Fabrication of a Split-Gate Quantum Wire Having a Ferromagnetic Dot

We have fabricated split-gate quantum wires having a buried ferromagnetic dot, by successively utilizing electron-beam (EB) and two-step scanning tunneling microscope (STM) fabrication. For STM fabrication, we used an STM/scanning electron microscope (SEM) combined with a system operated in high vacuum. The fabrication method is a kind of electrical evaporation with a tungsten (W) tip (top curvature is less than 50 nm). In the first step, a W tip was brought between the split-gate, and then a hole was fabricated by applying a pulse voltage between the W tip and the sample surface. In the second step, a W tip coated with nickel (Ni) was brought near the fabricated hole. Then by applying a pulse voltage between the Ni-coated W tip and sample surface, electrically evaporated Ni from the tip is buried into the hole. In a preliminary measurement at 0.3 K, we obtained the following unique transport properties. In a 4-terminal conductance (G 4t) as a function of gate voltage (V g), we observed a clear “kink” (an abrupt change of dG 4t/ dV g and step structures) before full pinch-off of the wire. In both regions of G 4t, that is, when V gk<V g (before the kink appears) and when V g<V gk (after the kink appears) (V gk is the gate voltage at which the “kink” appears), some step structures are seen. The step difference (ΔG 4t) is, however, different between the two regions. That is, ΔG 4t=2-4×(2e 2/h) before the kink appears, while ΔG 4t=(1/8)-(1/4)×(2e 2/h) after the kink appears.