Anisotropy, frustration and saddle point in the twisted Kagome antiferromagnet ErPdPb

The kagome lattice, with its inherent geometric frustration, provides a rich platform for exploring intriguing magnetic phenomena and topological electronic structures. In reduced-symmetry structures, such as twisted kagome systems involving rare earth elements, additional anisotropy can arise, enabling intriguing properties including spin-ice states, magnetocaloric effects, noncollinear magnetic ordering, and anomalous Hall effect. Here, we report the synthesis of single crystals of ErPdPb, which features a twisted kagome lattice net of Er atoms within the hexagonal ZrNiAl-type structure, and we investigate its magnetic, electronic, and thermal properties. The material exhibits antiferromagnetic ordering below 2.2 K, consistently observed in magnetic, transport, and heat capacity measurements. Magnetization measurements reveal 1/3 metamagnetic steps along the c-axis below the Néel temperature, suggesting an Ising-spin-like state on the twisted kagome lattice. A pronounced anisotropy between in-plane and out-of-plane resistivity is observed throughout the temperature range of 1.8-300 K, and the compound exhibits a significant frustration index of 13.6 (12.7) along the c-axis (ab-plane). Heat capacity measurements show a broad hump at 2.2 K, with an additional increase below 0.5 K. The anisotropic magnetic properties are further explored through density functional theory (DFT) calculations, which suggest strong easy-axis anisotropy, consistent with experimental magnetic measurements and crystal-field model expectations, and quasi-one-dimensional bands and a spin-split saddle point at the zone center.

💡 Research Summary

In this work the authors report the first growth of single‑crystal ErPdPb, a rare‑earth‑based twisted kagome antiferromagnet, and present a comprehensive study of its crystal structure, magnetic, transport, and thermodynamic properties together with density‑functional theory (DFT) calculations. ErPdPb crystallizes in the hexagonal ZrNiAl‑type structure (space group P 6̅2m) with lattice parameters a = 7.707 Å and c = 3.782 Å. The Er atoms form a buckled, “twisted” kagome net in the ab‑plane, breaking inversion symmetry and allowing strong spin‑orbit coupling (SOC) from the heavy Pb atoms to influence the magnetic anisotropy.

Magnetic susceptibility measured along the c‑axis (χ_c) and within the ab‑plane (χ_ab) shows pronounced anisotropy. χ_c rises sharply below ~100 K, while χ_ab remains relatively flat, displaying a subtle feature near 49 K that may indicate short‑range correlations or low‑dimensional fluctuations. Curie–Weiss fits in the high‑temperature regime give an effective moment μ_eff ≈ 9.4 μ_B (consistent with Er³⁺, J = 15/2) and negative Weiss temperatures θ_CW_c ≈ ‑2.2 K, θ_CW_ab ≈ ‑1.7 K, indicating overall antiferromagnetic interactions.

Heat‑capacity and resistivity data both reveal a clear transition at T_N ≈ 2.2 K, confirming antiferromagnetic ordering. Below the transition the specific heat shows a broad hump at 2.2 K and an additional upturn below 0.5 K, suggesting a possible secondary low‑temperature magnetic transition (e.g., spin‑glass freezing or a quantum critical point).

Isothermal magnetization measured with the field along the c‑axis displays a robust 1/3 magnetization plateau, characteristic of an Ising‑like spin system on a kagome lattice that stabilizes a ↑↑↓ three‑spin configuration. No comparable plateau is observed for fields in the ab‑plane, underscoring the strong easy‑axis anisotropy.

Electrical transport is highly anisotropic: ρ_ab is roughly an order of magnitude larger than ρ_c over the entire 1.8–300 K range, indicating that charge carriers propagate more efficiently along the c‑axis. Magnetoresistance (MR) is also direction‑dependent, with opposite signs for B‖c and B‖ab, reflecting the underlying anisotropic band structure and SOC‑induced spin splitting. Hall measurements confirm dominant electron‑like carriers.

First‑principles DFT+U calculations (U = 6 eV, J = 0.7 eV on Er‑4f) including SOC were performed using the experimentally refined crystal parameters. Constrained DFT enforcing Hund’s rules reproduces the correct 4f occupancy and yields a magnetic anisotropy energy that favors the c‑axis by ~0.8 meV per Er ion, in agreement with experiment. The calculated band structure shows quasi‑one‑dimensional dispersions along the c‑direction and a spin‑split saddle point at the Brillouin‑zone center (Γ). This saddle point, a hallmark of kagome geometry, lies close to the Fermi level and can enhance the density of states, potentially influencing the observed magnetic plateau and the low‑temperature heat‑capacity anomaly.

Overall, ErPdPb combines several intriguing features: a high frustration index (|θ_CW|/T_N ≈ 13), strong Ising‑type easy‑axis anisotropy, a 1/3 magnetization plateau, pronounced transport anisotropy, and a SOC‑driven saddle point near E_F. These attributes make it an excellent platform for exploring exotic low‑temperature phenomena such as quantum spin liquids, non‑Fermi‑liquid behavior, or pressure‑induced superconductivity. Future work involving pressure tuning, chemical substitution, or neutron scattering will be valuable to elucidate the nature of the low‑temperature anomaly and to probe possible emergent quantum phases.