Rare-Earth Mediated Cross-Over in Dimensionality of a Spin Density Wave in a Ruddlesden-Popper Nickelate

Nickelates allow for the investigation of charge and spin density waves (CDWs and SDWs), which are quantum states recognized to be of fundamental importance in modern condensed matter physics . An intriguing metal-metal transition occurs at temperature T_MMT in R₄Ni₃O₁₀ (R=trivalent rare earth). Recently, the metal-metal transition was argued to arise from the simultaneous formation of a strongly coupled quasi-2D SDW and 3D CDW based on analysis of incommensurate superlattice peaks observed in single-crystal neutron and X-ray diffraction of La₄Ni₃O₁₀ below T_MMT ≈ 148 K [1]. In this work, we investigated the effect of replacing non-magnetic La³⁺ cations with Pr³⁺ (4f²). Neutron and resonant x-ray scattering evidenced an ordered, induced SDW on the Pr³⁺ cations well below T_MMT which leads to a 2D-3D magnetic cross-over. We developed models of stacked SDW layers for the intermediate-temperature (Ni moment only) state on warming and cooling as well as the ground-state which contained SDWs on both the Ni and Pr³⁺ sites. The modelling determined that there was a reorientation of the spin direction in the ground-state as well as the phase shifts between layers that occurred due to exchange interactions along c. A Hamiltonian including the exchange interactions and single ion anisotropy was developed which explains observed thermal hysteresis.

[1] J. Zhang et al., Nat. Commun. 11, 6003 (2020).