Ferromagnetism in sp² carbon

The bulk, pristine sp² carbons, such as graphite, carbon nanotubes, and graphene, are usually assumed to be typical diamagnetic materials. However, over the past two decades, there have been many reports about the ferromagnetism in these sp² carbon materials, which have attracted intense interest for basic research and potential applications. In this review, we focus on the evidence and developments of the emergent ferromagnetism in sp² carbon revealed by nine kinds of experimental methods: magnetic force microscopy (MFM), magnetization measurements with physical property measurement system (PPMS), X-ray magnetic circular dichroism (XMCD), scanning tunneling microscopy (STM), miniaturized magnetic particle inspection (MPI), anomalous Hall effect (AHE), mechanical deflection of carbon nanotube cantilevers, magnetoresistance, and spin-related devices (spin field effect transistor and spin memory). The advantages, conclusions, challenges, and future of these methods are discussed. The ferromagnetism in sp² carbon will open a door to explore exotic physical phenomena and lay the basis for the development of integrated circuit of spintronics, which is fundamentally different from charge-based conventional electronics.