Crossover from commensurate to incommensurate antiferromagnetism in stoichiometric NaFeAs revealed by single-crystal 23Na,75As-NMR experiments

TL;DR

This study uses 23Na and 75As NMR experiments on NaFeAs single crystals to reveal a transition from commensurate to incommensurate antiferromagnetism, linked to structural changes and magnetic fluctuations.

Contribution

It provides the first detailed NMR evidence of a crossover from commensurate to incommensurate antiferromagnetic order in NaFeAs, highlighting anisotropic magnetic fluctuations.

Findings

Structural phase transition at 57 K

Antiferromagnetic transition at 45 K

Observation of commensurate-incommensurate crossover

Abstract

We report results of 23Na and 75As nuclear magnetic resonance (NMR) experiments on a self-flux grown high-quality single crystal of stoichiometric NaFeAs. The NMR spectra revealed a tetragonal to twinned-orthorhombic structural phase transition at T_O = 57 K and an antiferromagnetic (AF) transition at T_AF = 45 K. The divergent behavior of nuclear relaxation rate near T_AF shows significant anisotropy, indicating that the critical slowing down of stripe-type AF fluctuations are strongly anisotropic in spin space. The NMR spectra at low enough temperatures consist of sharp peaks showing a commensurate stripe AF order with a small moment \sim 0.3 muB. However, the spectra just below T_AF exhibits highly asymmetric broadening pointing to an incommensurate modulation. The commensurate-incommensurate crossover in NaFeAs shows a certain similarity to the behavior of SrFe2As2 under high pressure.