PHYSICAL REVIEW B 101, 224421 (2020) Magnetic structure and crystalline electric field effects in the triangular antiferromagnet CePtAl 4 Ge 2 S. Shin , 1 V. Pomjakushin, 2 L. Keller, 2 P. F. S. Rosa, 3 U. Stuhr, 2 C. Niedermayer, 2 R. Sibille, 2 S. Toth, 2 J. Kim, 1, 4 H. Jang, 1 S.-K. Son, 1 H.-O. Lee, 1 T. Shang , 5 M. Medarde, 5 E. D. Bauer, 3 M. Kenzelmann, 2 , * and T. Park 1 , † 1 Center for Quantum Materials and Superconductivity (CQMS) and Department of Physics, Sungkyunkwan University, Suwon 16419, South Korea 2 Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, CH-5232, Villigen PSI, Switzerland 3 Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 4 Institute of Basic Science, Sungkyunkwan University, Suwon 16419, South Korea 5 Laboratory for Multiscale Materials Experiments, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland (Received 3 September 2019; revised manuscript received 28 April 2020; accepted 26 May 2020; published 15 June 2020) Kondo metal CePtAl 4 Ge 2 exhibits long-range antiferromagnetic order below 2.3 K. Neutron powder- diffraction experiments reveal that Ce moments order antiferromagnetically with an incommensurate ordering wave vector k = (1.39, 0, 0.09). Inelastic neutron powder scattering experiments show a magnetic excitation at 14.5 meV, which corresponds to the first excited state due to the crystalline electric field splitting of the ground-state multiplet of Ce 3+ . The temperature and field dependence of the magnetization of CePtAl 4 Ge 2 is consistent with a doublet ground state with a dominant | j z = 1/2〉 character and a first excited doublet | j z = 3/2〉 at 14.5 meV. DOI: 10.1103/PhysRevB.101.224421 I. INTRODUCTION Magnetic frustration in metallic systems may induce re- markable emergent ground states, including nonmagnetic quantum spin liquids and chiral magnetic structures hosting anomalous Hall effect [1]. As Wannier proposed in 1950, geometrically frustrated triangular lattices with Ising mo- ments cannot satisfy all antiferromagnetic couplings, giving rise to magnetic frustration [2]. In the past half century, condensed-matter physics witnessed a flourish of theoreti- cal and experimental research on frustrated insulators [3–6]. More recently, the interplay between magnetic frustration and metallic degrees of freedom started to be investigated [7–9], and distinct frustration characteristics emerge due to the presence of itinerant electrons [10]. For the case of in- sulators, the magnetic moments vary their direction to avoid frustration and form a magnetically ordered/disordered state with exotic quantum magnetic properties, such as the quan- tum spin-liquid state [6]. In a metallic system, however, the magnetic moments may vary their amplitude as well as their direction [10]. In particular, magnetic frustration in f -electron metallic systems has been a promising route for the discovery of novel ground states. Experimentally, frustrated magnetic structures have been reported in a breadth of f -electron compounds including CePdAl [7,11], UNi 4 B[12], YbPtIn [13], and YbAgGe [13,14]. Magnetic frustration also gives rise to a skyrmion lattice in Gd 2 PdSi 3 [15], an electronic nematic state in CeRhIn 5 [16–18], and field-/pressure-induced * michel.kenzelmann@psi.ch † tp8701@skku.edu quantum criticality in CePdAl [19–22], CeAgGe [23–25], and CeRhSn [26–28]. Several theoretical works proposed that these electronic states in f -electron metallic systems could be ascribed to the interplay between the Ruderman-Kittel- Kasuya-Yosida (RKKY) interaction, the Kondo effect, and the strength of the magnetic frustration [29–31]. The recently reported Ce-based antiferromagnet CePtAl 4 Ge 2 is an ideal candidate to study the interplay among the competing orders because it crystallizes in the trigonal structure (R ¯ 3m) and shows a metallic behavior [32]. Electrical resistivity revealed a local minimum at 18 K due to the Kondo effect and a maximum at 6 K due to the onset of the Kondo coherent state. The magnetic Ce 3+ ions form a two-dimensional triangular lattice and order antiferromagnetically (AFM) below T N = 2.3K, with in plane being the magnetic easy plane. Magnetic specific-heat capacity shows an anomaly at T N and the magnetic entropy recovers only 61% of Rln2 due to Kondo screening effects and reduced dimensionality of the magnetism. The Kondo temperature is estimated as 2 K—the temperature where the magnetic entropy recovers 0.5Rln2. This is similar to the situation in CePdAl, where the geometrical frustration of Ce moments is responsible for the mixed ordered state with incommensurate ordering wave vector k = (0.5, 0, ∼ 0.35) [7]. Also in CePtAl 4 Ge 2 , the competing orders favored by RKKY interactions, the Kondo effect, and metallicity in the presence of magnetic frustration are expected to induce emergent quantum states near a T = 0 K AFM phase transition. Here we determine the magnetic structure of the trian- gular antiferromagnetic metallic compound CePtAl 4 Ge 2 via 2469-9950/2020/101(22)/224421(7) 224421-1 ©2020 American Physical Society