Evidence of field induced slow magnetic relaxation in cis-[Co(hfac)2(H2O)2] exhibiting tri-axial anisotropy with a negative axial component

Abstract

We report a combined experimental characterization and theoretical modeling of the hexa-coordinated high-spin Co(II) complex cis-[Co(hfac)₂(H₂O)₂] (I). The magnetic static field (DC) data and EPR spectra (measurements were carried out on the powder samples of diluted samples cis-[Co_0.02Zn_0.98 (hfac)₂(H₂O)₂]) were analyzed with the aid of the parametric Griffith Hamiltonian for the high-spin Co(II) supported by the ab initio calculations of the crystal field (CF) parameters, g-factors and superexchange parameters between H-bonded Co(II) ions in the neighboring molecules in a 1D network. This analysis suggests the presence of the easy axis of magnetic anisotropy and also shows the existence of a significant rhombic component. The detected frequency dependent (AC) susceptibility signal shows that complex I exhibits slow paramagnetic relaxation in the applied DC field belonging thus to the class of non-uniaxial field induced single ion magnets with a negative axial component of anisotropy. It is demonstrated that the main contributions to the relaxation come from the direct one-phonon process dominating at low temperatures, while the contribution of the two-phonon Raman process becomes important with increasing temperature.