Magnetic and transport properties of spin-dependent tunnel junctions

The structural, transport, and magnetic properties of ferromagnet/insulator/ferromagnet tunnel junction structures were examined. The insulating materials investigated were sputtered oxides of HfO₂, SiO₂, or MgO or native oxides of CoFe, Co, or Fe. The ferromagnetic electrodes in various combinations were Fe, Co, NiFe, and CoFe. Investigations using artificial sputtered oxide tunnel barriers demonstrated magnetoresistance up to 30% at 30K. The I-V curves of Pb/(sputtered oxide)/CoFe junctions at 4.2 K reflected the presence of the superconducting energy gap of Pb. Conductance measurements at 1.5 K revealed clear longitudinal and transverse phonon structure from the superconducting Pb as evidence of single-step, elastic tunneling. Oxidation experiments and the use of a Au base electrode independently showed, however, that the sputtered oxide barriers were permeable for thicknesses required for tunneling. The tunneling behavior observed in these junctions was due to a native oxide that formed on the base electrode during fabrication, i.e. the sputtered oxide was not the active barrier. Subsequently, superconducting tunneling and magnetoresistance were studied in junctions with native oxide barriers grown by exposing thin films of CoFe to ambient atmosphere. Junctions with Co counterelectrodes showed a maximum tunneling magnetoresistance of 2.5% at room temperature and 6% at 77K. In these junctions, the switching of the soft magnetic electrode can be influenced by magnetic coupling with the hard electrode. This was observed in NiFe/SiO₂(x)/CoFe unpatterned structures, which showed a large enhancement of the NiFe coercivity compared to a free NiFe film. The coercivity of the NiFe film decreased with an increasing SiO₂ spacer thickness. Magnetization experiments were consistent with a magnetostatic origin to the coupling. Lorentz imaging of hard CoFe films revealed a locking domain wall structure in fields less than the 140 Oe coercive field. The NiFe coercivity was enhanced due to coupling between stray flux from the CoFe walls and induced walls and ripple structure in the NiFe.