3d/4f metal clusters AC susceptibility DFT calculation X-ray diffraction [2 chloranilato cobalt coordination clusters coordination polymers crystal structure di-2 pyridyl ketone ferromagnetism graphene oxide heterometallic layers high pressure honeycomb layers iron complex lanthanide complex lanthanide ions lanthanides long-range magnetic ordering magnetism mixed metal Co/Ln clusters molecule-based magnets nickel(II) octacyanidoniobate(IV) pressure effect single-molecule magnet single-molecule magnets slow magnetic relaxation