Cascade of Bulk Magnetic Phase Transitions inNaxCoO2as Studied by Muon Spin Rotation

Abstract

Using muon spin rotation, well-defined bulk $\sim 100\%$ magnetic phases in ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ are revealed. A novel magnetic phase is detected for $x=0.85$ with the highest transition temperature ever observed for $x\ge 0.75$. This stresses the diversity of $x\ge 0.75$ magnetic phases and the link between magnetic and structural degrees of freedom. For the charge-ordered $x=0.50$ compound, a cascade of transitions is observed below 85 K. From a detailed analysis of our data, we conclude that the ordered moment varies continuously with temperature and suggest that the two secondary transitions at 48 and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.