MAGNETIC STUDIES OF MANGANESE OXIDE OCTAHEDRAL MOLECULAR-SIEVES - A NEW CLASS OF SPIN-GLASSES

Ac and dc magnetic susceptibility measurements of octahedral molecular sieves (OMS) of mixed-valent manganese oxide composition such as 81-1 .35Mn2+1.89-1.94Mn4+4.38-4.54)O12.4.47-4.55H2O for synthetic todorokit e and KMn8O16 for synthetic cryptomelane are reported here. Data for t emperature and frequency dependence of the real components of the ac s usceptibility suggest that synthetic cryptomelane (K-OMS-2) materials are spin glasses having a typical T(f)(omega) dependence. The imaginar y components of the ac susceptibility at various frequencies also show behavior that is typical of spin glass systems. Zero-field-cooled and field-cooled dc magnetization data as a function of temperature exhib it irreversibility, again consistent with the spin glass assignment fo r synthetic cryptomelane. Synthetic todorokite having a 3 x 3 MnO6 oct ahedral tunnel structure does not appear to be a true spin glass.