Evolution of Ferromagnetic and Antiferromagnetic States in Iron Nitride Clusters FenN and FenN2 (n = 1-10).

Abstract

First-principles density functional theory calculations on neutral and singly negatively and positively charged iron clusters Fen and iron nitride clusters FenN and FenN2 (n = 1-10) in the range of 1 ≤ n ≤ 10 revealed that there is a strong competition between ferromagnetic and antiferromagnetic states especially in the FenN20,±1 cluster series. This phenomenon was related to superexchange via a bridging N atom between two iron atoms in the FenN20,±1 cluster series and to a double superexchange effect via a Fe atom shared by two N atoms in the FenN20,±1 series. A thorough examination of the structure-energy-spin state relationships in these clusters is conducted, leading to new insights and confirmation of available experimental results on structural parameters and dissociation energetics. The bond energies of both nitrogen atoms in the FenN2 series are approximately the same. They weakly depend on the charge of the host cluster and fluctuate around 5.5 eV when moving along the series. The energy of N2 desorption is relatively small; it varies by about 1.0 eV and depends on the charge of the cluster. The experimental finding that N2 dissociates on the Fen+ clusters beginning with n = 4 was supported by the results of our computations. Our computed values of the Fen+-N bonding energies agree with the experimental data within the experimental uncertainty bars. It was found that the attachment of one or two N atoms does not seriously affect the polarizability, electron affinity, or ionization energy of the host iron clusters independent of the charge.