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Spin engineering in magnetic nanostructures

Increasing storage density requirements in computation has continued to motivate the exploration of new magnetic media. In the search for future ultrahigh density magnetic recording media, there are two approaches that are currently being studied intensively. One is based on conventional continuous film media but the storage density is increased through stabilising the nanoscale magnetic domains so as to resist superparamagnetic effect. Another way is by patterning the magnetic films into magnetically isolated dots.

In this project we introduce a new type of magnetic medium where the spin configurations are engineered in chemically homogenous magnetic films. Such a spin engineered media not only maintains the surface planarity but also the homogeneity of the magnetic materials. Substrates with laterally modulated single-crystal and polycrystalline surface regions were used to induce selective epitaxial growth of a ferromagnetic Ni film. The samples have a structure of GaAs(001)/Co(1.8nm)/Cu(70nm)/Ni(5nm)/Cu(5nm). Ultrathin NiO (1nm) patterns were used to modulate the GaAs substrate surface. The films that interfaced with GaAs grows epitaxially and the magnetisation of the Ni film oriented out-of-plane direction. The film that interfaced with NiO patterns have polycrystalline structure and the magnetisation of the Ni directed along the film plane. The co-existence of polycrystalline and epitaxial features in the films are confirmed by in situ reflection high-energy electron diffraction analyses. Atomic force microscope was used to examine the sample surface, yielding statistical roughness parameters in the range similar to that of epitaxial continuous Cu/Ni/Cu films. The co-existence of the in-plane and out-of plane magnetization regions in the films were imaged by magnetic force microscopy (see figure below) and also confirmed by magneto-optic Kerr effect and superconducting quantum-interference device measurements. The 90� transition between the in-plane and out-of-plane regions, which is due to spatially varying anisotropy properties in film, is a new type of magnetic wall. Micromagnetic simulations results show that the spin transition regions are asymmetric and has no mobility under external perturbation. We term this new feature as anisotropy constrained magnetic wall.


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