Regensburg 2004 – wissenschaftliches Programm

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O: Oberflächenphysik

O 12: Magnetismus in reduzierten Dimensionen

O 12.2: Vortrag

Montag, 8. März 2004, 16:00–16:15, H45

Spin effects in Coulomb blockade of Fe/MgO/Fe — •Wulf Wulfhekel¹, F. Zavaliche^1,2, M. Klaua¹, C.C. Kuo^1,3, M.T. Lin^1,3, and J. Kirschner¹ — ¹MPI für Mikrostrukturphysik, Weinberg 2, 06120 Halle — ²Departement de physique, Universite de Montreal, Quebec H3C 3J7, Canada — ³Department of Physics, National Taiwan University, Taipei 106, Taiwan

The Coulomb blockade (CB) of nanometer sized Fe islands grown on single crystalline MgO films on Fe(001) was studied using scanning tunneling microscopy (STM) at 25 K in ultra high vacuum. After deposition of 3-4 monolayers (ML) MgO, a sub-ML amount of Fe was deposited leading to the formation of Fe islands on the MgO films as observed by STM. Scanning tunneling spectroscopy was then used to locally measure the tunneling characteristics for tunneling from the tip via the island into the Fe substrate. The I(V) curves showed the characteristic steps of CB. The position of the steps in the I(V) characteristics of periodicity U_C are proportional to the Coulomb charging energy e/C, where e is the electron charge and C the islands capacitance. A plot of U_C versus e/C for the Fe islands, where C was estimated from the island size as observed with STM, reveals a linear dependence in agreement with CB. We, however, observe a constant and positive offset in U_C by 1 eV, which cannot be explained by spin independent tunneling. We propose a model for spin-dependent CB, that relates the offset to the exchange the additional electron feels in the Fe islands. No systematic offset in U_C has been found for identical experiments with Pd islands indicating that the observed offset is related to the island material.