Exchange Bias in Oxide-based Magnetic Nanoparticles

The exchange bias phenomenon forms the backbone of modern information technology, whose continued progress is crucial for the development of magnetic read-heads, high density magnetic storage media, etc. In this talk, experimental results of four multifunctional core-shell nanoparticle systems will be discussed where the exchange bias phenomenon has been studied. The possible technological implications of the observed results will also be pointed out. The studied nanoparticle systems can be categorized as : Category-I (La0.2Ce0.8CrO3 and BiFe0.8Mn0.2O3), and Category-II (Fe3O4 core/-Mn2O3 shell and BiFeO3 core-NiFe2O4 shell). The La0.2Ce0.8CrO3 nanoparticles showed a novel coexistence of the exchange bias field and magnetization sign-reversal phenomena, which was exploited to provide a new direction for designing a volatile magnetic memory and a thermally assisted magnetic memory. A significantly high value of the exchange bias field was observed in the BiFe0.8Mn0.2O3 nanoparticle system. Besides, a diluted antiferromagnetic (DAFF) nature of its shell part helped us to give a new methodology to rule out the role of a DAFF shell in the shift of the field-cooled (FC) hysteresis loop. The Fe3O4 core/-Mn2O3 shell nanoparticle system showed a novel magnetic proximity effect, however without the presence of any exchange bias effect. This study brought out the importance of the relative strength of an interface exchange coupling in governing the simultaneous occurrence of exchange bias as well as magnetic proximity effects. The experimental results of the BiFeO3 core/NiFe2O4 shell system showed an unusual shift of the zero field-cooled (ZFC) hysteresis loop along the magnetic field axis. An enhancement of the remanent magnetization along with a decrease in the coercivity was also observed in the FC case, as compared to the ZFC case, which was not found commonly in any conventional exchange-biased system. All these unusual features of the BiFeO3 core/NiFe2O4 shell nanoparticles were explained in terms of an interface exchange coupling between core and shell.