Complex ordering of Spin Density Waves in Ruddlesden-Popper oxides*

Pr4Ni3O10 is a metallic, trilayer, Ruddlesden-Popper oxide that possesses an anomalous metal-metal transition (MMT) at a temperature of ~ 158 K. Below the MMT, Pr4Ni3O10 develops a three-dimensionally correlated charge density wave similar to its cousin La4Ni3O10, along with a spin-density wave (SDW) that is weakly correlated along the c-axis. In addition, diffusive rods along [0,0,l] are observed in the magnetic signal from Neutron diffraction. We could successfully reproduce the diffuse scattering patterns by a numerical model of short-range SDW stacking. However, unlike La4Ni3O10, as the temperature is further lowered below ~ 30 K, the local magnetic fields created by the spin density wave induce magnetic moments on the Pr3+ cations. Exchange interactions couple the Pr and Ni moments which drives the spin density wave to cross over into a three-dimensionally correlated magnetic order that is commensurate along the c-axis but incommensurate and locked to the charge density wave-vector in the basal plane.