X-ray spectroscopy sheds light on the magnetism of oxygen-vacancy-rich nanostructures

Nanomagnetic materials – that is, nanoscale magnetic objects – are key components of various modern technologies, as they process information very efficiently. Those based on semiconductors are particularly attractive, as they provide the potential for “spintronic” control of both the electronic charge and the spin of an electron. For practical spintronic applications, these materials must also exhibit room-temperature ferromagnetism. In this regard, nanoscale titanium dioxide doped with cobalt (Co/TiO₂) and with abundant oxygen vacancies is a promising candidate, because it exhibits spontaneous ferromagnetism with a Curie temperature above 400 K – more than twice that of arsenic doped with gallium and manganese, which has been widely studied [1]. However, a significant challenge with Co/TiO₂ arises in the formation of subnanometre-sized cobalt clusters, which have Curie temperatures as high as 1360 K, and which are difficult to design and detect with conventional probes. This raises two key questions: Can dilute magnetic semiconductors be designed with rich oxygen vacancies but without atomic-level clustering? And what mechanisms drive the magnetism in a site-specific, single-atom-incorporated nanostructure?

This study addresses these questions with a combination of advanced synthesis methods, experimental techniques and first-principles calculations, including transmission electron microscopy (TEM), superconducting quantum interference device (SQUID) magnetometry, X-ray diffraction (XRD), high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS), X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS). The experiments were conducted at the ID26 beamline and the Pohang Accelerator Laboratory in Korea to probe the crystal structure, electronic and spin state, and overall magnetism. 

The TiO₂ nanostructures containing isolated single cobalt atoms were synthesized using a “wrap-bake-peel” strategy [2]. To investigate their structure, morphology, and composition, high-resolution TEM (Figure 1a-c), inductively coupled plasma atomic-emission spectroscopy, XRD, and Rietveld refinements were performed. The results revealed a uniform spherical morphology with an average particle size of 5 nm, and the cobalt content was determined to be 1.5 wt.%.

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Fig. 1: Representative TEM images of Co/TiO₂ at low (a) and (b) and high (c) magnifications. d) Ti K pre-edge HERFD spectra (top), and second derivatives (bottom). e) Co K-edge HERFD-XAS spectrum of Co/TiO₂ is compared with the simulation. f) EXAFS spectrum (dots) and theoretical fit (solid lines) of Co/TiO₂ at the Co K-edge. g) Magnetization (M) as a function of the applied field of Co/TiO₂ nanocrystals measured at room temperature. h) Co/TiO₂ spectrum (solid line) compared with the Co²⁺ high-spin reference (dashed line) in distorted octahedral geometry.

Element-selective HERFD-XAS measurements at the Ti K-edge confirmed the “anatase” phase of the nanocrystallites and showed no presence of Ti³⁺ ions, indicating a constant fraction of oxygen vacancies (Figure 1d). Cobalt K-edge HERFD-XAS experiments, supported by finite difference method near-edge structure simulations, confirmed the single-atom incorporation of cobalt and provided evidence for equatorial oxygen vacancies (Figure 1e). Additionally, extended X-ray absorption fine structure (EXAFS) measurements, which are sensitive to the local coordination of the absorbing atom, further validated the single-atom incorporation of cobalt and the presence of equatorial oxygen vacancies (Figure 1f).

SQUID measurements confirmed that Co/TiO₂ clearly exhibits room-temperature ferromagnetism (Figure 1g), whereas TiO₂ nanostructures, even with oxygen vacancies, exhibit none. This indicates a synergistic effect in which cobalt and oxygen vacancies are crucial for the intrinsic ferromagnetism.

To further investigate the mechanism of ferromagnetism and the spin state of cobalt, Kβ XES measurements were conducted. Comparison of the Kβ mainlines of Co/TiO₂ with experimental standards revealed that Co²⁺ exists in a high-spin configuration (Figure 1h) with distorted symmetry caused by oxygen vacancies.

