Vol 50, No 6 (2019)

Transition metal atoms adsorbed on surfaces (adatoms) behaving like magnets are important for potential applications in quantum computing and memory storage. Better insight into their magnetic properties, described by spin Hamiltonian (SH) parameters, is essential. Comprehensive modeling of SH parameters for Fe²⁺ with spin S = 2 adatoms on various surfaces is carried out using two approaches: (1) semiempirical CF/MSH [based on crystal field (CF) and microscopic spin Hamiltonian (MSH) theory], and (2) density functional theory (DFT). Here preliminary results of modeling of zero-field splitting parameters (ZFSPs) for Fe²⁺ on Cu₂N/Cu(100) surface [for short Fe²⁺@Cu₂N/Cu(100)] are presented. We focus on the orthorhombic second-rank ZFSPs in the conventional notation (D, E) measured for Fe²⁺@Cu₂N/Cu(100). The fourth-rank ZFSP in the Stevens notation (\(B_{k}^{q}\), k = 2, 4) measured for Fe²⁺ on CuN/Cu(100) surface are considered elsewhere. Using the CF/MSH approach within ⁵D approximation, the ZFSPs (k = 2, 4) and Zeeman g-factors are calculated for wide range of the microscopic parameters: spin–orbit (λ), spin–spin (ρ) coupling constants, and the crystal field energy levels (∆_i). The ρ-contributions and the fourth-rank ZFSPs are found important. Computations of the ZFSPs (D, E) are done using the SIESTA code by mapping of the physical energy levels to those of effective ZFS Hamiltonian. Comparison of the results enables bridging the gap between DFT methods and CF/MSH ones. The present results will also be utilized in ongoing studies of adatoms on other surfaces, single molecule magnets and single-ion magnets.

Barium tungstate crystals are an interesting and relatively new medium for stimulated Raman scattering for applications in Raman shifters of laser radiation. Good quality BaWO₄ crystals can be grown by Czochralski technique and doped with rare-earth ions. Doping with trivalent ions requires charge compensation which may be provided, for example, by structural defects or proper codoping with alkaline metal ions. Crystals possess scheelite-like structure with the space group I4₁/a. Results of the electron paramagnetic resonance (EPR) studies of BaWO₄:Ce crystals and crystals codoped with Na are presented in this paper. EPR study was performed in the low temperature range 3 K < T < 50 K for all three planes of single crystals. The EPR spectra were attributed to cerium ions with a fictitious spin S = 1/2. The spin Hamilton (SH) parameters were determined. The values of SH parameters indicate the occurrence of paramagnetic centers in axial and low symmetry. The number of observed EPR lines depends on the selected plane. Based on the roadmap, we have found that one center with axial symmetry and at least two centers with low symmetry appear. The linewidth, ∆B, vs temperature dependence revealed increasing exponential tendency with increasing temperature. It shows 1 phonon at the lower temperatures and Raman + Orbach effect at the higher temperatures. Exponential change of the ΔB could be connected with the spin–lattice relaxation processes involving excited states of Ce³⁺ ions.