A stable and strongly ferromagnetic Fe₁₇O₁₀ cluster with an accordion-like structure

Magnetic iron oxide nanoparticles are crucial in various applications, including targeted drug delivery, magneto-responsive therapy, and spintronics. Sub-nano iron oxide clusters offer tunable magnetism and chemical activity, making them ideal for high-density storage or spintronics microdevices. Understanding the structure-property relationship and gas-phase reactivity of these clusters is fundamental. While various iron oxide clusters have been studied, exhibiting diverse magnetic properties (antiferromagnetic, ferrimagnetic), the challenge has been to identify a stable and strongly ferromagnetic iron oxide cluster. Previous research has focused on clusters like Fe₁₀, Fe₁₃O₈, and Fe₁₂O₁₂; however, these often exhibited antiferromagnetic states. This study employs a deep-ultraviolet laser ionization mass spectrometry technique to investigate the reactions of iron clusters with oxygen, searching for a high-spin, ferromagnetic, and stable iron oxide cluster, thus bridging the gap between high stability and strong ferromagnetism. This work builds upon previous studies which looked at the reactivity of transition metal clusters with oxygen and found that certain magic numbers led to the formation of very stable compounds. The prior work on Cobalt and Nickel clusters showed that M₁₃O₈ (M = Co, Ni) clusters were particularly stable. This study aims to find if iron clusters form similar structures and if the properties are similar. This research is important because of its potential impact on the design of new materials for high-density storage and spintronics applications.

Extensive research exists on iron oxides' magnetic properties and their dependence on nuclearity and topology. FeO is antiferromagnetic at low temperatures; Fe₂O₃ is antiferromagnetic, while Fe₃O₄ is ferrimagnetic. Studies have focused on various oxo, peroxo, and superoxo isomers, dioxygen and oxygen complexes, and oxygen-rich clusters. However, in most of these oxide clusters (except a few (FeO)ₓ clusters), iron atoms are separated by oxygen, leading to antiferromagnetic properties. The magnetic moments of metal clusters are known to depend on their geometry and electron localization. Previous studies on polynuclear clusters, particularly magic-number iron and iron oxide clusters (like ring cluster Fe₁₀ and cubic Fe₁₃O₈), often reveal more stable antiferromagnetic states. This presents a contradiction: high stability and strong ferromagnetism seem incompatible in iron oxide clusters. Prior studies using mass spectrometry focused on oxidation reactions of transition metals such as Co, Ni, showing size-dependent reactivity and highlighting the stability of M₁₃O₈ (M = Fe, Co, Ni) clusters. However, there were experimental challenges in studying iron clusters due to their magnetic properties.

The experimental setup involved a custom-built reflection time-of-flight mass spectrometer (Re-TOFMS) with a mini flow tube reactor. Iron clusters (Feₙ⁻) were generated via laser ablation of an iron disk using a pulsed 532 nm Nd:YAG laser. The clusters were cooled during supersonic expansion with He carrier gas. Reactions with oxygen (10% O₂ in He) were conducted in the flow tube reactor at room temperature. The mass spectra provided information on the formed clusters. For theoretical calculations, the ab initio evolutionary algorithm USPEX, combined with the VASP software package, performed a global structure search for FeₙOₘ clusters. The Gaussian 09 software package, with BPW91 functional and 6-311G(d) basis set, optimized geometries, calculated energies, and vibrational frequencies. Transition states were checked, and intrinsic reaction coordinate (IRC) scans were performed. Gibbs free energy calculations were done at 298 K. Ab initio molecular dynamics (AIMD) simulations investigated the thermal stability of the discovered structure. Natural population analysis (NPA), density of states (DOS), and nucleus-independent chemical shift (NICS) analyses characterized the electronic structure and properties of the clusters.

Mass spectrometry revealed a prominent abundance of Fe₁₇O₁₀, suggesting its exceptional inertness. USPEX and VASP calculations determined the ground state structure of Fe₁₇O₁₀, an accordion-like C₂ᵥ structure. The cluster exhibits a remarkably high magnetic moment of 56 µB (55 unpaired electrons), corresponding to a strongly ferromagnetic state. DFT calculations using the Gaussian 09 software package show that oxygen molecule addition to Fe₁₇ leads to spontaneous O-O dissociation and formation of Fe₁₇O₁₀. In contrast, oxygen molecules on Fe₁₃ did not readily dissociate. Thermodynamic analysis (Figure 4g) shows that the formation of Fe₁₇O₁₀ is significantly faster than its further oxidation to Fe₁₇O₁₂. Ab initio molecular dynamics (AIMD) simulations show that the Fe₁₇O₁₀ structure remains stable up to 800K, indicating high thermal stability. Natural population analysis (NPA) showed similar negative charges on the µ₃-O atoms for Fe₁₇O₁₀ and Fe₁₃O₈, indicating similar oxygen bonding modes. However, DOS analysis reveals strong spin polarization in Fe₁₇O₁₀, contrasting with the symmetrical DOS pattern observed in Fe₁₃O₈. The strong spin polarization leads to a larger α-HOMO-LUMO gap in Fe₁₇O₁₀, contributing to its high-spin ferromagnetic state. Electron localization function (ELF) analysis further supports the compatibility of strong ferromagnetism and high stability in Fe₁₇O₁₀.

The study successfully identifies Fe₁₇O₁₀ as a stable and strongly ferromagnetic iron oxide cluster. The accordion-like structure facilitates the high spin state and contributes to its stability. The observed inertness of Fe₁₇O₁₀ in the mass spectrometry experiments is consistent with its thermodynamic stability and the kinetic barriers for further oxidation. The contrast between Fe₁₇O₁₀ and Fe₁₃O₈ highlights the structure-property relationship and the importance of cluster size in determining magnetic properties and reactivity. The findings challenge the prior notion of an incompatibility between high stability and strong ferromagnetism in iron oxide clusters. This discovery opens new avenues in exploring tailored molecule-based magnets and advancing the understanding of magnetic order evolution.

This research successfully synthesized and characterized Fe₁₇O₁₀, a stable and strongly ferromagnetic iron oxide cluster with an accordion-like structure and a magnetic moment of 56 µB. This unique cluster challenges the traditional understanding of the relationship between stability and ferromagnetism in iron oxide clusters. Its remarkable properties suggest potential applications in high-density storage and spintronics. Future work could explore the cluster's behavior in various environments and investigate its potential use in device fabrication.

The study primarily focuses on gas-phase reactions. The behavior of Fe₁₇O₁₀ in condensed phases or when embedded in a matrix remains to be investigated. The computational methods, while robust, rely on approximations, and more sophisticated methods could be used to refine the structural and electronic properties analysis. Further experimental validation of the structure and magnetic properties in condensed phase is needed.