Magnetization Blocking in Fe2 III Dy2 III Molecular Magnets: Ab Initio Calculations and EPR Spectroscopy

Veacheslav Vieru; Liviu Ungur; Valeriu Cemortan; Andrey Sukhanov; Amer Baniodeh; Christopher E Anson; Annie K Powell; Violeta Voronkova; Liviu F Chibotaru

doi:10.1002/chem.201803821

Magnetization Blocking in Fe₂ ^III Dy₂ ^III Molecular Magnets: Ab Initio Calculations and EPR Spectroscopy

Chemistry. 2018 Nov 7;24(62):16652-16661. doi: 10.1002/chem.201803821. Epub 2018 Oct 17.

Authors

Affiliations

¹ Theory of Nanomaterials Group, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001, Leuven, Belgium.
² Department of Chemistry, National University of Singapore, Block S8 Level 3, 3 Science Drive 3, Singapore, 117543, Singapore.
³ Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center of RAS, Kazan, Russian Federation.
⁴ Institute of Inorganic Chemistry, Karlsruhe Institute of Technology, Engesserstrasse 15, 76131, Karlsruhe, Germany.
⁵ Institute of Nanotechnology, Karlsruhe Institute of Technology, Postfach 3640, 76021, Karlsruhe, Germany.

PMID: 30107056
DOI: 10.1002/chem.201803821

Abstract

The magnetism and magnetization blocking of a series of [Fe₂ Dy₂ (OH)₂ (teaH)₂ (RC₆ H₄ COO)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.

Keywords: EPR spectroscopy; ab initio calculations; dysprosium; exchange interactions; iron; magnetic properties.

Abstract

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