Zero-field NMR for the Antiferromagnetic Phase of α -Mn at Ambient Pressure

Abstract

We performed detailed zero-field nuclear magnetic resonance measurements of α-Mn at ambient pressure to clarify its antiferromagnetic phase evolution. The 55 Mn internal magnetic field at site III vanishes continuously as the Néel temperature T N1 = 95 K is approached, which is consistent with a second-order antiferromagnetic transition. Between T N1 and T N2 ∼ 17.5 K, the observed number of inequivalent sites is consistent with those of the magnetic space group [Formula: see text] based on neutron diffraction. In addition, a clear change in the spectra at all sites indicates a subsequent phase transition at T N2 . Below T N2 , we observed significant variations in the integrated intensity ratio at site II (ranging from [Formula: see text] to [Formula: see text]). This may be explained based on two potential scenarios: a transition to a lower-symmetry F C m′m′2 phase, and phase coexistence model involving two [Formula: see text] phases with slightly different lattice constants. Our results resolve the long-standing discrepancies in the low-temperature magnetism of α-Mn.