π-electron weak ferromagnetism in potassium-intercalated 9-phenylanthracene

Intercalation of alkali metal is an effective way to tune the physicochemical property of polyaromatic hydrocarbons. Herein, we show for the first time that the weak ferromagnetism with a Curie temperature of 5.2 K can be realized in an all-carbon π-electron compound – potassium-intercalated 9-phenylanthracene. The magnetic measurements reveal that there exists an antiferromagnetic transition around 35 K before transition to the weak ferromagnetic phase. An in-depth theoretical investigation indicates that the weak ferromagnetism is produced by the intramolecular ferromagnetic and intermolecular antiferromagnetic spin couplings in the crystal unit cell. Our finding enriches the physical functionality of anthracene derivatives and adds a new member of organic magnetic materials.