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Interfacial Control of Orbital Occupancy and Spin State in LaCoO$_3$

Ellen M. Kiens, Nicolas Gauquelin, Arno Annys, Emma van der Minne, Iris C. G. van den Bosch, Matthijs A. van Spronsen, Zezhong Zhang, Annick de Backer, Sandra van Aert, Jo Verbeeck, Gertjan Koster, Bastian Mei, Frank M. F. de Groot, Christoph Baeumer

TL;DR

This study investigates how isopolar oxide interfaces can selectively tune the Co $d$-orbital occupancy and spin state in LaCoO$_3$. By fabricating LaCoO$_3$-based multilayers with LaTiO$_3$, LaMnO$_3$, LaNiO$_3$, and LaAlO$_3$ spacers and applying X-ray absorption spectroscopy with charge transfer multiplet (CTM) analysis, the authors quantify interfacial valence and spin changes and relate them to interfacial chemistry and structure. The key findings show partial $d^7$ occupancy at Ti/Mn interfaces, partial $d^5$ occupancy at Ni interfaces, and a predominantly $d^6$ low-spin state when charge transfer is blocked, with a strong correlation between out-of-plane strain and spin-state. Overall, the work demonstrates that simultaneous control of orbital occupancy and spin state in correlated oxides can be achieved through interfacial engineering, offering insights for spin-engineering and functional oxide heterostructures.

Abstract

Transition metal oxides exhibit a wide range of tunable electronic properties arising from the complex interplay of charge, spin, and lattice degrees of freedom, governed by their $d$ orbital configurations, making them particularly interesting for oxide electronics and (electro)catalysis. Perovskite oxide heterointerfaces offer a promising route to engineer these orbital states. In this work, we tune the Co $3d$ orbital occupancy in LaCoO$_3$ from a partial $d^7$ to a partial $d^5$ state through interfacial engineering with LaTiO$_3$, LaMnO$_3$, LaAlO$_3$ and LaNiO$_3$. Using X-ray absorption spectroscopy combined with charge transfer multiplet calculations, we identify differences in the Co valence and spin state for the series of oxide heterostructures. LaTiO$_3$ and LaMnO$_3$ interfaces result in interfacial charge transfer towards LaCoO$_3$, resulting in a partial $d^7$ orbital occupancy, while a LaNiO$_3$ interface results in a partial Co $d^5$ occupancy. Strikingly, a LaAlO$_3$ spacer layer between LaNiO$_3$ and LaCoO$_3$ results in a Co $d^6$ low spin state. These results indicate that the Co spin state, like the valence state, is governed by the interfacial environment. High-resolution scanning transmission electron microscopy imaging reveals a clear connection between strain and spin configuration, emphasizing the importance of structural control at oxide interfaces. Overall, this work demonstrates that interfacial engineering simultaneously governs orbital occupancy and spin state in correlated oxides, advancing spin-engineering strategies in correlated oxides and offering new insights for the rational design of functional oxide heterostructures.

Interfacial Control of Orbital Occupancy and Spin State in LaCoO$_3$

TL;DR

This study investigates how isopolar oxide interfaces can selectively tune the Co -orbital occupancy and spin state in LaCoO. By fabricating LaCoO-based multilayers with LaTiO, LaMnO, LaNiO, and LaAlO spacers and applying X-ray absorption spectroscopy with charge transfer multiplet (CTM) analysis, the authors quantify interfacial valence and spin changes and relate them to interfacial chemistry and structure. The key findings show partial occupancy at Ti/Mn interfaces, partial occupancy at Ni interfaces, and a predominantly low-spin state when charge transfer is blocked, with a strong correlation between out-of-plane strain and spin-state. Overall, the work demonstrates that simultaneous control of orbital occupancy and spin state in correlated oxides can be achieved through interfacial engineering, offering insights for spin-engineering and functional oxide heterostructures.

Abstract

Transition metal oxides exhibit a wide range of tunable electronic properties arising from the complex interplay of charge, spin, and lattice degrees of freedom, governed by their orbital configurations, making them particularly interesting for oxide electronics and (electro)catalysis. Perovskite oxide heterointerfaces offer a promising route to engineer these orbital states. In this work, we tune the Co orbital occupancy in LaCoO from a partial to a partial state through interfacial engineering with LaTiO, LaMnO, LaAlO and LaNiO. Using X-ray absorption spectroscopy combined with charge transfer multiplet calculations, we identify differences in the Co valence and spin state for the series of oxide heterostructures. LaTiO and LaMnO interfaces result in interfacial charge transfer towards LaCoO, resulting in a partial orbital occupancy, while a LaNiO interface results in a partial Co occupancy. Strikingly, a LaAlO spacer layer between LaNiO and LaCoO results in a Co low spin state. These results indicate that the Co spin state, like the valence state, is governed by the interfacial environment. High-resolution scanning transmission electron microscopy imaging reveals a clear connection between strain and spin configuration, emphasizing the importance of structural control at oxide interfaces. Overall, this work demonstrates that interfacial engineering simultaneously governs orbital occupancy and spin state in correlated oxides, advancing spin-engineering strategies in correlated oxides and offering new insights for the rational design of functional oxide heterostructures.

Paper Structure

This paper contains 11 sections, 13 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Methods
  3. Experimental
  4. Charge transfer multiplet calculations
  5. Results and discussion
  6. Conclusions
  7. Pulsed laser deposition parameters
  8. Charge transfer multiplet calculations parameters
  9. Thin film characterization
  10. Additional XAS data and fitting
  11. EELS data

Figures (13)

  • Figure 1: Family of samples to study the four LaBO$_3$-LaCoO$_3$ interfaces, where B is (a) Ti, (b) Mn, (c) Al, (d) Ni.
  • Figure 2: X-ray diffraction measurements of the LaCoO$_3$ multilayers: (a) 2$\theta$-$\omega$ scans. (b) Reciprocal space maps of the 103 reflections of the multilayers containing the (b) LaNiO$_3$-LaCoO$_3$ interface, (c) LaAlO$_3$-LaCoO$_3$ interface, (d) LaMnO$_3$-LaCoO$_3$ interface, (e) LaTiO$_3$-LaCoO$_3$ interface.
  • Figure 3: (a) Co L$_{2,3}$ edges for different LaCoO$_3$ and a 10 nm single LaCoO$_3$ film (solid lines). Dashed lines represent a linear combination fit of the simulated spectra. (b) Simulated Co L$_{2,3}$ edges for different valence and spin states.
  • Figure 4: HAADF STEM images and EDX maps of the multilayers containing the (a) LaNiO$_3$-LaCoO$_3$ interface, (b) LaAlO$_3$-LaCoO$_3$ interface, (c) LaMnO$_3$-LaCoO$_3$ interface, (d) LaTiO$_3$-LaCoO$_3$ interface.
  • Figure 5: Out-of-plane strain mapping of the multilayers containing the (a) LaNiO$_3$-LaCoO$_3$ interface, (b) LaAlO$_3$-LaCoO$_3$ interface, (c) LaMnO$_3$-LaCoO$_3$ interface, (d) LaTiO$_3$-LaCoO$_3$ interface. White outlined regions are used to calculate the average out of plane strain per layer. (e) Average out of plane strain as a function of distance from the interface. Averaging is performed horizontally across the white area outlined in (a)-(d). The LaCoO$_3$ layer is indicated by the red shaded region, while the neighboring layers are indicated by the grey shaded region. The white region corresponds to the LaNiO$_3$. Error bars indicate standard errors. The legend labels indicate the neighboring layer to the LaCoO$_3$ layer.
  • ...and 8 more figures