Magnetic properties and excitation spectrum of the S = 1/2 three-legged spin ladder, LiMoOP2O7
We report a comprehensive experimental study including density functional theory calculations detailing the magnetic ground state of the $S = 1/2$ Mo$^{5+}$ pyrophosphate, LiMoOP2O7, and its magnetic excitations. Neutron powder diffraction measurements confirm the $P2_1/n$ structural model which is comprised of a three-legged ladder of distorted Mo$^{5+}$ octahedra. Magnetometry and specific heat reveal a long-range magnetic ordering transition at $T_\textrm{N} = 3.5$~K and a field-induced transition at $μ_{0}$H$_\textrm{c}$ = 1 T. This zero-field long-range magnetic order was determined to be a commensurate $κ= 0$ order with an ordered moment that is reduced from the full $S = 1/2$ magnetic moment by 30%. Through inelastic neutron scattering measurements and density functional theory calculations, we define the minimal microscopic model and resolve the spin Hamiltonian up to the seventh leading exchange parameter, resulting in a model with dominant antiferromagnetic leg interactions, weaker ferromagnetic rung interactions, and a significant interlayer ferromagnetic coupling.