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).