Size-dependent exchange bias in vortex-stabilized Py/IrMn hemispherical nanocaps

The size-dependent exchange-bias in Py/Ir₂₃Mn₇₇(IrMn) magnetic nanocaps was investigated. The magnetic cap structures were fabricated by depositing magnetic thin films of layer stack Py (40 nm)/Ir₂₃Mn₇₇(t= 3 nm, 5 nm, 7 nm)/Pt (5 nm) onto large, closely packed arrays of polystyrene spheres with diameters of 175 nm, 607 nm, and 1020 nm. By zero-field cooling below the blocking temperature of the antiferromagnetic IrMn grains, the spin structure of the Py layer is imprinted into the IrMn, which manifests as an increase in the vortex nucleation and annihilation fields. Under field cooling, either a displaced vortex state or a C-state can be stabilized. Furthermore, we investigated the distribution of blocking temperatures across the Py/IrMn sample series. Blocking temperature measurements indicated a distribution of IrMn grain stabilities, where the mean blocking temperature was found to increase with IrMn thickness but decrease with increasing nanocap diameter. This study demonstrates that the exchange bias in Py/IrMn magnetic vortex caps can be effectively tailored by varying the diameter of the cap structures and the thickness of the IrMn layer.