Magnetization Blocking in (Fe2Dy2III)-Dy-III Molecular Magnets: Ab Initio Calculations and EPR Spectroscopy

Abstract

The magnetism and magnetization blocking of a series of [Fe2Dy2(OH)2(teaH)2(RC6H4COO)6] complexes was investigated, in which teaH3=triethanolamine and R=meta-CN (1), para-CN (2), meta-CH3 (3), para-NO2 (4) and para-CH3 (5), by combining ab initio calculations and EPR measurements. The results of broken-symmetry DFT calculations show that in all compounds the Fe–Fe exchange interaction is antiferromagnetic and stronger by far than the Fe–Dy and Dy–Dy interactions. As a result, the lowest two exchange doublets probed by EPR spectroscopy mostly originate from the Ising interaction of the dysprosium ions in all compounds. A correct quantitative description of the splitting of these two doublets requires, however, an explicit account of the Fe–Dy and Fe–Fe interactions. Due to the inversion symmetry of the complexes, the doublets under consideration are described by a collinear Ising exchange interaction. This picture is also supported by the EPR spectra, which could be simulated with parameters close to those extracted from the calculations. The magneto-structural analysis shows an increase of the antiferromagnetic Fe–Fe exchange interaction with increasing Fe-O-Fe angle and Fe–Fe distance. For the Dy–Fe interaction, the opposite tendency is observed, that is, a decrease of antiferromagnetic exchange coupling with increasing Dy-O-Fe angle and Dy–Fe distance. The transversal g factors extracted from the ab initio calculations have values in the range of 0.01–0.2, testifying to the lack of high axiality of the ground state of the dysprosium ions. This explains the lack/poor single-molecule magnetic behavior of this series of compounds at the investigated temperatures of a few Kelvin. Due to a very small gap (fractions of a wavenumber) between the ground and first-excited exchange doublet, relaxation takes place by magnetic moment reversal at individual dysprosium sites in the considered temperature domain.