Orbital Magnetic Moment and Single-Ion Magnetic Anisotropy of the S = 1/2 K₃[Fe(CN)₆] Compound: A Case Where the Orbital Magnetic Moment Dominates the Spin Magnetic Moment

Abstract

The potassium hexacyanoferrate(III), K 3 [Fe III (CN) 6 ], is known for its exceptional magnetic anisotropy among the 3d transition metal series. The Fe(III) ions are in the S = 1/2 low spin state imposed by the strong crystal field of the cyanido ligands. A large orbital magnetic moment is expected from previous publications. In the present work, X-ray magnetic circular dichroism was recorded for a powder sample, allowing direct measurement of the Fe(III) orbital magnetic moment. A combination of molecular multiconfigurational ab initio and atomic ligand field multiplets calculations provides the spin and orbital magnetic moments for the [Fe III (CN) 6 ] 3− isolated cluster, the crystallographic unit cell, and the powder sample. The calculations of the angular dependencies of the spin and orbital magnetic moments with the external magnetic induction direction reveal easy magnetization axes for each S = 1/2 molecular entity and the crystal. It also shows that the orbital magnetic moment dominates the spin magnetic moment for all directions. Our measurements confirm that the orbital magnetic moment contributes to 60% of the total magnetization for the powder, which is in excellent agreement with our theoretical predictions. An orbital magnetic moment greater than the spin magnetic moment is exceptional for 3d transition metal ions. The impact of crystal field strength and distortion, π back-bonding, spin−orbit coupling, and external magnetic induction was analyzed, leading to a deeper understanding of the spin and orbital magnetic anisotropies.

Sensing the Spin State of Room-Temperature Switchable Cyanometallate Frameworks with Nitrogen-Vacancy Centers in Nanodiamonds

“…5 M(H) at 300 K does not saturate for as-synthesized or illuminated powders, indicating the presence of antiferromagnetic interactions in the roomtemperature paramagnetic regime. EPR spectroscopy found negligible response for as-synthesized green powders of 2a due to the presence of significant spin−orbit coupling, 41 and fast spin−lattice relaxation. 42 Upon illumination, a signal appeared at g = 2.03, consistent with prior observations by Cai et al (see Figure 6B).…”

“…The reduced ODMR and MM contrast upon light illumination of 2a could originate from many sources. The movement of a largely orbital dominant moment in Fe III to the spin RO + • R radical, as well as the room-temperature moment on the Eu III ion interacting with the RO + • R radical could affect the NV – sensing contrast. Moreover, evidence for hyperfine structure in the EPR spectrum of the illuminated state (Figure B), observed for similar systems in previous reports, , suggests coupling between the RO + • R radical and spin active nuclei, such as 151 Eu (47.8%) and 153 Eu (52.2% abundance) both with 5 / 2 nuclear spin, which could increase the importance of the nuclear spin-bath interactions with NV – centers. , Although the detailed mechanism underlying photoinduced contrast reduction remains unclear, our conjecture is analogous to spin-noise mediated reduction in NV – relaxation times reported for other chemical systems in the literature. ,, Contrast reduction was also shown to be dependent on the illumination time (Figure S18), similar to the evolution of the IR spectrum observed by Cai et al .…”