Signatures of polarized chiral spin disproportionation in rare earth nickelates (2408.17450v2)

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.

• The paper identifies a chiral magnetic ground state using circular dichroism in polarized resonant X-ray scattering.
• It quantifies a significant spin disproportionation with a magnetic moment ratio of 3.2 ± 0.4 between inequivalent Ni sites.
• The findings imply potential multiferroic coupling and pave the way for advanced spintronic and quantum device applications.

Signatures of Polarized Chiral Spin Disproportionation in Rare Earth Nickelates

The paper of rare earth nickelates (RENiO$_3$) presents intriguing insights into the interplay between lattice dynamics, electronic properties, and magnetic interactions. The research paper titled "Signatures of Polarized Chiral Spin Disproportionation in Rare Earth Nickelates" provides an in-depth exploration of the multiferroic coupling phenomena observed in these materials, emphasizing the unique antiferromagnetic (AFM) characteristics within their bond/charge disproportionated insulating phases.

Overview and Methodology

The primary focus of the paper is on the detailed examination of the spin-disproportionated states within RENiO$_3$, particularly in NdNiO$_3$ thin films. Leveraging polarized resonant magnetic X-ray scattering, the authors successfully discern a chiral magnetic ground state characterized by significant macroscopic chirality. The research method emphasizes utilizing circular dichroism (CD) to distinguish between collinear and noncollinear magnetic structures, thereby unveiling a helical magnetic configuration compatible with a $P_{2a}2_1$ space group symmetry.

Experimentation involved probing both site-resolved X-ray spectra and the azimuthal angle dependence of scattering intensity. The azimuthal angle dependence revealed noncollinear spiral magnetic structures, while the magnetic space group analysis supported a helical-like structure distinctively observed in (101)$_\mathrm{ortho}$ NNO films.

Results and Implications

The paper reports significant findings concerning the AFM state of RENiO$_3$, characterized by a period-four magnetic supercell with distinct Ni$_{A,B}$ sites and notable circular dichroism. The results indicate a remarkable chiral imbalance in domain populations (64% right-handed, 36% left-handed), suggesting inherent stochastic properties while also pointing to potential multiferroic characteristics within these systems due to the interplay between magnetism and lattice dynamics.

In terms of numerical results, the authors ascertain a significant difference in magnetic moment sizes at the two inequivalent nickel sites, noted as a magnetic moment ratio $$|\mathbf{m}_\mathrm{A}|/|\mathbf{m}_\mathrm{B}| = 3.2 \pm 0.4$$. This sizeable spin disproportionation reflects antiferromagnetically coupled spins forming spatially modulated Zhang-Rice singlet-like states, further corroborated by theoretical double-cluster calculations.

Theoretical Speculations and Future Prospects

The insights drawn from this paper broaden the understanding of symmetry breaking phenomena in complex oxide materials. Specifically, the documented macroscopic chirality and the potential multiferroic nature suggest promising directions for future research. Understanding the precise mechanisms of spin-lattice interaction opens avenues for tailoring materials with advanced functionalities such as efficient spintronic or quantum computing devices.

Moreover, the exploration of noncollinear AFM orders could inspire leveraging the magnetoelectric coupling in engineered heterostructures, rendering RENiO$_3$ and similar compounds as potent candidates for next-generation electronic components. Future research could particularly delve into the connection between magnetic texture and improper ferroelectric behavior.

Conclusion

This research marks a significant stride in unraveling the intricate electronic and magnetic interactions within rare earth nickelates, particularly unraveling the chiral spin disproportionation signature in NdNiO$_3$. Although further exploration is necessary to demystify the comprehensive nature of observed phenomena and potential practical applications, the paper positions RENiO$_3$ nickelates as compelling models for multiferroic research and a deeper exploration into fundamental physics governing correlated electron systems.