Giant Barkhausen jumps in exchange biased bulk nanocomposites sinterd fom core-shell Fe3O4-CoO nanoparticles

The magnetic behavior of spark plasma sintered Fe3O4-CoO nanoparticles is studied. The samples sintered at 500°C exhibit density over 90% and average magnetite grain size about 100 nm. When the nanocomposite is field cooled below the Néel temperature (TN=291 K for CoO), hysteresis loops shows the expected shift with an exchange field of 80 mT at 100 K that drops down to zero approaching TN. The coercivity at 100 K reaches 0.4 T, ten times larger than nanostructured magnetite prepared in the same conditions. When the sample is zero field cooled down to 90 K, the hysteresis loops exhibits giant Barkhausen jumps, an anomalous feature never observed before to our knowledge. The density of jumps gradually decrease on heating and disappear between 150 and 170 K. The stochastic character of the jumps is visible in the plot of the differential permeability. This new phenomenon is thought that it could be related to self-field cooling.