Tailoring exchange bias with magnetic nanostructures

Exchange bias in antiferromagnet/ferromagnet bilayers can be tuned by their lateral dimensions when they are below typical magnetic domain sizes. Bilayers of uniform-FeF₂/nanoporous Fe-network have been successfully fabricated by a copolymer nanolithography technique. Feature sizes of about 200 {} have been achieved over a macroscopic 1 cm² area. Exchange bias larger than that in uniform FeF₂/Fe film has been observed, with a similar temperature dependence. Magnetic hysteresis loops and anisotropic magnetoresistance both show pronounced asymmetry that vanishes at the FeF₂ N\'eel temperature. The asymmetry is due to a magnetization rotation process that builds up moments perpendicular to the saturation magnetization directions. The characteristics of the exchange bias originates from the nanostructure of the bilayer.