Time-of-flight elastic and inelastic neutron scattering studies on the localized 4d electron layered perovskite La₅Mo₄O₁₆

The magnetic structure and spin-wave excitations in the quasi-square-lattice layered perovskite compound La₅Mo₄O₁₆ were studied by a combination of neutron diffraction and inelastic neutron scattering techniques using polycrystalline sample. Neutron powder diffraction refinement revealed that the magnetic structure is ferrimagnetic in the ab plane with antiferromagnetic stacking along the c-axis where the magnetic propagation vector is k ¼ ð0;0;1=2Þ. The ordered magnetic moments are estimated to be 0.54(2)μ_B for Mo⁵⁺ (4d¹) ions and 1.07(3)μ_B for Mo⁴⁺ (4d²) ions at 4K, which are about half of the expected values. The inelastic neutron scattering results display strong easy-axis magnetic anisotropy along the c-axis due to the spin–orbit interaction in Mo ions evidenced by the spin gap at the magnetic zone center. The model Hamiltonian consisting of in-plane anisotropic exchange interactions, the interlayer exchange interaction, and easy-axis single-ion anisotropy can explain our inelastic neutron scattering data well. Strong Ising-like anisotropy and weak interlayer coupling compared with the intralayer exchange interaction can explain both the high-temperature magnetoresistance and long-time magnetization decay recently observed in La₅Mo₄O₁₆.