Proton NMR study of atacamite Cu2₂Cl(OH)₃

The proton NMR spectrum of Cu2₂Cl(OH)₃ at low temperature in zero applied field has two sharp peaks. This result means that the majority of magnetic moments order antiferromagnetically at low temperature. The magnetic structure determined by comparing the observed proton NMR spectrum with the numerical calculated ones in several magnetic models shows that the magnetic moments order ''all-in all-out'' orienting to body center. This magnetic structure is similar to antiferromagnetic pyrochlore with geometric frustration, and different from the one of atacamite Mn2Cl(OH)₃, though having the same crystal structure. The magnitude of magnetic moments of Cu(I) is 1.12-B and Cu(II) 0.25-B. In increasing temperature, the number of moments deviated from ''all-in all-out'' directions becomes large. However, the clear evidence of the disordering or spin-glass moments is not detected from observed NMR spectrum.