Incommensurate and commensurate antiferromagnetic states in CaMn2As2 and SrMn2As2 revealed by As-75 NMR

We carried out As-75 nuclear magnetic resonance (NMR) measurements on the trigonal CaMn2As2 and SrMn2As2 insulators exhibiting antiferromagnetic (AFM) ordered states below Neel temperatures T-N = 62 and 120 K, respectively. In the paramagnetic state above T-N, typical quadrupolar-split As-75 NMR spectra were observed for both systems. The As-75 quadrupolar frequency nu(Q) for CaMn2As2 decreases with decreasing temperature, while nu(Q) for SrMn2As2 increases, showing an opposite temperature dependence. In the AFM state, the relatively sharp and distinct As-75 NMR lines were observed in SrMn2As2 and the NMR spectra were shifted to lower fields for both magnetic fields H parallel to c axis and H parallel to ab plane, suggesting that the internal fields B-int at the As site produced by the Mn ordered moments are nearly perpendicular to the external magnetic field direction. No obvious distribution of B-int was observed in SrMn2As2, which clearly indicates a commensurate AFM state. In sharp contrast to SrMn2As2, broad and complex NMR spectra were observed in CaMn2As2 in the AFM state, which clearly shows a distribution of B-int at the As site, indicating an incommensurate state. From the analysis of the characteristic shape of the observed spectra, the AFM state of CaMn2As2 was determined to be a two-dimensional incommensurate state where Mn ordered moments are aligned in the ab plane. A possible origin for the different AFM states in the systems was discussed. Both CaMn2As2 and SrMn2As2 show very large anisotropy in the nuclear spin-lattice relaxation rate 1/T-i in the paramagnetic state. 1/T-i for H parallel to ab is much larger than that for H parallel to c, indicating strong anisotropic AFM spin fluctuations in both compounds.