Spectroscopic and magnetic properties of Fe2+ (3d6; S= 2) ions in Fe(NH4)2(SO4)2·6H2O – Modeling zero-field splitting and Zeeman electronic parameters by microscopic spin Hamiltonian approach

Abstract Utilizing the package MSH/VBA, based on the microscopic spin Hamiltonian (MSH) approach, spectroscopic and magnetic properties of Fe 2+ (3d 6 ; S = 2) ions at (nearly) orthorhombic sites in Fe(NH 4 ) 2 (SO 4 ) 2 ·6H 2 O (FASH) are modeled. The zero-field splitting (ZFS) parameters and the Zeeman electronic (Ze) factors are predicted for wide ranges of values of the microscopic parameters, i.e. the spin-orbit (λ), spin-spin (ρ) coupling constants, and the crystal-field (ligand-field) energy levels (Δ i ) within the 5 D multiplet. This enables to consider the dependence of the ZFS parameters b k q (in the Stevens notation), or the conventional ones (e.g., D and E ), and the Zeeman factors g i on λ, ρ, and Δ i . By matching the theoretical SH parameters and the experimental ones measured by electron magnetic resonance (EMR), the values of λ, ρ, and Δ i best describing Fe 2+ ions in FASH are determined. The novel aspect is prediction of the fourth-rank ZFS parameters and the ρ(spin-spin)-related contributions, not considered in previous studies. The higher-order contributions to the second- and fourth-rank ZFSPs are found significant. The MSH predictions provide guidance for high-magnetic field and high-frequency EMR (HMF-EMR) measurements and enable assessment of suitability of FASH for application as high-pressure probes for HMF-EMR studies. The method employed here and the present results may be also useful for other structurally related Fe 2+ ( S = 2) systems.