Exchange bias in barium ferrite/magnetite nanocomposites

Exchange bias which accompanies a magnetic hysteresis loop shift along field axis or increase in coercivity, occurs due to exchange interactions between ferromagnetic (FM) and antiferromagnetic (AFM) or in ferrimagnetic nanoparticles/nanolayers systems. Mixture of barium ferrite (BaFe 12 O 19 ) and graphite was mechanically milled for different times. Phase analysis, particles morphology, magnetic properties at room temperature and magnetic properties after field cooling at 4 k were measured via XRD, HRTEM, VSM and SQUID, respectively. A nanocomposite of BaFe 12 O 19 /Fe 3 O 4 forms after 20 and 40 h of milling. HRTEM images revealed that the nanocomposite consists of crystallites of both phases in intimate contact with crystallite sizes below 20 nm after 20 h milling. Field cooling of the 20- and 40-h milled samples up to 4 k resulted in exchange bias phenomenon. The shift in hysteresis loop for 20- and 40-h milled samples was 204 and 254 Oe, respectively. In spite of the mostly observed exchange coupling systems being ferromagnetic/antiferromagnetic systems, in this research the exchange coupling occurred between ferrimagnetic phases. The large difference between coercivity values at 300 and 4 k revealed that superparamagnetic particles constitute a large volume fraction of the milled nanocomposites.