We are interested in patterning of perpendicular anisotropy magnetic films into nanostructures to explore the scaling behavior of the magnetic properties, with relevance to magnetic random access memories. Magnetic tunnel junction films were patterned into pillars using electron beam lithography or block copolymer lithography combined with ion-beam etching. Sharp hysteresis loops were recorded using conductive atomic force microscopy, from which the variation of magnetoresistance, switching field, and switching current can be determined.

Perpendicuar magnetic tunnel junction film patterned into pillars using block copolymer lithography on the left, and into 40 nm pillars using electron beam lithography, right.

We have also examined the domain wall dynamics in perpendicular magnetic anisotropy (PMA) films, which offer lower thresholds for domain wall propagation. We have worked on PMA [Co/Pd]_n multilayers (K_u ~ 3   10⁶ erg cm^-3) and L1₀-FePt (K_u ~ 7  10⁷ erg cm^-3) films. In a [Co (0.6 nm)/Pd (1.2 nm)]₁₀ nanowire of linewidth 200 nm with top Ta (7 nm)/Au (130 nm) electrodes, a current density of 3.7  10¹¹ A cm^-2 passing through the Au electrode orthogonal to the nanowire produces an Oersted field which induces a local domain wall in the nanowire of coercivity 400 Oe. At applied fields of H_z = 550 Oe and H_z = -550 Oe, domains magnetized up and down, respectively, propagated from the edges towards the centre of the nanowire. The effects of edge tapering on domain nucleation have also been explored.

Domains in Co/Pd. (a) SEM image showing the Au conduction electrode pads on the nanowire of linewidth 200 nm. MFM image of (b) local domain wall formation via a current-induced Oersted field through the Au electrode orthogonal to the nanowire (c) domain propagation in the presence of an out-of plane field H_z = 550 Oe and (d) H_z = -550 Oe.