π-Electron ferromagnetism of a purely organic radical crystal

Transition to ferromagnetic order was found in the early 1990’s with a crystal of a purely organic compound without any metallic elements. The process of the research is reviewed starting from the pre-stage study to obtain the designing strategies to bring out ferromagnetic interaction in organic crystal and ending at the discovery of the ferromagnetism in the crystal of p-nitrophenyl nitronyl nitroxide (p-NPNN; C(13)H(16)N(3)O(4)).