Collinear and Noncollinear Antiferromagnetic Ordering in Decorated Square Kagomé Lattice Antiferromagnets of the Nabokoite Family
Glazkov V.N., Rebrov Ya.V., Dubovitskii M.A., Markina M.M., Zakharov K.V., Murtazoev A.F., Berdonosov P.S., Vasiliev A.N.

Received: August 31, 2026

DOI: 10.31857/S0044451026120121

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Nabokoite family compounds @cee ACu7(TeO4)(SO4)5X (A=Na, K, Rb, Cs; X=Cl, Br) host frustrated two-dimensional square kagomé lattice layers decorated by additional inter-layer magnetic ions. Despite the high Curie-Weiss temperatures Θ _CW\simeq 100-150 K, all nabokoite family compounds order only at T_C\simeq 3-6 K\ll Θ _CW, highlighting the role of magnetic frustration in nabokoites. We study magnetic order in nabokoites with multi-frequency electron spin resonance spectroscopy and thermodynamic measurements (specific heat, magnetization, and dielectric permittivity). Our study reveals that the choice of the low-temperature ground state is qualitatively different in light-alkali-ion (A=K, Na) and heavy-alkali-ion (A=Rb, Cs) compounds. Heavy-alkali-ion nabokoites order in a conventional collinear antiferromagnetic pattern with easy-axis anisotropy. The parameters of the ordered antiferromagnetic state are very close for the whole heavy-alkali-ion subfamily: zero-field magnon gap at 1.7 K is about 110-115 GHz and spin-flop field is about 3.5-4.0 T. Light-alkali-ion members of nabokoite family demonstrate a much more complicated route to the ordered state: firstly, a ferroelectric transition at 25-90 K partially lifts the frustration and thus pre-cooks the low-temperature ordering; secondly, an unusual noncollinear magnetic order develops via a two-step phase transition with the first transition temperature T_C1\simeq 5-6 K and the second transition at T_C2\simeq 3-4 K. Noncollinear order is evidenced by observation of a characteristic non-Larmor antiferromagnetic resonance mode. Spin dynamics of light-alkali-ion nabokoites is characterized by three zero-field magnon gaps and two spin-reorientation fields and, contrary to the heavy-alkali-ion subfamily, the values of magnon gaps and critical fields are quite different for different light-alkali-ion compounds. The finite-size cluster modeling of the pyramidal structural block of nabokoite structure combined with DFT estimates suggests that the critical closeness of the nabokoite exchange coupling parameters to the border-line between different quantum ground states of the pyramidal building block of nabokoite structure could be the clue to the choice of qualitatively different ordered states in light- and heavy-alkali-ion nabokoites. Keywords: two-dimensional decorated square kagomé lattice, magnetic frustration, antiferromagnetic order, electron spin resonance