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, L2,3-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 FeI dopant ions to be linearly coordinated, occupying a D6h 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 L2,3-edge XAS from which the energy reduction of 3dz2 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, MJ = ±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.
Highlights
Coordinated transition metal ions can exhibit rstorder spin–orbit coupling which gives rise to large magnetic anisotropy barriers and bi-stability of magnetisation
The results demonstrate that dopant sites within solid-state host lattices could offer a viable alternative to rare-earth bulk magnets and high-performance single-ion magnets (SIMs), where the host matrix can be tailored to impose high symmetry and control lattice induced relaxation effects
In this paper we apply the range of X-ray spectroscopies that selectively characterise different aspects of electronic structure, from which we identify Li2(Li1ÀxFex)N is a high symmetry solid state SIM clear of stoichiometric vacancies where Fe lattice sites are geometrically equivalent
Summary
Coordinated transition metal ions can exhibit rstorder spin–orbit coupling which gives rise to large magnetic anisotropy barriers and bi-stability of magnetisation. An example of this is Fe doped in lithium nitride (Li2(Li1ÀxFex)N). Calculations, based on the crystal structure, propose a a1g2e2g3e1g2 ground state con guration with an almost unquenched L 1⁄4 2 orbital angular momentum These calculations are supported and found consistent with Mossbauer spectroscopy data[7] and highresolution single-crystal crystallography provides the rst experimental evidence of 3dz[22] electron occupation from electron density analysis.[8] despite the increasing number of reports of new linear transition metal SIMs9 there have been very few experimental studies beyond the characterisation of orientation averaged magnetism. The geometric and electronic structure of Li2(Li1ÀxFex)N is compared against molecular based SIMs and important insights into the origin of high temperature magnetic blocking and exceptionally long magnetic relaxation times observed in Li2(Li1ÀxFex)N are obtained
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