Substrate-controlled ferromagnetism in iron phthalocyanine films due to one-dimensional iron chains

Using the magneto-optical Kerr effect, superconducting quantum interference device magnetometry, and magnetic circular dichroism spectroscopy, ferromagnetism was found below 4.5 K in iron phthalocyanine thin films grown by molecular-beam epitaxy. The stacking of the molecules can be controlled via deposition temperature and substrate choice. The molecules self-assemble such that quasi-one-dimensional iron chains form. The chains are limited in length by the grain size or film thickness, which are manipulated using judicious growth methods. Magnetic hysteresis loops show distinct features in the saturation magnetization and susceptibility that depend on the thin-film structure and/or substrate. We find two regimes for the magnetic behavior: below temperatures of similar to 25 K, intrachain interactions couple the Fe ions and produce nontraditional paramagnetic behavior, while at low temperatures below similar to 4.5 K interactions between the one-dimensional chains produce hysteresis, magnetic order, and slow magnetization dynamics.