We report unconventional exchange bias in oxide-coated manganese nanoparticles, in which the Curie temperature of Mn3O4 shell is lower than the Néel temperature of the antiferromagnetic core. The coercivity (873kA∕m) of the nanoparticles, which is more than four times greater than that of bulk Mn3O4, has been enhanced significantly. A considerable enhancement in Curie temperature compared to the bulk was also observed for Mn3O4 in nanoscale. An exchange bias field as large as 400kA∕m was observed due to the strong interfacial exchange coupling. A simple phenomenological model is given to understand these phenomena in this ferri/antiferromagnetic system.

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Amorphous carbon nanofibers (ACNFs) have been synthesized by a thermal chemical vapor deposition technique. The ACNFs grow as two branches perpendicular to {111} facets of a catalytic copper nanoparticle. The carbon nanofibers are composed of disordered localized nanofragments which in turn consist of several graphene layers. The ACNFs show a paramagnetic characteristics at 2, 5, and 10 K. The magnetic moments are suggested to originate from a large amount of defects in the graphene layers of the nanofragments.