GMAG Student Dissertation Award: Amplitude mode in a spatially anisotropic 2D lattice: α-NaMnO2

The orbitally active system, α-NaMnO₂, has long been intriguing to researchers for its potential as a cathode material; however it is also known to possess intriguing magnetic properties. The lattice is fundamentally comprised of a triangular lattice of edge-sharing MnO₆ octahedra separated by sheets of Na cations. A cooperative Jahn-Teller distortion of the MnO₆ octahedra (Mn³⁺, t_2g³e_g¹, S=2) restructures this hexagonal plane and directly affects the electronic (ferro-orbital) and magnetic (antiferromagnetic) properties. The Jahn-Teller distortion results in the formation of an anisotropic triangular motif in the basal plane, where nearest neighbor Mn atoms, along the b-axis, are much closer than next-nearest neighbor Mn atoms. The magnetic lattice thus behaves as weakly coupled spin chains with predominately 1D spin excitations. The magnon dispersion resulting from linear spin wave theory of a J₁-J₂ Heisenberg model with single-ion anisotropy, D, fits well to the expected transverse modes in the inelastic channel. Additionally, careful polarized neutron scattering experiments were carried out to confirm the surprising presence of an additional mode in the spin excitation spectra, which was determined to be longitudinally polarized. This coherent and long-lived excitation is an unusual mode for a nominally classical antiferromagnet and its origins will be discussed.