Magnetic force microscopy analysis and quantum transport in ferromagnetic dot structure embedded in semiconductor quantum wires

We have made ferromagnetic (Ni) dot structure embedded in semiconductor quantum wires by two-step surface modifications with STM. The wire was a kind of split-pate type made on pseudomorphic (PM) HEMT surface and a hole into which the Ni was buried has a size of 200 nm upper diameter and 100 nm depth reaching two-dimensional electron gas (2DEG) plane located at 60 nm depth from the surface. To investigate the magnetic properties of the dot, which is tightly related to those of transport, magnetic force microscopy (MFM) measurements were carried out at ambient conditions. The observations suggest a domain wall formation inside the dot before magnetization. It disappeared after magnetization, which implies a single domain formation in the entire dot. In low temperature transport measurements, we observed two distinct magnetization-dependent features: before magnetization, observed were aperiodic conductance oscillations against gate voltage sweep, while periodic and reproducible oscillations were recorded after magnetization. Moreover, in the current-voltage characteristics, Coulomb blockade-like features were observed and the Coulomb gap reduced to about a half by applying an external magnetic fields of +/-5000 G, which seems to mean the dot size expansion. On the basis of those results, we propose a domain related transport model which essentially interprets the observed transport properties. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.