Magnetic anisotropy and the phase diagram of chiralMnSb2O6

The magnetic phase diagram and low-energy magnon excitations of structurally\nand magnetically chiral MnSb2O6 are reported. The specific heat and the static\nmagnetization are investigated in magnetic fields up to 9 and 30 T,\nrespectively, while the dynamicmagnetic properties are probed by X-band as well\nas tunable high-frequency electron spin-resonance spectroscopy. Below TN = 11.5\nK, we observe antiferromagnetic resonance modes which imply small but finite\nplanar anisotropy showing up in a zero-field splitting of 20 GHz. The data are\nwell described by means of an easy-plane two-sublattice model with the\nanisotropy field BA = 0.02 T. The exchange field BE = 13 T is obtained from the\nsaturation field derived from the pulsed-field magnetization. A crucial role of\nthe small anisotropy for the spin structure is reflected by competing\nantiferromagnetic phases appearing, at T = 2 K, in small magnetic fields at BC1\n$\\approx$ 0.5 T and BC2 = 0.9 T. We discuss the results in terms of spin\nreorientation and of small magnetic fields favoring helical spin structure over\nthe cycloidal ground state which, at B = 0, is stabilized by the planar\nanisotropy. Above TN, short-range magnetic correlations up to >=60 K and\nmagnetic entropy changes well above TN reflect the frustrated triangular\narrangement of Mn2+ ions in MnSb2O6.\n