Perpendicular Exchange Bias of [Pt/Co]5/IrMn Multilayers on Self-Organized Hexagonally Patterned Nanodots

[Pt/Co]₅/IrMn multilayers were deposited on hexagonally patterned nanodots on the backside of anodized alumina oxide (AAO) templates and thermally oxidized Si substrates. AAO templates with different diameters of about 50, 60, 130 and 400 nm nanodots were used to fabricate the exchange-biased nanostructures and investigate the size effect on the perpendicular exchange bias. With decreasing nanodots diameter, the multilayers exhibited large variations in both exchange bias field H_E and coercivity H_C. Compared to the flat film, H_E of the nanodots samples was significantly enhanced, which can be attributed to the reduction in antiferromagnetic and ferromagnetic domain sizes induced by the curved morphology and domain wall pinning between two adjacent magnetic nanocaps. Unexpectedly, the 50 nm nanodot sample had higher thermal stability than the flat film sample.