Commensurate and incommensurate antiferromagnetic orderings in SrMn₂As₂ and CaMn₂As₂revealed 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.  Among the Mn compounds, CaMn₂As₂ and SrMn₂As₂ have been reported to be antiferromagnetic (AFM) insulators with T_N = 62 K and 120 K, respectively. In this study, we have carried out ⁷⁵As nuclear magnetic resonance (NMR) measurements on those systems to investigate the magnetic properties from a microscopic point of view. In the case of SrMn₂As₂, no obvious distribution of the internal field at the As site was observed in the AFM state. In contrast, broad and complex NMR spectra were observed in CaMn₂As₂ in the AFM state, which clearly shows a distribution of internal field at the As site. Those results indicate that the antiferromagnetic states are commensurate and incommensurate in SrMn₂As₂ and CaMn₂As₂, respectively.