Abstract
Modeling of spin Hamiltonian parameters enables correlation of crystallographic, spectroscopic, and magnetic data for transition ions in crystals. In this paper, based on the crystallographic data and utilizing the point-charge model and superposition model, the crystal field parameters (CFPs) are estimated for Ni2+(3d 8) ions in the Haldane gap system Y2BaNiO5. The CFPs serve as input for the perturbation theory expressions and the crystal field analysis package for microscopic spin Hamiltonian modeling of the zero-field splitting parameters (ZFSPs) D and E. Results of an extensive literature search of the pertinent crystallographic data, experimental ZFSPs, and model parameters are briefly outlined. The modeling aims at verification of the experimental ‘single ion anisotropy’ parameters and explanation of the controversy concerning the maximal rhombic distortion |E/D| ≈1/3 reported for Ni2+ ions in Y2BaNiO5. The preliminary results call for reanalysis of some magnetic studies of the Haldane gap systems.
Highlights
The Haldane gap systems exhibit several key features [1,2,3,4,5], namely, (1) integer spin (S = 1, 2) on the transition ions, (2) one-dimensional anisotropic antiferromagnetic (AF) chains, (3) no long range magnetic ordering observed down to very low T; (4) a nonmagnetic ‘‘spin-liquid’’ ground state separated from a branch of triplet excitations by a finite energy gap D, so-called Haldane gap, conjectured by Haldane in 1983 [6]
Two complementary approaches are utilized, i.e., the Point-Charge Model (PCM) and Superposition Model (SPM), to estimate the crystal field parameters (CFPs) based on the knowledge of the crystallographic data
The CFPs serve as input for the perturbation theory (PT) expressions and the Crystal Field Analysis (CFA) package for Microscopic Spin Hamiltonian (MSH) modeling of the zero-field splitting parameters (ZFSPs) D and E for Ni2? ions at orthorhombic symmetry sites
Summary
Existence of the Haldane gap in the energy spectrum has profound implications for magnetic and spectroscopic properties of these systems. No such gap exists for the half-integer spin systems. Modeling of ZFSPs, which enables correlation of crystallographic, spectroscopic, and magnetic data for transition ions in crystals, may provide a better insight into properties of these systems. The modeling aims at verification of the single ion anisotropy data and explanation of the controversy concerning the maximal rhombicity ratio |E/D| & 1/3 reported by some authors for YBNO [8,9,10], which contradicts the first INS results [11] indicating a large axial D value with the E term not considered. The preliminary results call for re-analysis of some magnetic studies of the Haldane gap systems
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