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

Abstract

We used high-resolution resonant inelastic x-ray scattering (RIXS) at the Ni L3 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 RNiO3 (R = Nd, Pr, La) with pseudocubic perovskite structure. With the support of calculations based on a doublecluster 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 RNiO3 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)pc direction that exhibit collinear magnetic order. These results give insight into the interplay of different collective ordering phenomena in a prototypical 3d transition metal oxide and establish RIXS as a powerful tool to quantitatively study several order parameters and the corresponding collective excitations within one experiment.

Probing photoinduced rearrangements in the NdNiO3 magnetic spiral with polarization-sensitive ultrafast resonant soft x-ray scattering

“…Most notably, the size of m LB /m SB is found to decrease at early time delays to a value below 1, followed by a gradual nanosecond recovery back to equilibrium. This suggests that the light excitation creates a state where the LB moment becomes smaller than the SB moment, which is inverse to the spin disproportionation found in the equilibrium low-energy spin model [18,20]. This state also differs from the equilibrium metallic state, as in this case no bond disproportionation between the two nickel sites is expected.…”

“…Powder neutrondiffraction measurements indicated a collinear AFM structure of the spins (↑↑↓↓) [13,14], while polarization-sensitive resonant soft x-ray scattering [15][16][17] and inelastic x-ray scattering [18] on thin films have found a noncollinear spin spiral structure (↑→↓←). Recent x-ray scattering studies on thin films have also found the spins to have a canting angle less than 90°, with a collinear structure emerging in the limit of monolayer thickness [19,20].…”

Oxygen Hole Character and Lateral Homogeneity in PrNiO2+δ Thin Films

“…Figure 2A shows the XAS measurements taken in the standard experimental setup at the Ni-L edge for both pristine and reduced pieces of the same sample. For the pristine piece, we observe the well-known double-peak structure at the Ni-L 3 edge, indicative of charge fluctuations in the metallic phase [23][24][25]. Upon topotactical reduction, both the Ni-L 2 and L 3 edges shift towards lower energies, suggesting the transition from a Ni 3+ oxidation state towards Ni 2+ and Ni 1+ [11].…”

On the magnetic structure and magnetic behaviour of the most distorted member of the series of RNiO3 perovskites (R = Lu)

“…By contrast, for the subsequent members, the TMI increases from PrNiO3 (TMI = 130 K) to LuNiO3 (TMI =599 K) and the TN shows a monotonic variation. The nature of the electronic transition has been systematically studied by spectroscopic and diffraction techniques [19][20][21][22][23] . Charge disproportionation 15,[24][25][26][27][28] and charge ordering 29,30 are the main mechanisms proposed to explain charge localization in the insulating regime, as the smaller rare-earth derivatives (R= Y, Dy  Lu) display a concomitant transition from the more regular perovskite high-temperature orthorhombic (Pbnm) structure to the low-temperature insulating monoclinic (P21/n) structure, which shows an alternating breathing-like distortion of the NiO6 octahedral arrangement.…”