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We model the magnetic behavior of the thiacalixarene complexes MnII2GdIII2 (I) and CoII2EuIII2 (II). The nonmonotonic dependence of χT versus T for I (exhibiting a rotational band of spin excitations) is attributed to the competitive ferromagnetic Mn−Gd and antiferromagnetic Mn−Mn interactions. Complex II exhibits strong uniaxial anisotropy of the CoII ions and isotropic exchange in the CoII2 pair. The dominating anisotropic contribution with an easy plane of magnetization in II is described by an anisotropic effective spin-1/2 Hamiltonian.
In view of a wide perspective of 3d–4f complexes in single-molecule magnetism, here we propose an explanation of the magnetic behavior of the two thiacalixarene tetranuclear heterometallic complexes MnII2GdIII2 and CoII2EuIII2. The energy pattern of the MnII2GdIII2 complex evaluated in the framework of the isotropic exchange model exhibits a rotational band of the low-lying spin excitations within which the Landé intervals are affected by the biquadratic spin–spin interactions. The nonmonotonic temperature dependence of the χT product observed for the MnII2GdIII2 complex is attributed to the competitive influence of the ferromagnetic Mn–Gd and antiferromagnetic Mn–Mn exchange interactions, the latter being stronger (J(Mn, Mn) = −1.6 cm–1, Js(Mn, Gd) = 0.8 cm–1, g = 1.97). The model for the CoII2EuIII2 complex includes uniaxial anisotropy of the seven-coordinate CoII ions and an isotropic exchange interaction in the CoII2 pair, while the EuIII ions are diamagnetic in their ground states. Best-fit analysis of χT versus T showed that the anisotropic contribution (arising from a large zero-field splitting in CoII ions) dominates (weak-exchange limit) in the CoII2EuIII2 complex (D = 20.5 cm–1, J = −0.4 cm–1, gCo = 2.22). This complex is concluded to exhibit an easy plane of magnetization (arising from the CoII pair). It is shown that the low-lying part of the spectrum can be described by a highly anisotropic effective spin-1/2 Hamiltonian that is deduced for the CoII2 pair in the weak-exchange limit
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