Giant Barkhausen Jumps in Exchange Biased Bulk Nanocomposites Sintered from Core-Shell $\hbox {Fe}_{3}\hbox {O}_{4}{-}\hbox {CoO}$ Nanoparticles

The magnetic behavior of spark plasma sintered Fe₃O₄-CoO nanoparticles is studied. The samples sintered at 500 exhibit density over 90% and average magnetite grain size about 100 nm. When the nanocomposite is field cooled below the Néel temperature of CoO, hysteresis loops shows an exchange field of 80 mT at 100 K together with a coercivity of 0.4 T, ten times larger than nanostructured magnetite prepared in the same conditions. When the sample is zero field cooled, the hysteresis loops exhibits giant Barkhausen jumps. 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 phenomenon is thought to be due to self-field cooling.