Discovery of room temperature ferromagnetism in metal-free organic semiconductors

In the early scenario of quantum mechanics, the ferromagnetism must arise from the electrons with the principal quantum number being greater than two. Since the 1990s, chemists have been persistently exerting efforts to synthesize organic radical ferromagnets carrying only s and p orbitals, and thus far the spontaneous magnetization of such materials was solely observed below 36 Kelvin. In order to improve the Curie temperature, heavy atom radicals and insulating metal-organic coordination complexes were introduced. However, room temperature ferromagnetism along with semiconducting properties in organic materials composed merely of carbon, hydrogen, oxygen, and nitrogen have hitherto not been found. Here we report the discovery of ferromagnetism in the metal-free perylene diimide (PDI) semiconductor, whose Curie temperature is higher than 400 Kelvin. We use a solvothermal approach to concurrently reduce and dissolve the rigid-backbone PDI crystallites to the soluble dianion species with remarkably high reduction potential. The drop-casted PDI films comprising the radical anion aggregates were fabricated by the subsequent self-assembly and spontaneous oxidation process. Room temperature magnetic measurements exhibit the strong ferromagnetic ordering with the saturated magnetization of 0.48 $\mu_{\rm B}$ per molecule and the appreciable magnetic anisotropy in the PDI films. X-ray magnetic circular dichroism spectra further suggest the ferromagnetism stems from $\pi$ orbitals of unpaired electrons in the PDI radical anions. Our findings unambitiously demonstrate the long-range ferromagnetic ordering can survive at room temperature in organic semiconductors, although which are intuitively regarded to be nonmagnetic. This discovery paves a new way toward room temperature magnetic semiconductors.