New magnetic orderings in Na${_x}$CoO${_2}$

Na${_x}$CoO${_2}$ has a rich phase diagram and intriguing physical properties. Very recently, it has been found that there exist further magnetic orderings at low temperatures in additional to the known 22 K antiferromagnetic phase. In this paper, we report two distinct types of ordering occurring at 8 K and 15 K for $x$=0.834 respectively, revealed by the specific heat and magnetization measurements. Both orderings are of metamagnetism.