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-${\mathrm{FeF}}_2$/nanoporous Fe-network have been successfully fabricated by a copolymer nanolithography technique. Feature sizes of about 200 Å have been achieved over a macroscopic 1 ${\mathrm{cm}}^2$ area. Exchange bias larger than that in uniform ${\mathrm{FeF}}_2/\mathrm{F}\mathrm{e}$ film has been observed, with a similar temperature dependence. Magnetic hysteresis loops and anisotropic magnetoresistance both show pronounced asymmetry that vanishes at the ${\mathrm{FeF}}_2$ Néel 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.