Commensurate and incommensurate antiferromagnetic orderings in CaMn₂P₂ and SrMn₂P₂revealed by NMR

Recently much attention has been paid to the Mn-based compounds with AMn₂Pn₂ (A= Sr, Ca, Ba, and Pn = P, As, Sb, Bi) since the systems show a rich variety of magnetic properties with different crystal structures. Different from BaMn₂As₂ which shows the body-centered tetragonal ThCr₂Si₂-type structure, (Ca,Sr)Mn₂Pn₂ (Pn = P, As, Sb, Bi) crystallize in the trigonal CaAl₂Si₂-type structure where the Mn ions form a triangular lattice bilayer which can be considered as a corrugated Mn honeycomb sublattice. CaMn₂P₂ and SrMn₂P₂ have been reported to exhibit first-order antiferromagnetic (AFM) transitions at T_N = 69.8 K and 53 K, respectively [1]. In this study, we have carried out ³¹P nuclear magnetic resonance (NMR) measurements on those systems to investigate the magnetic properties from a microscopic point of view. In the case of CaMn₂P₂, we observed a clear signature of first-order AFM transition at T_N in ³¹P NMR data and found distinct splittings of ³¹P NMR lines in the AFM state. In contrast, broad NMR spectra were observed in the AFM state of SrMn₂P₂, indicating a distribution of internal fields at the P site. Those results indicate that the AFM states are commensurate and incommensurate in CaMn₂P₂ and SrMn₂P₂, respectively.



[1] N. S. Sangeetha, et al., Proc. Natl. Acad. Sci. U.S.A. 118, e2108724118 (2021).