Europhys. Lett.
Volume 63, Number 3, August 2003
Page(s) 419 - 425
Section Condensed matter: electronic structure, electrical, magnetic, and optical properties
Published online 01 November 2003
DOI: 10.1209/epl/i2003-00549-4
Europhys. Lett., 63 (3) , pp. 419-425 (2003)

Spin-polarized vacuum tunneling at small gap widths

H. F. Ding, W. Wulfhekel, U. Schlickum and J. Kirschner

Max-Planck Institut für Mikrostrukturphysik - Weinberg 2, 06120 Halle, Germany

(Received 17 September 2002; accepted in final form 5 June 2003)

We studied spin-polarized tunneling through a vacuum barrier using spin-polarized scanning tunneling microscopy on $\chem{Co(0001)}$. By varying the tip-to-sample distance in a controlled way, the tunneling magnetoresistance, i.e., the tunneling current asymmetry for parallel and antiparallel configuration of tip and sample magnetization, was measured as a function of the gap width. At large gap widths the asymmetry is constant. At gap widths below $\approx 4.5$ $\un{\AA}$, a drop of the asymmetry was found in contrast to predictions of the Jullière model. The drop of the current asymmetry in spin-polarized tunneling is found to correlate with drop of the imaginary electron momentum inside the barrier and we compare our experimental results with the predictions of the Slonczewski model.

72.25.Mk - Spin transport through interfaces.
68.37.Ef - Scanning tunneling microscopy (including chemistry induced with STM).

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