Complex Cation and Spin Orders in the High-Pressure Ferrite CoFe3O5.

A ferrite in the Sr2Tl2O5-type MFe3O5 family with M = Co has been synthesized at 12 GPa pressure. Neutron diffraction shows the sample to be Co deficient with composition Co0.6Fe3.4O5. The Co/Fe cation distribution is found to be profoundly different from those of MFe3O5 analogs and lies between normal and inverse limits, as Co2+ substitutes across trigonal prismatic and one of the two octahedral sites. CoFe3O5 shows complex magnetic behavior with weak ferromagnetism below TC1 ≈ 300 K and a second transition to ferrimagnetic order at TC2 ≈100 K. Spin scattering of carriers leads a substantial increase in the hopping activation energy below TC1, and a small negative magnetoresistance is observed at low temperatures.