Mesoscopic Conductance Fluctuations in Cobalt Nanoparticles

We~present measurements of mesoscopic conductance fluctuations in Cobalt particles of diameter 200nm. Samples are made by e-beam lithography and shadow metal deposition. Co particles are not single domain; domain walls are nucleated at the contacts between Co and Cu-reservoirs. We obtain the dependence of~peaks in differential resistance with the applied voltage and the magnetic field during the magnetization reversal process at 0.03K temperature. The conductance fluctuations with the magnetic field are caused by a mechanism different from the usual Aharonov-ohm effect. In particular, domain walls are found to generate significant mesoscopic fluctuations. We obtain that electron transfer across the domain wall is associated with a phase change of about 5$\pi $. We explain how this phase-shift arises from a not perfectly parallel spin-transport across domain walls. The dephasing time is very short, $\tau _\phi \sim ps$. Fast dephasing is correlated with the strong magnetocrystalline anisotropy in Co. This work was performed in part at the Georgia-Tech electron microscopy facility. We thank P. Brouwer for valuable discussions. This research is supported by the David and Lucile Packard Foundation grant 2000-13874 and Nanoscience/Nanoengineering Research Program at Georgia-Tech.