Site-Selective Probe of Magnetic Excitations in Rare-Earth Nickelates Using Resonant Inelastic X-ray Scattering

Abstract

We use high-resolution resonant inelastic x-ray scattering (RIXS) to study a thin film ofNdNiO3, a compound whose unusual spin- and bond-ordered electronic ground state has been of long-standing interest. Below the magnetic ordering temperature, we observe well-defined collective magnon excitations along different high-symmetry directions in momentum space. The magnetic spectra depend strongly on the incident photon energy, which we attribute to RIXS coupling to different local electronic configurations of the expanded and compressedNiO6octahedra in the bond-ordered state. Both the noncollinear magnetic ground state and the observed site-dependent magnon excitations are well described by a model that assumes strong competition between the antiferromagnetic superexchange and ferromagnetic double-exchange interactions. Our study provides direct insight into the magnetic dynamics and exchange interactions of the rare-earth nickelates and demonstrates that RIXS can serve as a site-selective probe of magnetism in these and other materials.

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Plain Language Summary

Future spintronic devices, which use the spin of electrons in addition to their charge to store and transfer information, could outperform conventional electronics. Much recent research in this field has focused on how spins intermingle in transition metal oxides. A key issue for understanding this complex physics is how to selectively probe the electronic and magnetic properties of the different components of these compounds. We leverage the latest advances in instrumentation to probe the magnetic dynamics in a rare-earth nickelate,NdNiO3.

TheRNiO3family (whereRstands for “rare earth”) has gained a lot of attention recently because of its rich phase diagram, comprising a metal-insulator transition and an unusual magnetic ground state. After decades of studies on this system, our resonant inelastic x-ray scattering (RIXS) data finally yield the strength and spatial range of the magnetic exchange interactions, thereby providing a new basis for microscopic models of these materials and an explanation for their unique noncollinear magnetic order.

Specifically, we show that the partial cancellation between ferromagnetic and antiferromagnetic interactions reduces the net exchange coupling between nearest-neighbor Ni spins, so longer-range interactions dominate. The unique noncollinear magnetic ground state ofRNiO3is thus a result of competing short-range and long-range interactions.

Alongside this key contribution to a long-standing scientific problem, we show that RIXS can be used as a site-selective probe of magnetic excitations in metal oxides with different metal valence states, a capability that goes beyond those of previous experiments and that will surely find many applications in other oxides and complex systems.