Proton NMR Study of Pyrochlore-Like Atacamite Mn2Cl(OH)3

The proton NMR spectrum of Mn 2 Cl(OH) 3 with zero applied field has rather sharp peaks, and a glassy property is not observed. The temperature dependence of NMR frequency shows that antiferromagnetic transition occurs at T N1 (=3.4 K), and that no anomaly appears at T N2 (=2.7 K). However, the change in frequency is considerably larger than the normal change caused by thermal fluctuation. The magnetic structure of Mn 2 Cl(OH) 3 determined by comparison with the observed proton NMR spectrum and the numerical calculated ones of various magnetic models shows that magnetic moments do not order in an “all-in all-out” orientation relative to the body center like antiferromagnetic pyrochlore, and not parallel to the local crystal field. The coexistence of antiferromagnetic and spin-glass moments does not reproduce the observed NMR spectrum. Our result suggests that the magnetic structure of atacamite Mn 2 Cl(OH) 3 is antiferromagnetic, but different from that of pyrochlore, although it is similar to the crystal...