Spin density on ligands O2- and covalency of Fe3+ ions in octahedral sitesof the Ca3Fe2Ge3O12 garnet: A polarised neutron diffraction study

By means of polarised neutron diffraction in an external magnetic field of 7.5 T at 2 K the covalent spin density is unambiguously observed on the O2- ligands of the Fe3+ ions that occupy only the octahedral 16a sites in the antiferromagnetic Ca3Fe2Ge3O12 garnet. The maximum entropy method is used t o reconstruct these spin density peaks. The moment value of the field-induc ed component is found by the least-squares refinement on the flipping ratio s of the reflections affected only by oxygen. The ligand moment is determin ed to be 0.116(18) mu(B) by extrapolation of this component to the spin-fli p (exchange) field. The corresponding reduction of the Fe3+ moment is 14(2) %. An impurity moment of 0.0085(13) mu(B) in 24c sites (probably 0.21% of F e2+) can explain the partial magnetic disorder of this material.