To further understand the origin of magnetic interactions, first-principles calculations were performed, taking into account the electronic structure and spin state identified through X-ray spectroscopy. As shown in Figure 2a, various locations of oxygen vacancies (V_O) relative to cobalt were examined: (i) two nearest-neighbour positions (labelled 1-1 and 1-2), (ii) five next-nearest-neighbour positions (labelled 2-1 to 2-5), and (iii) a configuration where Co_Ti and V_O are widely separated. In all cases, the cobalt remains robustly in the 2+ oxidation state with a high-spin configuration (t⁵_2ge²_g, S = 3/2).

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Fig. 2: a) Position of a single Co ion and a single oxygen vacancy (V_O). 1−N and 2−N (N = 1 to 4) indicate the nearest- and next-nearest-neighbouring positions, respectively. b) Energies of single Co and single VO in TiO₂, for both high-spin and low-spin. c) Schematic crystal field splitting of Co d bands. d) Projected density of states onto Co d, Ti d, and O p orbitals for the most stable configuration 1-1. The Fermi level is set to zero, and U(Ti) = U(Co) = 0.  e) The experimental RIXS (left), and theoretical RIXS (right) for high-spin Co²⁺ in a distorted octahedral symmetry. f) Energies of two 1Co_Ti + 1V_O as a function of Co–Co distance, for both FM and AFM configurations. g) ∆E = E[AFM]−E[FM] as a function of electron doping per Co. h) Graphical representation of the magnetic model. FM interactions are mediated by Co_Ti + V_O states as a result of the electron doping.

1s3p RIXS measurements were performed to provide a clearer understanding of the population and symmetry of the cobalt states. During the RIXS process, a core 1s electron is excited into an unoccupied 3d state (1s → 3d), followed by the relaxation of a core 3p electron (3p → 1s). Multiplet calculations, incorporating the t_2g-e_g crystal field splitting of cobalt orbitals in tetragonal symmetry, were used to interpret the data. The strong agreement between the simulations and experimental results confirms that Co²⁺ exists in a high-spin state (Figure 2b-e).

Using detailed information from X-ray spectroscopy experiments and simulations to investigate the origin of ferromagnetic (FM) properties, the magnetic interactions between two Co²⁺ ions were analysed. The calculated FM-AFM energy difference (ΔE) was examined as a function of doped electrons per cobalt atom (Figure 2f-g). The results revealed that electron doping stabilizes ferromagnetism and significantly enhances ferromagnetic interactions.

This research confirms that the single-atom incorporation strategy enables the design of oxygen-vacancy-rich magnetic semiconductors while avoiding atomic-level clustering. By employing advanced synchrotron-based X-ray spectroscopy techniques and calculations, the findings challenge the prevailing assumption that ferromagnetism arises from Ti³⁺ + V_O carriers.

The results show that the FM stability between Co²⁺ ions is very weak. However, electron doping from additional oxygen vacancies can significantly enhance it, which explains the observed room-temperature ferromagnetism.

Furthermore, this research underscores the potential of single-atom-incorporated nanostructures in offering a foundation for designing dilute semiconductor oxides. These materials hold promise for applications in multifunctional nanospintronics, nanoelectronics, and neuromorphic devices.

Principal publication and authors
Ferromagnetic stability optimization via oxygen-vacancy control in single-atom Co/TiO₂ nanostructures, V.K. Paidi (a), B.-H. Lee (b), A.T. Lee (c), S. Ismail-Beigi (c), E. Grishaeva (a), S. Vasala (a), P. Glatzel (a), W. Ko (d,e), D. Ahn (f), T. Hyeon (d,e), Y. Kim (f), K.-S. Lee (f), Proc. Natl. Acad. Sci. 121 (48) (2024); https://doi.org/10.1073/pnas.2409397121
(a) ESRF  
(b) Korea Institute of Science and Technology, Seoul (Republic of Korea)
(c) Yale University, New Haven (USA)
(d) Institute for Basic Science, Seoul (Republic of Korea)
(e) Seoul National University, Seoul (Republic of Korea)
(f) Pohang Accelerator Laboratory, Pohang (Republic of Korea)

References
[1] T. Dietl, Nat. Mater. 9, 965-974 (2010).
[2] B.-H. Lee et al., Nat. Mater. 18, 620-626 (2019).