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 semiconducto r quantum wires by two-step surface modifications with STM. The wire w as a kind of split-pate type made on pseudomorphic (PM) HEMT surface a nd a hole into which the Ni was buried has a size of 200 nm upper diam eter and 100 nm depth reaching two-dimensional electron gas (2DEG) pla ne located at 60 nm depth from the surface. To investigate the magneti c properties of the dot, which is tightly related to those of transpor t, magnetic force microscopy (MFM) measurements were carried out at am bient conditions. The observations suggest a domain wall formation ins ide the dot before magnetization. It disappeared after magnetization, which implies a single domain formation in the entire dot. In low temp erature transport measurements, we observed two distinct magnetization -dependent features: before magnetization, observed were aperiodic con ductance oscillations against gate voltage sweep, while periodic and r eproducible 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 re lated transport model which essentially interprets the observed transp ort properties. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.