Dynamics and thermodynamics of the S = 5/2 almost-Heisenberg triangular lattice antiferromagnet K2Mn(SeO3)2

We report calorimetric, magnetic, and neutron scattering studies on an S = 5/2, nearly Heisenberg triangular-lattice antiferromagnet K2Mn(SeO3)2 with weak XXZ easy-axis anisotropy. Multiple magnetic phases are identified, including a non-collinear Y phase in zero field, a field-induced collinear m = 1/3 magnetization plateau, and a high-field V phase. In the Y phase, the magnetic excitation spectrum exhibits both single-magnon excitations and an extended high-energy continuum. Both features are well described by non-linear spin wave theory. In the field-induced phases, complex effects of the spectrum renormalization even for large S = 5/2 material are clearly detectable. These results underscore the essential role of magnon-magnon interactions in the dynamics of large-S Heisenberg spin systems on a triangular lattice.

💡 Research Summary

K₂Mn(SeO₃)₂ is a quasi‑two‑dimensional triangular‑lattice antiferromagnet composed of Mn²⁺ ions (S = 5/2) that form an ABC‑stacked network of equilateral triangles. The crystal belongs to space group R‑3 m, and the magnetic layers are separated by non‑magnetic K⁺ ions, giving the system a pronounced two‑dimensional character. Bulk measurements reveal antiferromagnetic interactions with a Curie–Weiss temperature Θ_W ≈ ‑27 K and an ordering transition at T_N ≈ 4 K, indicating moderate frustration (Θ_W/T_N ≈ 7).

Magnetization curves measured up to 23.5 T show a clear 1/3 magnetization plateau when the magnetic field is applied along the crystallographic c‑axis (the easy‑axis). The plateau appears between H_c1 ≈ 7.4 T and H_c2 ≈ 10.0 T and is absent for fields in the basal plane, where the magnetization rises smoothly to saturation. Using the saturation field and the critical field for the onset of the plateau, the exchange parameters of a nearest‑neighbour XXZ Hamiltonian were extracted: J_xy ≈ 0.106 meV, J_zz ≈ 0.115 meV, giving an anisotropy ratio Δ = J_xy/J_zz ≈ 0.92, i.e., the system is very close to the isotropic Heisenberg limit.

Specific‑heat measurements as a function of temperature and magnetic field map out the magnetic phase diagram. For H ∥ c, the main anomaly at T_N shifts to higher temperature with increasing field up to ≈ 6.5 T and then recedes. A second low‑temperature feature emerges above 2 T, coinciding with the onset of the 1/3 plateau. For H ∥ a the overall behavior is similar but the field‑induced shifts are smaller, and the low‑temperature feature appears at higher fields, consistent with the weaker coupling in the plane.

Neutron diffraction confirms three‑sublattice magnetic order. In zero field magnetic Bragg peaks appear at Q = (1/3, 1/3, L) and symmetry‑related positions, consistent with the non‑collinear Y‑state predicted for an easy‑axis XXZ model. At 8 T the intensity pattern matches the up‑up‑down (uud) configuration of the 1/3 plateau, and at 10.8 T the V‑state is realized. The in‑plane correlation length is resolution limited, while the out‑of‑plane (L) direction shows a broad peak corresponding to a correlation length of only ≈ 11 Å, indicating very weak interlayer coupling and quasi‑2D magnetic order.

Inelastic neutron scattering performed on the CAMEA spectrometer provides a comprehensive view of the spin dynamics. In the Y‑phase a well‑defined magnon branch is observed at low energies, but a broad high‑energy continuum extending up to ~3 meV is also present. Linear spin‑wave theory (LSWT) fails to capture this continuum, whereas non‑linear spin‑wave theory (NLSWT) that incorporates magnon‑magnon interactions reproduces both the magnon dispersion and the intensity of the continuum quantitatively. In the uud plateau the continuum is strongly suppressed, and in the V‑phase it persists only weakly. Moreover, the magnon energies exhibit a non‑monotonic renormalization with increasing field, in excellent agreement with NLSWT predictions that account for the field‑dependent self‑energy corrections.

These findings demonstrate that even for a large‑spin (S = 5/2) system, quantum magnon‑magnon interactions play a decisive role in shaping the excitation spectrum. The presence of a high‑energy continuum in the non‑collinear phase, its suppression in the collinear plateau, and the field‑driven renormalization of magnon modes all point to significant quantum fluctuations beyond the semiclassical picture. K₂Mn(SeO₃)₂ thus serves as an ideal platform to study quantum dynamics in nearly Heisenberg triangular antiferromagnets, bridging the gap between the well‑studied S = 1/2 compounds and classical large‑S materials.