Spin-glass behavior in hexagonalNa0.70MnO2

Abstract

In this paper, we present an experimental study of the hexagonal compound ${\mathrm{Na}}_{0.70}\mathrm{Mn}{\mathrm{O}}_{2}$. 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 ${\mathrm{Na}}_{0.70}\mathrm{Mn}{\mathrm{O}}_{2}$ undergoes a spin-glass transition at ${T}_{f}=39\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The spin-glass order parameter $q(T)$ determined from the dc spin susceptibility exhibits the relation $q(T)\ensuremath{\propto}(1\ensuremath{-}T∕{T}_{f})$, 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 $\ensuremath{\lambda}∕{t}_{w}^{\ensuremath{\mu}}$. The magnetic entropy extracted from the specific heat implies that the spin degrees of freedom of ${\mathrm{Mn}}^{3+}∕{\mathrm{Mn}}^{4+}$ ions are completely frozen at low temperatures, and the origin of this spin-glass behavior is attributed to the mixture of ${\mathrm{Mn}}^{3+}∕{\mathrm{Mn}}^{4+}$ ions and geometrical frustrations on the triangular lattices. Comparisons with the magnetic properties of ${\mathrm{Na}}_{0.70}\mathrm{Co}{\mathrm{O}}_{2}$ are also made.