Growth and characterization of magnetic nanostructures

Spintronic devices such as giant magnetoresistive (GMR) reading head and magnetic tunneling junction (MTJ) play important roles in magnetic recording and sensor industry. We have studied the essential physics that affect the performance of such devices from different points of view.; In searching for an ideal material (100% polarization of conduction electrons at interface) for the electrodes of MTJ, the surface magnetic properties of polycrystalline NiMnSb thin films were studied. Although 100% spin-polarization was not found near the Fermi level, our surface preparation method leads to electronic structure similar to that of a cleaved bulk single crystal surface. The fabrication process of tunneling barrier of MTJ is optimized through the measurements of critical oxidation depth of Aluminum, whose oxide is widely used in MTJs. The annealing effects on ferromagnetic oxide have been studied to explain the enhancement of tunneling magnetoresistance (TMR) upon annealing as encountered in MTJs with over-oxidized tunneling barriers. The results suggest the diminishment of such oxide upon annealing to ∼300°C. The physics of exchange-biased system, which is an integral part of both GMR and MTJ devices, have also been studied to yield information important to both science and technology. Such investigations start from the magnetic anisotropy of antiferromagnetic thin films, whose magnetic properties are much less understood than their ferromagnetic counterparts. A crystal-lattice-distortion-induced magnetic anisotropy is observed in epitaxial NiO/Ag(001) thin films and explained with an atomic model semi-quantitatively. The attempts to image antiferromagnetic domains of the above films yield magnetic contrast consistent with the magnetic anisotropy due to the lattice distortions. Finally, the exchange-bias is found to establish not only the well-known unidirectional anisotropy in ferromagnetic layer but also a uniaxial anisotropy in antiferromagnetic layer.; Through the above studies, the macroscopic performances of MTJ devices are tightly connected to their microscopic properties. Moreover, these studies have lead to many new discoveries such as those related to the magnetic properties of anti ferromagnetic thin films.