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

Q. -P. Ding; N. S. Sangeetha; Abhishek Pandey; D. C. Johnston; Y. Furukawa

doi:10.1103/PhysRevB.104.224413

paper

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

arXiv:2112.00836 · doi:10.1103/PhysRevB.104.224413

Abstract

We carried out $^{75}$ As nuclear magnetic resonance (NMR) measurements on the trigonal CaMn $_{2}$ As $_{2}$ and SrMn $_{2}$ As $_{2}$ insulators exhibiting antiferromagnetic (AFM) ordered states below Néel temperatures $T_{N}$ = 62 K and 120 K, respectively. In the paramagnetic state above $T_{N}$ , typical quadrupolar-split $^{75}$ As-NMR spectra were observed for both systems. The $^{75}$ As quadrupolar frequency $ν_{Q}$ for CaMn $_{2}$ As $_{2}$ decreases with decreasing temperature, while $ν_{Q}$ for SrMn $_{2}$ As $_{2}$ increases, showing an opposite temperature dependence.In the AFM state, the relatively sharp and distinct $^{75}$ As NMR lines were observed in SrMn $_{2}$ As $_{2}$ and the NMR spectra were shifted to lower fields for both magnetic fields $H$ $∣∣$ $c$ axis and $H$ $∣∣$ $ab$ 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 SrMn $_{2}$ As $_{2}$ , which clearly indicates a commensurate AFM state. In sharp contrast to SrMn $_{2}$ As $_{2}$ , broad and complex NMR spectra were observed in CaMn $_{2}$ As $_{2}$ 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 CaMn $_{2}$ As $_{2}$ 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 CaMn $_{2}$ As $_{2}$ and SrMn $_{2}$ As $_{2}$ show very large anisotropy in the nuclear spin-lattice relaxation rate 1/ $T_{1}$ in the paramagnetic state. 1/ $T_{1}$ for $H$ $∣∣$ $ab$ is much larger than that for $H$ $∣∣$ c$, indicating strong anisotropic AFM spin fluctuations in both compounds.

10 pages, 11 figures, accepted for publication in Phys. Rev. B

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Linear magnetoelectricity in the Zintl phase pnictides (Ba, Ca, Sr) $Mn_{2} (P, As, Sb)_{2}$ from first principles calculations