Spin-glass behavior in hexagonal Na0.70MnO2

In this paper, we present an experimental study of the hexagonal compound Na0.70MnO2. Zero-field-cooled and field-cooled susceptibilities display divergences at low temperatures, and the magnetic measurements of frequency dependence of ac susceptibility, hysteresis effect, and long-time relaxation are performed, indicating that Na0.70MnO2 undergoes a spin-glass transition at T-f=39 K. The spin-glass order parameter q(T) determined from the dc spin susceptibility exhibits the relation q(T)proportional to(1-T/T-f), in agreement with the prediction of conventional spin-glass theory. Spin dynamics in the spin-glass state is carefully examined, and the time decay of the thermoremanent magnetization can be well scaled with a reduced effective waiting time lambda/t(w)(mu). The magnetic entropy extracted from the specific heat implies that the spin degrees of freedom of Mn3+/Mn4+ ions are completely frozen at low temperatures, and the origin of this spin-glass behavior is attributed to the mixture of Mn3+/Mn4+ ions and geometrical frustrations on the triangular lattices. Comparisons with the magnetic properties of Na0.70CoO2 are also made.