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

TL;DR

This work investigates the dynamics and thermodynamics of a near-Heisenberg, $S=5/2$ triangular-lattice antiferromagnet, K$_2$Mn(SeO$_3$)$_2$, using calorimetry, magnetometry, neutron diffraction, and inelastic neutron scattering. The authors map the phase diagram, extract exchange parameters for an XXZ Hamiltonian, and test linear and non-linear spin-wave theories against detailed spectra across zero and high magnetic fields. In zero field, a non-collinear Y phase exhibits a high-energy magnon continuum that non-linear spin-wave theory quantitatively reproduces, while field-induced phases (uud plateau and V phase) show renormalized magnon energies and a weakened continuum; the continuum is largely suppressed in the plateau phase. The results demonstrate that magnon-magnon interactions remain essential even for large spins on a triangular lattice and provide a stringent, quantitative benchmark for interacting magnon theories in frustrated magnets.

Abstract

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.

Figures (10)

• Figure 1: (a) Crystal structure of K$_2$Mn(SeO$_3$)$_2$. (b) Top view of a triangular plane consisting of MnO$_6$ octahedra connected by SeO$_3$. Inset: Photo of a K$_2$Mn(SeO$_3$)$_2$ single crystal on a millimeter grid paper.
• Figure 2: (a) Magnetic susceptibility as a function of temperature with $\mu_0H = 0.1$ T along the crystallographic $c$ and $a$ axis, respectively. Solid lines are the Curie-Weiss fits. Inset shows the expanded view of the low temperature regime. (b) Left axis: Magnetization as a function of magnetic fields measured using pulsed-fields at $T = 1.3$ K (solid lines) and Faraday balance magnetometry at $T = 0.2$ K (green circles). Dashed lines denote the saturation magnetization for both field orientations and 1/3 of the saturated magnetization for $H \parallel c$. Right axis: $dM/dH$ as a function of magnetic field. $H_{c1}$ and $H_{c2}$ denote the critical fields of the onset and end of the 1/3 plateau phase, respectively.
• Figure 3: (a),(b) False color plots of specific heat $C_p/T$ of K$_2$Mn(SeO$_3$)$_2$ measured as a function of temperature and magnetic fields along the $c$ and $a$ axis, respectively. Inset arrows illustrate the magnetic structure in each phase. Blue circles are phase boundaries determined by the neutron diffraction data. Red squares are critical fields $H_{c1}$ and $H_{c2}$ extracted from $dM/dH$ of the pulsed-field magnetization. Green diamonds are critical fields obtained from Faraday balance magnetometry. Solid lines are guides to the eye. The dash line is a crossover. (c),(d) Typical temperature dependent specific heat curves of K$_2$Mn(SeO$_3$)$_2$ measured with fields along the $c$ and $a$ axis. Arrows denote the lower-temperature Y-to-uud phase transition or inverted-Y to $\Psi$ crossover as described in the text. Each curve is offset from one another by 17 J mol$^{-1}$ K$^{-1}$ for visibility.
• Figure 4: (a) $H$- and (b) $L$-scans over magnetic reflection $\mathbf{Q}$ = (2/3,2/3,0) measured at $\mu_0H$ = 0 and 8 T at $T=0.08$ K. Solid lines in (a) are the fits by Gaussian functions. Solid lines in (b) are the fits by Voigt functions as described in the text. Dashed lines are the background. The gray bar represents the instrumental resolution estimated using a nearby nuclear peak (1,1,0). A 40$^{\prime}$ horizontal collimator is placed after the sample to improve the Q-resolution for the $L$ scans. (c),(d) Temperature dependence of the intensity of magnetic reflection (2/3,2/3,0) measured at $\mu_0H$ = 0 and 8 T, respectively. Dashed lines are the background. No collimator is used.
• Figure 5: False color plots of magnetic excitation spectra of K$_2$Mn(SeO$_3$)$_2$ measured at $T = 0.2$ K in (a) $H=0$ in the Y phase, (b) $\mu_0H = 8$ T in the uud plateau phase, and (c) $\mu_0H = 10.8$ T in the V phase, along high-symmetry directions in the reciprocal space. Solid lines are the dispersion relation calculated by LSWT using parameters summarized in Table \ref{['tab:exchange_paramters']}. White hexagon in (a) represents the boundary of the first Brillouin zone. White arrows in (a) highlight the high-energy continua. The data have been averaged over all equivalent paths as shown in Fig. \ref{['fig:reciprocal_path']}, Appendix \ref{['AppendixA']}.