Magnetic anisotropy in cyano-bridged bimetallic ferromagnets synthesized from the [Mo(CN)7]4– precursor

Abstract

The goal of this feature article is to introduce the dimension ‘magnetic anisotropy’ in the field of molecule-based magnets. For that, we have focused on three cyano-bridged Mn^IIMo^III compounds synthesized from the [Mo^III(CN)₇]^4– precursor. The pentagonal bipyramid structure of this precursor is not compatible with a cubic lattice, as found in the Prussian blue phases. In this precursor, Mo^III has a low-spin configuration, with a local spin S_Mo = 1/2, and a strongly anisotropic g tensor. Two of the compounds have a three-dimensional structure. Their formulas are Mn₂(H₂O)₅Mo(CN)₇·nH₂O, with n = 4 for the α phase, and n = 4.75 for the β phase. One of the compounds, of formula K₂Mn₃(H₂O)₆[Mo(CN)₇]₂·6H₂O, has a two-dimensional structure, with K⁺ cations and water molecules located between double-sheet layers. The compounds crystallize in the monoclinic system, and the lattice symmetries are very low. For the three compounds, we have succeeded in growing well shaped single crystals suitable for magnetic anisotropy measurements, and we have investigated the magnetic properties as follows: first, we have determined the magnetic axes by looking for the extremes of the magnetization in the three crystallographic planes ab, bc, and ac. Then, we have measured the temperature and field dependences of the magnetization in the dc mode along the three magnetic axes. These measurements have revealed the existence of several magnetically ordered phases for the three-dimensional compounds, and of field-induced spin reorientations for the three compounds. For the very first time in the field of molecular magnetism, we have been able to determine the magnetic phase diagram for each compound. We have obtained additional information from magnetic data recorded in the ac mode, with both zero and non-zero static fields. Finally, we have found that when the non-coordinated water molecules are released, the long-range magnetic ordering temperatures are shifted toward higher temperatures. Irrespective of the structural details, the Mo^III–C–N–Mn^II interaction has been found to be ferromagnetic. We have discussed this unexpected result, and proposed a mechanism accounting for this. We have also discussed the factors governing the magnetic anisotropy of the compounds.