Nanoscale magnetoelectronic devices via electron beam patterning of diblock copolymer films
A new technique for fabricating magnetic nanowire transport devices is developed by exposing an ordered and microphase separated P(S-b-MMA) diblock copolymer film with an electron beam. The volume fraction of the PS (polystyrene) block of the diblock copolymer is chosen to be 0.7 to get cylindrical microphase separation, where the PMMA (polymethyl methacrylate) cylinders are aligned perpendicular to the substrate by an external electric field. Electron beam exposure of the aligned diblock copolymer crosslinks the PS matrix and degrades the PMMA inside the cylindrical regions, creating a laterally patterned hexagonal array of nanopores (12 run in diameter with a lattice spacing of 24 nm). A unique magnetotransport device is fabricated, consisting of an array of electrodeposited Co nanowires standing atop a thin gold film patterned into a four-probe resistor configuration. Magnetoresistance measurements are performed at various temperatures and magnetic field orientations. This technique is further extended to a multi-level lithographic processing which allowed to fabricate and study the magnetotransport properties through a few vertical Co nanowires in electrical contact. Magnetization reversal of Co nanowires is studied via anisotropic magnetoresistance (AMR) measurements.