The magnetic phase diagram and low-energy magnon excitations of structurally and magnetically chiral ${\text{MnSb}}_2{\text{O}}_6$ are reported. The specific heat and the static magnetization are investigated in magnetic fields up to 9 and 30 T, respectively, while the dynamic magnetic properties are probed by X-band as well as tunable high-frequency electron spin-resonance spectroscopy. Below $T_{\mathrm{N}}=11.5$ K, we observe antiferromagnetic resonance modes which imply small but finite planar anisotropy showing up in a zero-field splitting of 20 GHz. The data are well described by means of an easy-plane two-sublattice model with the anisotropy field $B_A=0.02$ T. The exchange field $B_{\mathrm{E}}=13$ T is obtained from the saturation field derived from the pulsed-field magnetization. A crucial role of the small anisotropy for the spin structure is reflected by competing antiferromagnetic phases appearing, at $T=2$ K, in small magnetic fields at $B_{\mathrm{C}1} \approx 0.5$ T and $B_{\mathrm{C}2}=0.9$ T. We discuss the results in terms of spin reorientation and of small magnetic fields favoring helical spin structure over the cycloidal ground state which, at $B=0$, is stabilized by the planar anisotropy. Above $T_{\mathrm{N}}$, short-range magnetic correlations up to $\gtrsim 60$ K and magnetic entropy changes well above $T_{\mathrm{N}}$ reflect the frustrated triangular arrangement of ${\mathrm{Mn}}^{2+}$ ions in ${\text{MnSb}}_2{\text{O}}_6$.

• Received 6 June 2016
• Revised 9 August 2016

DOI:https://doi.org/10.1103/PhysRevB.94.104408