Tools for magnetostructural correlations for the 3d⁸(³A2state) ions at orthorhombic sites: Comparative study with applications to Ni²⁺ions in Y2BaNiO5and Nd2BaNiO5

Authors:

• P. Gnutek,
• M. Açikgöz,
• Czesław Zygmunt Rudowicz

Abstract

Three approaches are employed to study magnetostructural correlations for the 3d⁸(³A₂state) ions at orthorhombic sites in crystals: (i) the higher-order perturbation theory (PT) of the microscopic spin Hamiltonian (MSH) parameters, (ii) the crystal field (CF) analysis (CFA) within all 3d⁸states combined with the superposition model (SPM) calculations of CF parameters, and (iii) the second-order PT of MSH parameters. A comparative study is carried out to assess the merit of each modeling approach. These approaches enable predictions of the orthorhombic zero-field splitting parameters (ZFSPs) for the 3d⁸ions at orthorhombic sites. Hence, correlation of the magnetic and spectroscopic properties with the structural ones may be considered. The approach (i) and (iii) take into account only the spin-orbit coupling (SOC) and a limited set of low lying states. Analysis of the expressions used in the approach (i) reveals discrepancies concerning: the sign of the SOC parameter, the cubic crystal field parameter Dq, the energy levels sequence, and numerical errors, which diminish its reliability. The distinction between the first- and second-kind orthorhombic symmetry is also elucidated. The approaches (i)-(iii) are applied for Ni²⁺(S=1) ions in the Haldane gap systems Y₂BaNiO₅and Nd₂BaNiO₅. The contributions to the ZFSPs due to the spin-spin and spin-other-orbit interactions considered using the approach (ii) are found nearly insignificant as compared with the dominant SOC ones. The results indicate that the approach (i) - corrected and (iii) may be employed only as an approximation. The approach (ii) together with the SPM/CFP modeling appear to be preferable and more reliable tools to study magnetostructural correlations and thus spectroscopic and magnetic properties of the 3d⁸(³A₂state) ions at orthorhombic sites in crystals.