Self-assembled Magnetic Nanostructures: Epitaxial Ni on TiN (001) Surface

Abstract

Systems that contain single domain magnetic particles have been receiving intensive attentions over recent years since they are possible candidates for applications in ultrahigh-density data storage and magnetoelectronic devices. The focus of this research is self-assembly growth of magnetic nickel nanostructures by domain matching epitaxy under Volmer-Weber mode. The growth was conducted by pulsed laser deposition (PLD) technique using epitaxial titanium nitride film as the template, which was in turn grown on silicon (100) substrate. The structural characterization includes X-ray diffraction and both cross-sectional and plan-view transmission electron microscopy. The results showed that the nickel islands formed exhibit a self-assembled nature, i.e., a certain degree of uniformity in orientation, shape, and size. The orientation relationship observed is Ni [100] // TiN [100] // Si [100], the so-called 'cube-on-cube' relationship. The islands are faceted, forming truncated pyramids with walls of (111) planes and a flat top of (100) plane. The base of islands is rectangular with the two principal edges parallel to two orthogonal 011 directions. The size distribution is relatively narrow, comparable to that obtained from self-assembled islands grown under Stranski-Krastanow (S-K) mode. A certain degree of self-organization was also found in the island lateral distribution: island chains were observed along the directions close to 011, which are also the edge directions. The island faceting could be explained by surface energy minimization. The interaction of island edge induced strain field between neighboring islands is believed to be responsible for the size uniformity and the lateral ordering. Magnetic measurements were also conducted on these crystallographically aligned nickel islands using superconducting quantum interference device (SQUID) magnetometer, and the results were compared with that obtained from the ensemble of randomly oriented nickel islands, which were grown on polycrystalline/amorphous Al2O3 matrix layer. It is found that both blocking temperature and coercivity of aligned nickel islands are significantly higher than that of the randomly oriented nickel islands. The enhancement in ferromagnetism is attributed to the increased collective effects resulting from the particle interactions in the ensemble of aligned islands, which are self-assembled and self-organized to some degree.