The magnetic anisotropy of some lanthanide ions can hamper reversal of the molecular magnetic moment and in recent years there has been growing interest in including lanthanides in transition metal magnetic materials in order to modify their properties. The main impact to anisotropy in magnetic materials comprising lanthanides results from the effects of the crystal field on the rare earth's 4f wavefunction. Using 57Fe Mössbauer spectroscopy we have investigated the magnetic interaction between the DyIII ion and 57Fe nuclei in several Fe-Dy containing clusters [Fe2Dy2(OH)2(teaH)2(R-C6H4COO)6] [1]. The central [Fe2(OH)2] unit is antiferromagnetically coupled and thus has an S = 0 ground state. This was proved by measurement of the Mössbauer spectra in an external magnetic field of isostructural yttrium(III) containing compounds [Fe2Y2(OH)2(teaH)2(R-C6H4COO)6]. This means that even under an applied field there is no orientation of the zero spin of the cluster and each antiferromagnetically coupled dimer is still relaxing fast on the Mössbauer time scale. Since the dysprosium derivatives are isostructural with the yttrium ones, then we conclude that the interaction between Fe-Fe in compounds [Fe2Dy2(OH)2(teaH)2(R-C6H4COO)6] should also be antiferromagnetic. This raises the question: - Why the internal field experienced by the iron nuclei is so big giving a magnetic sextet? (see Figure 1 (zero field)) The only impact to the internal hyperfine field at iron nuclei in compounds [Fe2Dy2(OH)2(teaH)2(R-C6H4COO)6] can result from the magnetic interaction with the anisotropic DyIII ions, which provide sufficient magnetic anisotropy to “block” the hyperfine field experienced by the iron nuclei. But what is more unexpected is that when an external magnetic field is applied, then the magnetic onsets in spectra of [Fe2Dy2] compounds vanish with increasing field and the Mössbauer spectra at high fields (> 1 T) exhibit patterns typical of a diamagnetic complex (Figure 1, down). Here again emerges a question: - Why when a dc field is applied in Mössbauer spectra the sextets are vanishing? On applying an external magnetic field, it appears that the applied field affects the ground state of DyIII by a slight mixing of the excited state wave functions into the ground state lowering the energy of the system and resulting in a change of its direction of polarization. In this way the anisotropy of the DyIII ions is suppressed and no more interaction with iron ions occurs. Remarkably, the observed effects on the magnetic anisotropy and relaxation are dependent not only on the nature of the substituents, but also their position in the benzoic ring – para or meta [2]. In addition, they show a simple correlation to the Hammett constant of the ligand widely used for the study and interpretation of organic reactions and their mechanisms. These and several other results are analyzed in combination with ac magnetic susceptibility measurements and ab-initio calculations.Figure 1: 57Fe Mössbauer spectra of [Fe2Dy2(OH)2(teaH)2(R-C6H4COO)6], (R=2xNO2) in zero- and applied external magnetic fields 0.03 – 4 T at 3.0 K.