  We have investigated spin accumulation in Ni/Au/Ni single-electron
transistors assembled by atomic force microscopy. The fabrication technique is
unique in that unconventional hybrid devices can be realized with unprecedented
control, including real-time tunable tunnel resistances. A grid of Au discs, 30
nm in diameter and 30 nm thick, is prepared on a SiO2 surface by conventional
e-beam writing. Subsequently, 30 nm thick ferromagnetic Ni source, drain and
side-gate electrodes are formed in similar process steps. The width and length
of the source and drain electrodes were different to exhibit different coercive
switching fields. Tunnel barriers of NiO are realized by sequential Ar and O2
plasma treatment. Using an atomic force microscope with specially designed
software, a single non-magnetic Au nanodisc is positioned into the 25 nm gap
between the source and drain electrodes. The resistance of the device is
monitored in real-time while the Au disc is manipulated step-by-step with
Angstrom-level precision. Transport measurements in magnetic field at 1.7 K
reveal no clear spin accumulation in the device, which can be attributed to
fast spin relaxation in the Au disc. From numerical simulations using the
rate-equation approach of orthodox Coulomb blockade theory, we can put an upper
bound of a few ns on the spin-relaxation time for electrons in the Au disc. To
confirm the magnetic switching characteristics and spin injection efficiency of
the Ni electrodes, we fabricated a test structure consisting of a Ni/NiO/Ni
magnetic tunnel junction with asymmetric dimensions of the electrodes similar
to those of the SETs. Magnetoresistance measurements on the test device
exhibited clear signs of magnetic reversal and a maximum TMR of 10%, from which
we deduced a spin-polarization of about 22% in the Ni electrodes.
