Anisotropic magnetism and electronic structure of trigonal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>EuAl</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Ge</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math> single crystals
Santanu Pakhira, Asish K. Kundu, Farhan Islam, M. A. Tanatar, Tufan Roy, Thomas Heitmann, Turgut Yilmaz, E. Vescovo, Masahito Tsujikawa, Masafumi Shirai, R. Prozorov, David Vaknin, D. C. Johnston
Abstract
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 ${\mathrm{EuAl}}_{2}{\mathrm{Ge}}_{2}$ crystallizing in the trigonal ${\mathrm{CaAl}}_{2}{\mathrm{Si}}_{2}$-type structure. We report zero-field neutron diffraction, temperature $T$- and magnetic-field $H$-dependent magnetic susceptibility $\ensuremath{\chi}(T,H)$, isothermal magnetization $M(T,H)$, heat capacity ${C}_{\mathrm{p}}(T,H)$, and electrical resistivity $\ensuremath{\rho}(T,H)$ measurements, together with $T$-dependent angle-resolved photoemission spectroscopy (ARPES) measurements complemented with first-principles calculations. ${\mathrm{EuAl}}_{2}{\mathrm{Ge}}_{2}$ undergoes second-order A-type antiferromagnetic (AFM) ordering below ${T}_{\mathrm{N}}=27.5(5)$ K, with the Eu moments (${\mathrm{Eu}}^{2+},S=7/2$) 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 $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$, 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 ${H}_{\text{c}1}=2.5(1)$ kOe. The $\ensuremath{\rho}(T)$ measurements reveal metallic behavior transforming into a slight resistivity increase on cooling towards ${T}_{\mathrm{N}}$. A pronounced loss of spin-disorder scattering is observed below ${T}_{\mathrm{N}}$. The ARPES results show that ${\mathrm{EuAl}}_{2}{\mathrm{Ge}}_{2}$ is metallic both above and below ${T}_{\mathrm{N}}$, and the Fermi surface is anisotropic with two hole pockets at the zone center and one small electron pocket at each $\mathrm{M}$ 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 ${T}_{\mathrm{N}}$. 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 ${Z}_{2}$ topological numbers $1;(000)$.