Magnetoresistance and domain wall tunneling in mesoscopic ferromagnets

We have investigated the magnetoresistance in one and two dimensional Ni structures. The measurements showed that the resistivity of these small structures were very sensitive to the relative orientation between the measurement current and the magnetization. We have explained some of the observations in terms of the two band model. However, we found that the two current model did not always agree with our measurements, especially when the measurement current was parallel to the direction of magnetization. In addition, the magnetoresistance of a thin wire under this measurement conditions showed reproducible discontinuities. We provide experimental evidence indicating that these discontinuities are from pinning and de-pinning of a single domain wall trapped by structural defects. The de-pinning process of various samples showed quantum tunneling at temperatures below 2-6 K and thermal activation at higher temperatures, which is about an order of magnitude higher than predicted by the theory