Collective Superexchange and Exchange Coupling Constants in the Hydrogenated Iron Oxide Particle Fe₈O₁₂H₈

Motivated by the fact that Fe₂O₃ nanoparticles are used in the treatment of cancer, we have examined the role of ligands on the magnetic properties of these particles by focusing on (Fe₂O₃)₄ as a prototype system with H as ligands. Using the Broken-Symmetry Density Functional Theory, we observed a strong collective superexchange in the hydrogenated Fe₈O₁₂H₈ cluster. The average antiferromagnetic exchange coupling constant between the four iron–iron oxo-bridged pairs was found to be −178 cm^–1, whereas coupling constants between hydroxo-bridged pairs were much smaller. We found that despite the apparent symmetry of the iron atom framework, it is not reasonable to assume this symmetry when fitting the exchange coupling constants. We also analyzed the geometrical and magnetic properties of Fe₈O₁₂H_<i>n</i> for <i>n</i> = 0–12 and found that hydrogenating oxo-bridges would generally inhibit the Fe–O–Fe antiferromagnetic superexchange interactions. Antiferromagnetic lowest total energy states become favorable only when specific distributions of hydrogen atoms are realized. The (HO)₄–Fe₄(all spin-up)–O₄–Fe₄(all spin-down)–(OH)₄ configuration in Fe₈O₁₂H₈ presents such an example. This symmetric configuration can be considered a superdiatomic system

The Intrinsic Ferromagnetism in a MnO₂ Monolayer

The Mn atom, because of its special electronic configuration of 3d⁵4s², has been widely used as a dopant in various two-dimensional (2D) monolayers such as graphene, BN, silicene and transition metal dichalcogenides (TMDs). The distributions of doped Mn atoms in these systems are highly sensitive to the synthesis process and conditions, thus suffering from problems of low solubility and surface clustering. Here we show for the first time that the MnO₂ monolayer, synthetized 10 years ago, where Mn ions are individually held at specific sites, exhibits <i>intrinsic ferromagnetism</i> with a Curie temperature of 140 K, comparable to the highest <i>T</i>_C value achieved experimentally for Mn-doped GaAs. The well-defined atomic configuration and the intrinsic ferromagnetism of the MnO₂ monolayer suggest that it is superior to other magnetic monolayer materials

A Theoretical and Mass Spectrometry Study of Dimethyl Methylphosphonate: New Isomers and Cation Decay Channels in an Intense Femtosecond Laser Field

Using both mass spectrometry with intense femtosecond laser ionization and high-level computational methods, we have explored the structure and fragmentation patterns of dimethyl methylphosphonate (DMMP) cation. Extensive search of the geometries of both neutral and positively charged DMMP yields new isomers that are appreciably lower in total energy than those commonly synthesized using the Michaelis–Arbuzov reaction. The stability of the standard isomer with CH₃PO­(OCH₃)₂ topology is found to be due to the presence of high barriers to isomer interconversion that involves several transition states. Our femtosecond laser ionization experiments show that the relative yields of the major dissociation products as a function of peak laser intensity correlate well with the theoretical estimates for the energies of the DMMP⁺ decay via various channels. In contrast, the peak laser intensities required for observation of minor dissociation products exhibit no correlation with the computed decay energies, which suggests that barrier heights and/or excited electronic states of DMMP⁺ determine its preferred fragmentation pathways in an intense femtosecond laser field

Structural Patterns in Carbon Chemisorption on an Icosahedral 2 Iron Cluster

Carbon chemisorption on iron nanoparticles at small carbon coverage has been studied by using a Fe₁₃ particle as a model because it possesses a nearly icosahedral geometry, and complications with additional effects associated with the surface inhomogeneity do not arise. The electronic and geometrical structures of Fe₁₃C_<i>n</i> are computed for <i>n</i> = 0–20 using an all-electron density functional theory with generalized gradient approximation and a rather large basis set. It is found that the energetically preferred structures correspond to the formation of carbon dimers up to Fe₁₃C₁₂ and trimers up to Fe₁₃C₁₈ in octahedral configurations of the dimers and trimers with the Fe₁₃ cluster being endohedral. The trend for the formation of carbon tetramers breaks at Fe₁₃C₂₀. We found that the dependence of the total energy on the total spin is nearly the same for Fe₁₃ and Fe₁₃C₈. When the number of chemisorbed carbon atoms exceeds 6, chemisorption quenches the total magnetic moment to 36 μ_B from the value of 44 μ_B in the ground-state Fe₁₃ cluster. We used natural atomic orbital populations to understand why the quenching does not depend on the number of chemisorbed atoms. Free C_<i>n</i> species were reoptimized at the same level of theory to calculate the dissociation energies of C_<i>n</i> and Fe₁₃C_<i>n</i>. It is found that the largest fragmentation energy of 12 eV belongs to the Fe₁₃C₁₂ → Fe₁₃ + C₁₂ channel. Finally, we found that atomization energies for the carbon chemisorbed on the iron particle are larger by approximately 10 eV than atomization energies of the corresponding free carbon particles, which can be related to the catalytic strength of the Fe₁₃ particle