Exploring a low temperature glassy state, exchange bias effect, and high magnetic anisotropy in Co₂C nanoparticles

It is interesting to explore the connections between the exchange bias effect (EBE) and magnetic anisotropy (MA). It is often found that materials exhibiting a strong EBE also have enhanced MA. Here we explore 40 nm diameter Co₂C nanoparticles (NPs) that exhibit ferromagnetism with a blocking temperature exceeding 300 K. We report the first observation of EBE in these Co₂C NPs below 50 K. The effect arises from the exchange coupling of frozen ferromagnetic spins with a freely rotatable spin component. The dynamics of the freely rotatable component freezes in a temperature range between 5 K to 20 K resulting in low-temperature coexistence of a glassy behavior along with ferromagnetism. In fact, Co₂C displays a unique separation of onset temperatures of spin freezing (∼20 K), vanishing of EBE (∼50 K), and magnetic blocking (⩾450 K). Our calculations show that Co₂C NPs have a core-shell structure. Our study suggests that modifying chemical co-ordination in the shell is one of the effective routes to manipulating MA compared to manipulating EBE.