Exchange bias tuned by cooling field in nanoparticles

We report the magnetic properties in the nanosized charge ordering manganite La0.2Ca0.8MnO3 with an average particle size ∼50 nm. The sample exhibits ferromagnetism at low temperatures. The exchange bias phenomenon is observed when the sample is cooled down in an external magnetic field. Moreover, the exchange bias field is dependent on the cooling field and shows a maximum of ∼520 Oe under a cooling field ∼5 kOe. The exchange bias effect can be attributed to the exchange coupling between the ferromagnetic shell and antiferromagnetic core. The decrease of exchange bias field in high cooling field can be attributed to the growth of ferromagnetic component under high cooling field.