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Single-ion magnetism in the extended solid-state: insights from X-ray absorption and emission spectroscopy

Abstract : Large single-ion magnetic anisotropy is observed in lithium nitride doped with iron. The iron sites are twocoordinate, putting iron doped lithium nitride amongst a growing number of two coordinate transition metal single-ion magnets (SIMs). Uniquely, the relaxation times to magnetisation reversal are over two orders of magnitude longer in iron doped lithium nitride than other 3d-metal SIMs, and comparable with high-performance lanthanide-based SIMs. To understand the origin of these enhanced magnetic properties a detailed characterisation of electronic structure is presented. Access to dopant electronic structure calls for atomic specific techniques, hence a combination of detailed single-crystal X-ray absorption and emission spectroscopies are applied. Together K-edge, L 2,3-edge and Kb X-ray spectroscopies probe local geometry and electronic structure, identifying iron doped lithium nitride to be a prototype, solid-state SIM, clean of stoichiometric vacancies where Fe lattice sites are geometrically equivalent. Extended X-ray absorption fine structure and angular dependent single-crystal X-ray absorption near edge spectroscopy measurements determine Fe I dopant ions to be linearly coordinated, occupying a D 6h symmetry pocket. The dopant engages in strong 3dp-bonding, resulting in an exceptionally short Fe-N bond length (1.873(7)Å) and rigorous linearity. It is proposed that this structure protects dopant sites from Renner-Teller vibronic coupling and pseudo Jahn-Teller distortions, enhancing magnetic properties with respect to molecular-based linear complexes. The Fe ligand field is quantified by L 2,3-edge XAS from which the energy reduction of 3d z 2 due to strong 4s mixing is deduced. Quantification of magnetic anisotropy barriers in low concentration dopant sites is inhibited by many established methods, including far-infrared and neutron scattering. We deduce variable temperature L 3edge XAS can be applied to quantify the J ¼ 7/2 magnetic anisotropy barrier, 34.80 meV ($280 cm À1), that corresponds with Orbach relaxation via the first excited, M J ¼ AE5/2 doublet. The results demonstrate that dopant sites within solid-state host lattices could offer a viable alternative to rare-earth bulk magnets and high-performance SIMs, where the host matrix can be tailored to impose high symmetry and control lattice induced relaxation effects.
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Submitted on : Friday, November 20, 2020 - 5:02:11 PM
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Myron S Huzan, Manuel Fix, Matteo Aramini, Peter Bencok, J. Frederick W. Frederick W Mosselmans, et al.. Single-ion magnetism in the extended solid-state: insights from X-ray absorption and emission spectroscopy. Chemical Science , The Royal Society of Chemistry, 2020, 11 (43), pp.11801-11810. ⟨10.1039/d0sc03787g⟩. ⟨hal-03017107⟩



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