Jochen Schneider

Fabrication and properties of Fe/Fe oxide and Co/Co oxide films

Evaporation of Fe and Co at a residual O 2 pressure of 10-5to 10-4Torr yielded magnetic films of high coercivities up to 500 Oe and magnetizations of about 1000 EMU/cm3. While the Co films were continuously deposited in the O 2 atmosphere, a sandwich structure of Fe and Fe 2 O 3 was formed by intermittently altering the evaporation conditions. Both kinds of films were produced in the same geometrical arrangement. The Fe/Fe 2 O 3 films showed shape anisotropy caused by the oblique incidence deposition, while no uniaxial anisotropy was observed on the Co films. For both, Co films and Fe/Fe 2 O 3 sandwich films, it was possible to adjust the magnetic properties within a certain range. In both cases the coercivity increased considerably by annealing at 200°C.

Magnetic first-order phase transition in MnAs films

For polycrystalline and epitaxial films of MnAs, the magnetic phase transformation from the ferromagnetic to the paramagnetic phase was studied. It was found that the films which are removed from the substrate undergo an abrupt loss of magnetization near 38°C within a temperature interval of 0.2°C. They exhibit a temperature hysteresis of 5°C. Films clamped to thick inflexible substrates show a second-order phase transition and a shift in Curie temperature of 35°C per 1-percent volume change due to a difference in thermal expansion coefficient between film and substrate. MnAs films on highly flexible thin plastic foils exhibit a first-order-like phase transition. First-order transition films show a drastic change in resistivity ρ due to microcracks. At 20°C ρ of the virgin film is 1.93 × 10-4Ω cm and increases irreversibly to 6.0 × 10-4Ω cm after 15 transitions. The reversible change varies between a factor of 2.5-3.5. The Hall voltage U_{H} = (R_{0}H + R_{1}M) i/d was measured and the Hall constants R 0 and R 1 were calculated to be 2 × 10-3and 0.33 cm3/A.s respectively, at 20°C.