# Resonant inelastic x-ray scattering study of bond order and spin excitations in nickelate thin-film structures

## TL;DR

This study uses high-resolution RIXS to analyze bond order and spin excitations in nickelate thin films, revealing how different growth directions influence magnetic order and collective excitations, advancing understanding of complex oxide physics.

## Contribution

It demonstrates the capability of RIXS to simultaneously quantify bond order and magnetic excitations in nickelate heterostructures, and distinguishes effects of spatial confinement and growth orientation.

## Key findings

- Robust non-collinear spin spiral order in (001)-oriented superlattices
- Magnons with flat dispersion and reduced energy in (111)-oriented superlattices
- RIXS effectively probes multiple order parameters in transition metal oxides

## Abstract

We used high-resolution resonant inelastic x-ray scattering (RIXS) at the Ni $L_3$ edge to simultaneously investigate high-energy interband transitions characteristic of Ni-O bond ordering and low-energy collective excitations of the Ni spins in the rare-earth nickelates $R$NiO$_3$ ($R$ = Nd, Pr, La) with pseudocubic perovskite structure. With the support of calculations based on a double-cluster model we quantify bond order (BO) amplitudes for different thin films and heterostructures and discriminate short-range BO fluctuations from long-range static order. Moreover we investigate magnetic order and exchange interactions in spatially confined $R$NiO$_3$ slabs by probing dispersive magnon excitations. While our study of superlattices (SLs) grown in the (001) direction of the perovskite structure reveals a robust non-collinear spin spiral magnetic order with dispersive magnon excitations that are essentially unperturbed by BO modulations and spatial confinement, we find magnons with flat dispersions and strongly reduced energies in SLs grown in the $(111)_{\text{pc}}$ direction that exhibit collinear magnetic order. These results give insight into the interplay of different collective ordering phenomena in a prototypical 3$d$ transition metal oxide and establish RIXS as a powerful tool to quantitatively study several order parameters and the corresponding collective excitations within one experiment.