Effects of 3d-4f magnetic exchange interactions on the dynamics of the magnetization of Dy(III)-M(II)-Dy(III) trinuclears clusters

[{Dy(hfaC)(3)}(2)(Fe(bpca)(2))}]center dot CHCl(3) ([Dy(2)Fe]) and [{Dy(hfac)(3)}(2){Ni-(bpca)(2)}]center dot CHCl(3) ([Dy(2)Ni]) (in which hfac(-) = 1,1,1,5,5,5-hexafluoroacetylacetonate and bpca-=bis(2-pyridylcarbonyl)amine anion) were synthesized and characterized. Single-crystal X-ray diffraction shows that [Dy(2)Fe] and [Dy(2)Ni] are linear trinuclear complexes. Static magnetic susceptibility measurements reveal a weak ferromagnetic exchange interaction between Ni(II) and Dy(III) ions in [Dy(2)Ni], whereas the use of the diamagnetic Fen ion leads to the absence of magnetic exchange interaction in [Dy(2)Fe]. Dynamic susceptibility measurements show a thermally activated behavior with the energy barrier of 9.7 and 4.9 K for the [Dy(2)Fe] and [Dy(2)Ni] complexes, respectively. A surprising negative effect of the ferromagnetic exchange interaction has been found and has been attributed to the structural conformation of these trinuclear complexes.