare earth moments develop long range commensurate magnetic order below 13.2 K. Upon further cooling the system undergoes additional transitions at 9.4 K, 2.9 K, and 1.77 K. The ordering wave vector is [0,0,1]. In the ground state the spins are in the a-b plane, and the transition at 1.77 K is primarily a spin reorientation transition where the spins rotate to the c-axis, extinguishing the magnetic [0,0,L] Bragg peaks. The magnetic structures in all the phases have been investigated in detail, and the transitions agree well with our transport and susceptibility data. The possible explanations for the topological Hall effect in this system, such as skyrmion, a fluctuation driven mechanism like YMn₆Sn₆ [1], or a CDW mechanism such as for EuGa₂Al₂ [2], will be discussed.

[1] Ghirime, N., et al. Sci. Adv. 6, 51, abe2680 (2020).

[2] Moya, J., et al. Phys. Rev. Mater. 6, 074201(2022).