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 $S=1\mathrm{/}2$ Mo${}^{5+}$ pyrophosphate, LiMoOP2O7, and its magnetic excitations. Neutron powder diffraction measurements confirm the $P{2}_1\mathrm{/}n$ 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 $T_{\text{N}}=3.5$~K and a field-induced transition at ${\mu }_0$H${}_{\text{c}}$ = 1 T. This zero-field long-range magnetic order was determined to be a commensurate $\kappa =0$ order with an ordered moment that is reduced from the full $S=1\mathrm{/}2$ 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.