Direct observation of a ferri-to-ferromagnetic transition in a fluoride-bridged 3d–4f molecular cluster

Abstract

We report on the synthesis, crystal structure and magnetic characterisation of the trinuclear, fluoride-bridged, molecular nanomagnet [Dy(hfac)₃(H₂O)–CrF₂(py)₄–Dy(hfac)₃(NO₃)] (1) (hfacH = 1,1,1,5,5,5-hexafluoroacetylacetone, py = pyridine) and a closely related dinuclear species [Dy(hfac)₄–CrF₂(py)₄]·½CHCl₃ (2). Element-specific magnetisation curves obtained on 1 by X-ray magnetic circular dichroism (XMCD) allow us to directly observe the field-induced transition from a ferrimagnetic to a ferromagnetic arrangement of the Dy and Cr magnetic moments. By fitting a spin-Hamiltonian model to the XMCD data we extract a weak antiferromagnetic exchange coupling of j = −0.18 cm⁻¹ between the Dy^III and Cr^III ions. The value found from XMCD is consistent with SQUID magnetometry and inelastic neutron scattering measurements. Furthermore, alternating current susceptibility and muon-spin relaxation measurements reveal that 1 shows thermally activated relaxation of magnetisation with a small effective barrier for magnetisation reversal of Δ_eff = 3 cm⁻¹. Density-functional theory calculations show that the Dy–Cr couplings originate from superexchange via the fluoride bridges.