Neutron Scattering Studies of Magnetic Structure and Excitations in Na$_{5/8}$MnO$_2$

Na$_x$TMO$_2$ (TM=transition metal) materials consist of alternating layers of Na and TM ions with the TM ions arranged on a geometrically frustrated triangular lattice. Na can be easily and reversibly removed from these materials, making them of interest for application in rechargeable batteries and allowing for exploration of their rich phase diagrams as a function of Na concentration. Na ordering is an important factor in ground state selection, and is driven by electrostatic interactions in many Na$_x$TMO$_2$ systems. The series Na$_x$MnO$_2$ differs in that Na ordering is driven by a cooperative Jahn-Teller effect, due to the coexistence of Jahn-Teller active Mn$^{3+}$ and non-active Mn$^{4+}$ ions. We have recently shown the existence of a charge stripe ordering a in the material Na$_{5/8}$MnO$_2$ [1]. At low temperatures a magnetic stripe order also develops. We present neutron diffraction and inelastic scattering measurements and examine the details of the magnetic structure and excitations in the magnetic stripe ordered phase. [1] X. Li \textit{et al.} Nature Mater. \textbf{13}, 586 (2014)