Anisotropic magnetism and electronic structure of trigonal EuAl2Ge2 single crystals

Understanding the interplay between magnetic and electronic degrees of freedom is of profound recent interest in different Eu-based magnetic topological materials. In this paper, we studied the magnetic and electronic properties of the layered Zintl-phase compound EuAl2Ge2 crystallizing in the trigonal CaAl2Si2-type structure. We report zero-field neutron diffraction, temperature T- and magnetic-field H-dependent magnetic suscepti-bility chi(T, H), isothermal magnetization M(T, H), heat capacity Cp(T, H), and electrical resistivity rho(T, H) measurements, together with T-dependent angle-resolved photoemission spectroscopy (ARPES) measurements complemented with first-principles calculations. EuAl2Ge2 undergoes second-order A-type antiferromagnetic (AFM) ordering below TN = 27.5(5) K, with the Eu moments (Eu2}, S = 7/2) aligned ferromagnetically in the ab plane while these layers are stacked antiferromagnetically along the c axis. The critical fields at which all moments become parallel to the field are 37.5(5) and 52.5(5) kOe for H II ab and H II c, respectively. The H = 0 magnetic structure consists of trigonal AFM domains associated with ab-plane magnetic anisotropy and a field-induced reorientation of the Eu spins in the domains is also evident at T = 2 K below the critical field Hc1 = 2.5(1) kOe. The rho(T) measurements reveal metallic behavior transforming into a slight resistivity increase on cooling towards TN. A pronounced loss of spin-disorder scattering is observed below TN. The ARPES results show that EuAl2Ge2 is metallic both above and below TN, and the Fermi surface is anisotropic with two hole pockets at the zone center and one small electron pocket at each M point. In the AFM phase, we directly observe folded bands in ARPES due to the doubling of the magnetic unit cell along the c axis with an enhancement of quasiparticle weight due to the complex change in the coupling between the magnetic moments and itinerant electrons on cooling below TN. The observed electronic structure is well reproduced by first-principles calculations, which also predict the presence of nontrivial electronic states near the Fermi level in the AFM phase with Z2 topological numbers 1; (000).