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.