Mapping of single-site magnetic anisotropy tensors in weakly coupled spin clusters by torque magnetometry

Abstract

Single-crystal torque magnetometry performed on weakly-coupled polynuclear systems provides access to a complete description of single-site anisotropy tensors. Variable-temperature, variable-field torque magnetometry was used to investigate triiron(III) complex [Fe₃La(tea)₂(dpm)₆] (Fe₃La), a lanthanum(III)-centred variant of tetrairon(III) single molecule magnets (Fe₄) (H₃tea = triethanolamine, Hdpm = dipivaloylmethane). Due to the presence of the diamagnetic lanthanoid, magnetic interactions among iron(III) ions (s_i = 5/2) are very weak (<0.1 cm⁻¹) and the magnetic response of Fe₃La is predominantly determined by single-site anisotropies. The local anisotropy tensors were found to have D_i > 0 and to be quasi-axial with |E_i/D_i| ∼ 0.05. Their hard axes form an angle of approximately 70° with the threefold molecular axis, which therefore corresponds to an easy magnetic direction for the molecule. The resulting picture was supported by a High Frequency EPR investigation and by DFT calculations. Our study confirms that the array of peripheral iron(III) centres provides substantially noncollinear anisotropy contributions to the ground state of Fe₄ complexes, which are of current interest in molecular magnetism and spintronics.