Site Split of Antiferromagnetic $\alpha$-Mn Revealed by $^{55}$Mn Nuclear Magnetic Resonance

The magnetic structure of antiferromagnetic $\alpha$-Mn has been unclarified for almost 70 years since its magnetism was discovered. We measured the zero-field nuclear magnetic resonance spectra of antiferromagnetic $\alpha$-Mn to obtain further insight into magnetism below $T_{\text{N}} = 95$ K. The site II spectra split into two sites with five subpeaks owing to quadrupole interaction, and this shows that the ordered moments at site II are slightly tilted from the $[001]$ direction. The site III spectra revealed that this site splits into four sites below $T_{\text{N}}$. These findings clearly demonstrate that the antiferromagnetic $\alpha$-Mn symmetry is lower than previously considered.