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

Myron S Huzan; Manuel Fix; Matteo Aramini; Peter Bencok; J Frederick W Mosselmans; Shusaku Hayama; Franziska A Breitner; Leland B Gee; Charles J Titus; Marie-Anne Arrio; Anton Jesche; Michael L Baker

doi:10.1039/d0sc03787g

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

Chem Sci. 2020 Oct 7;11(43):11801-11810. doi: 10.1039/d0sc03787g.

Authors

Myron S Huzan^{1

2}, Manuel Fix³, Matteo Aramini⁴, Peter Bencok⁴, J Frederick W Mosselmans⁴, Shusaku Hayama⁴, Franziska A Breitner³, Leland B Gee⁵, Charles J Titus⁶, Marie-Anne Arrio⁷, Anton Jesche³, Michael L Baker^{1

2}

Affiliations

¹ The University of Manchester at Harwell, Diamond Light Source Harwell Campus OX11 0DE UK michael.baker@manchester.ac.uk.
² Department of Chemistry, The University of Manchester Manchester M13 9PL UK.
³ EP VI, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg D-86159 Augsburg Germany.
⁴ Diamond Light Source, Harwell Science and Innovation Campus Chilton Didcot OX11 0DE UK.
⁵ Department of Chemistry, Stanford University Stanford CA 94305 USA.
⁶ Department of Physics, Stanford University Stanford CA 94305 USA.
⁷ Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, CNRS, Sorbonne Université, IRD, MNHN, UMR7590 75252 Paris Cedex 05 France.

Abstract

Large single-ion magnetic anisotropy is observed in lithium nitride doped with iron. The iron sites are two-coordinate, 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 Kβ 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 3dπ-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 L₃-edge XAS can be applied to quantify the J = 7/2 magnetic anisotropy barrier, 34.80 meV (∼280 cm^-1), that corresponds with Orbach relaxation via the first excited, M _J = ±5/2 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.