Diamond Annual Review 2020/21

98 99 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Spectroscopy Group Beamline I20-Scanning (and beamline I10 from theMagnetic Materials Group) When iron atoms behave like rare-earth elements - andwhy Related publication: HuzanM. S., FixM., Aramini M., Bencok P., Mosselmans J. F.W., Hayama S., Breitner F. A., Gee L. B.,Titus C. J., ArrioM.-A., Jesche A. & Baker M. L. Single-ionmagnetism in the extended solid-state: insights fromX-ray absorption and emission spectroscopy. Chem. Sci. 11 , 11801–11810 (2020). DOI: 10.1039/D0SC03787G Publication keywords: Single-ionmagnetism; Solid state; Magnetic anisotropy; Electronic structure;Transitionmetal dopant; X-ray spectroscopy T he clean energy revolution demands large quantities of rare-earth elements in applications ranging from wind turbines to electric car motors. However, rare-earth metals are toxic, and extraction is an energy-intensive and environmentally damaging process. Iron- doped lithium nitride has extraordinary magnetic properties, with single-ion magnetism that exceeds any other transition metal system and a magnetic coercivity field that surpasses even the largest values observed in rare-earth-based permanent magnets. However, unanswered questions concerning the fundamental geometric and electronic structure of the iron sites limit its impact. Researchers used a combination of element-sensitive X-ray techniques on Diamond Light Source’s Scanning Branch of beamline I20 to investigate the iron sites. They used K-edge X-ray Absorption Near Edge Structure (XANES) to quantify iron coordination geometry, Extended X-ray Absorption Fine Structure (EXAFS) to deduce the bond lengths around iron sites and Kβ X-ray Emission Spectroscopy (XES) to measure iron oxidation state as a function of dopant concentration. To directly access the electrons that contribute to the magnetism at iron sites, they used I10: Beamline for Advanced Dichroism Experiments (BLADE). The insights gained into the electronic structure will inform the preparation of improved bulkmagnets that do not require rare-earth elements. The results of this study provide insights into how transition metals within host lattices could one day offer a viable alternative to rare-earth bulk magnets. It may also become possible to use the direction of magnetisation at the iron atoms to store binary information at the atomic scale, for future high-density information storage applications and advances in quantum computing. The magnetic properties of Fe doped in lithium nitride are extraordinary, with a magnetic coercivity field greater than values observed in rare-earth- based permanent magnets 1 . Unusually, the displayed coercivity field persists down to extremely low Fe doping concentration inferring that the magnetism isofatomic,single-ion,nature.Single-ionmagnetismoccursduetoaneffective anisotropic barrier to the reversal of atomic magnetic moments. The observed single-ion magnetism in Fe doped lithium nitride exceeds any other transition metal compound and is comparable to high-performance rare-earth-based single-ion magnets. While this effect does not occur at room temperature, it arises below 50 K and is clearly observed below 10 K, a temperature that can be reached in the lab using liquid helium. An understanding into the origin of the displayed magnetic properties have been inhibited by limited knowledge of the geometric and electronic structure at the Fe dopant sites. Theoretical studies have considered either a situation where Fe sites occupy Li+ positions, resulting in a two-coordinate Fe with a +1 oxidation and general formula Li 2 [Li 1-x Fe x ]N, or due to coupling with additional Li+ vacancies within the lattice, a +2 oxidation state 2 . Defining the geometric and electronic structure of Fe dopants in lithium nitride is motivated by a desire to further understand how transition metals could be used to replace rare-earths in high-performance magnetic materials. Improved understanding of electronic structure also informs research into how magnetic operating temperatures could be raised. This observation of single-ion magnetism within Fe doped lithiumnitride is of relevance to atomic-scale information storage applications, since the direction of magnetisation at Fe sites could be harnessed in the storage of binary information at the atomic level. The random distribution of Fe sites within the lithium nitride crystal prohibitsaccesstothestructureofdopantsitesbydiffractionmethods.Accessing the dopant sites, therefore, requires element selective techniques, hence a combination of Fe X-ray absorption and emission spectroscopies at Diamond were applied. Each of the selected