Cluster glass behavior of frustrated birnessites AxMnO2*yH2O (A = Na, K)

We report on the synthesis and magnetic properties of frustrated Na0.22MnO20.39H2O and K0.6MnO20.48H2O with the birnessite structure. The structure, static and dynamic magnetic properties of the compounds are investigated in detail. A combination of DC and AC magnetic susceptibility measurements and magnetization decay measurements reveal cluster glass behavior below the freezing temperature of 4 K for Na-birnessite and 6 K for K-birnessite. The frequency dependence of the freezing temperature is analyzed on the basis of dynamic scaling laws including the critical slowing down formula and the Vogel-Fulcher law, which further confirm cluster glass formation in both compounds.