Magnetic nanostructures from block copolymer lithography: Hysteresis, thermal stability, and magnetoresistance

A series of two dimensional close-packed Co, NiFe, and CoFe/Cu/NiFe magnetic particle arrays, in which the particles have mean diameters of 34 nm, thicknesses of 5-20 nm, and periodicity of 56 nm, were made using a process based on self-assembled polystyrene-b-polyferrocenyldimethylsilane block copolymer templates. Interparticle magnetostatic interactions lead to the thermally assisted collective reversal of small groups of particles. The switching field distribution, whose width decreases as the thickness increases, has been modeled as a result of the distribution of particle size, shape, and microstructure. For multilayered particles, interlayer magnetostatic interactions stabilize flux-closed states with antiparallel alignment of the CoFe and NiFe layers at remanence. The multilayer particles show a greater thermal stability than single-layer particles, and a magnetoresistance comparable to that of the unpatterned film.