techniques carries a unique sensitivity due to the specific transitions and core-holes involved that together enable a holistic characterisation of both geometric and electronic structure (Fig. 1a). Fe K-edge extended X-ray absorption fine structure (EXAFS) measurements at the I20-Scanning beamline were applied to analyse local bond lengths around the Fe sites, fromwhich two extremely short Fe-N 1.873 Å bonds were determined (Fig. 1b and Fig. 2). Fe K-edge X-ray absorption near edge structure (XANES) measurements were performed to probe the Fe coordination geometry via an analysis of Fe 4p ligand-field splitting. An intense low-energy peak centred at 7113 eV is assigned as corresponding with electric dipole transitions into a degenerate pair of 4p x,y orbitals; a signature for a linear coordination geometry (Fig. 1c). This assignment was then confirmed by DFT calculations based on a linear structure using the Fe-N bond lengths determined by EXAFS (Fig. 1c). A lack of suitable reference spectra meant that Fe K-edge measurements could not be analysed to conclude the Fe oxidation state. Furthermore, predictions in the literature of a minority divalent species and a monovalent majority species with a ratio that changes as a function of dopant concentration added additional complexity to the characterisation 2 .To solve this particular question Fe Kβ X-ray emission spectroscopy (XES) measurements were performed with Fe doping concentrations (x) ranging from 18 % down to 0.2 % in Li 2 [Li 1-x Fe] N. The spectra show no variation in the intensity ratio or splitting between the Kβ’ and Kβ 1,3 emission lines, confirming that the Fe oxidation state does not vary with dopant concentration (Fig. 1d). Furthermore, measurements in the valence-to-core region of the spectra confirm no variation in bonding at Fe sites with respect to dopant concentration (Fig. 1e). Finally, to confirm the oxidation state of Fe sites, Fe L 2,3 -edge X-ray absorption spectroscopy (XAS) measurements were performed at the I10-BLADE beamline to directly probe the 3d valence shell. These measurements were analysed using ligand-field multiplet simulations. The Fe sites are confirmed to have a +1 oxidation state with a 4 D 7/2 ground state of 4 E symmetry. The energetic order of the 3d orbitals are found to be affected by strong Fe 4s–3d z 2 mixing that lowers the energy of the 3d z 2 orbital resulting in a a 1g 2 e 2g 3 e 1g 2 ground state configuration. The Fe L 2,3 -edge XAS analysis enabled experimental quantification of 3d z 2 energy reduction.The linear ligand field with large 3d z 2 energy reduction results in the first-order spin-orbit coupled ground-state that gives rise to the rare-earth like magnetic anisotropy barrier in Fe doped lithium nitride. Together the X-ray spectroscopy results identify the Fe dopant sites in lithium nitride to be clean of stoichiometric vacancies and geometrically equivalent. It is proposed that the combination of a short Fe-N bond, related strong 3d bonding and high point symmetry imposed by the hexagonal lithium nitride lattice contribute to suppress vibronic effects, resulting in increased magnetic coercivities with respect to other linear single-ion magnets.These findings are key to furthering understanding of how transition metal ion dopants could be used to replace toxic and expensive rare-earth ions in magnets and next-generation nanoscale information storage devices. References: 1. Jesche A. et al. Giant magnetic anisotropy and tunnelling of the magnetization in Li2(Li1−xFex)N. Nat. Commun. 5 , 3333 (2014). DOI: 10.1038/ncomms4333 2. Xu L. et al. Spin-reversal energy barriers of 305 K for Fe2+ d6 ions with linear ligand coordination. Nanoscale 9 , 10596–10600 (2017). DOI: 10.1039/C7NR03041J Funding acknowledgement: We acknowledge the support of the Royal Society of Chemistry (RM1802- 4019) and Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Grant No. JE748/1. Corresponding authors: Dr Michael L. Baker, Department of Chemistry, the University of Manchester, and The University of Manchester at Harwell, michael.baker@manchester.ac.uk ; Mr Myron S. Huzan, Department of Chemistry, the University of Manchester, and the University of Manchester at Harwell Figure 1: (a) The Fe X-ray spectroscopic transitions accessed during the study of Li 2 [Li 1-x Fe x ]N at the I20-Scanning beamline; (b) Background subtracted k 3 -weighted EXAFS; (c) Single crystal Fe K-edge XANES with calculated DFT spectrum; (d) Kβ XES for Fe dopant concentrations ranging from 0.2 to 18 %; (e) The valence-to-core region of the Kβ XES. Figure 2: The structure of lithium nitride including an Fe impurity, Li 2 [Li 1-x Fe x ]N. Fe, brown; N, green; Li, grey.