Unique Fermi Surface and Emergence of Charge Density Wave in EuGa₄ and EuAl₄

Abstract

EuGa 4 and EuAl 4 with the BaAl 4 -type tetragonal structure are Eu-divalent antiferromagnets with the Néel temperature T N = 16.5 K and 15.4 K, respectively. We clarified the Fermi surface properties from the results of de Haas-van Alphen experiments for EuGa 4 and EuAl 4 and energy band calculations of a non-4f reference compound SrGa 4 . Among two kinds of Fermi surfaces in EuGa 4 , there exists a cube-like electron Fermi surface with a vacant space in center, which most likely drives a charge density wave (CDW) via a nesting effect. This is realized in EuGa 4 under pressures, where the Fermi surface is expected to be modified so that the CDW transition is more easily induced. The similar CDW phenomenon is observed blow T CDW = 140 K in EuAl 4 at ambient pressure. Therefore, the cube-like Fermi surface below T CDW in EuAl 4 dose not exist at low temperatures because of the Fermi surface reconstruction, revealing only small ellipsoidal Fermi surfaces.

153Eu Zero-field NMR Study of Antiferromagnetic State in EuAl4

“…The K iso and K aniso shift to negative side with decreasing temperature due to the RKKY interaction. These temperature dependences follow the Curie-Weiss law with θ P = +14 K, which is consistent with that of the magnetic susceptibility [3,4]. From the K − χ plot, the values of the hyperfine fields H hf iso and H hf aniso are -3.231 and -0.162 kOe/µ B for Al(I) site, and -1.823 and -0.264 kOe/µ B for Al(II) site, respectively.…”

“…1. EuAl 4 and EuGa 4 orders antiferromagnetically at almost the same Néel temperature of T N ≈ 16 K with effective magnetic moments of 7.89 and 7.86 µ B , respectively, which are close to a divalent value of 7.94 µ B /Eu [3,4].…”

“…In the paramagnetic (PM) phase, the magnetic susceptibilities of EuAl 4 and EuGa 4 follow the Curie-Weiss law with positive Curie-Weiss temperatures θ P = +14 and +3 K, respectively [3,4]. The low-temperature magnetic susceptibility of EuAl 4 indicates four successive antiferromagnetic (AFM) transitions at T N1 (= T N ) = 15.4 K, T N2 = 13.2 K, T N3 = 12.2 K, and T N4 = 10.0 K. A steep susceptibility drop is observed at T N3 and T N4 , indicating that the changes in the AFM structures are of the first order.…”

“…On the other hand, an easy-axis in the magnetization of EuGa 4 is the [100] direction [4]. The AFM states in EuAl 4 and EuGa 4 are changed into the field induced ferromagnetic (FM) states at critical fields H c of approximately 2 and 7 T, respectively [3,4]. Recently, the 27 Al NMR measurement of EuAl 4 in the PM and FM states was carried out at temperatures between 2 K and 300 K, applying an external magnetic field of approximately 6.5 T [5].…”

NMR Study of Characteristic CDW Transition in SrAl4

“…Many measurements were made to clarify the magnetism and electronic states of the divalent Eu intermetallic compound EuAl 4 , indicating the antiferromagnetic transitions at T N ≈ 16 K due to the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction [1][2][3]. Additionally, the charge density wave (CDW) transition was found to occur below 145 K at ambient pressure in EuAl 4 with the incommensurate wave vector q c = (0, 0, 0.18), which was confirmed by the X-ray diffraction measurements [2][3][4].…”