Signatures of polarized chiral spin disproportionation in rare earth nickelates

TL;DR

This study reveals chiral spin-disproportionation in rare earth nickelates, showing how ligand hole pairing leads to complex magnetic order and potential multiferroic behavior, confirmed through resonant X-ray scattering and theoretical calculations.

Contribution

It provides experimental evidence of chiral spin order and ligand hole pairing in NdNiO$_3$, advancing understanding of the microscopic origin of phase transitions in rare earth nickelates.

Findings

Observation of chiral spin spirals propagating along (101)$_\mathrm{ortho}$

Coupling of spin spirals to X-ray helicity indicating macroscopic chirality

Confirmation of spin-disproportionated state with ligand hole singlets through theoretical calculations

Abstract

In rare earth nickelates (RENiO$_3$), electron-lattice coupling drives a concurrent metal-to-insulator and bond disproportionation phase transition whose microscopic origin has long been the subject of active debate. Of several proposed mechanisms, here we test the hypothesis that pairs of self-doped ligand holes spatially condense to provide local spin moments that are antiferromagnetically coupled to Ni spins. These singlet-like states provide a basis for long-range bond and spiral spin order. Using magnetic resonant X-ray scattering on NdNiO$_3$ thin films, we observe the chiral nature of the spin-disproportionated state, with spin spirals propagating along the crystallographic (101)$_\mathrm{ortho}$ direction. These spin spirals are found to preferentially couple to X-ray helicity, establishing the presence of a hitherto-unobserved macroscopic chirality. The presence of this chiral magnetic configuration suggests a potential multiferroic coupling between the noncollinear magnetic arrangement and improper ferroelectric behavior as observed in prior studies on NdNiO$_3$ (101)$_\mathrm{ortho}$ films and RENiO$_3$ single crystals. Experimentally constrained theoretical double-cluster calculations confirm the presence of an energetically stable spin-disproportionated state with Zhang-Rice singlet-like combinations of Ni and ligand moments.