Spin-polarized vacuum tunneling at small gap widths

– We studied spin-polarized tunneling through a vacuum barrier using spinpolarized scanning tunneling microscopy on 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 ≈ 4.5 Å, 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. In 1971, Tedrow and Meservey opened up the field of spin-polarized tunneling (SPT) by studying electron tunneling between a ferromagnet and a superconductor under an applied magnetic field [1]. Later, Jullière found that due to spin-polarized tunneling between two ferromagnetic layers, the tunneling resistance depends on the relative orientation of magnetization of the layers [2]. Since then, SPT has been studied intensively from the fundamental point of view to illuminate the underlying physical mechanisms [3–9] and has stimulated many new applications [10–13]. Nevertheless, SPT is still far from complete understanding. The difficulties to obtain a coherent picture of the effect are partly related to the complex structure of the tunneling junctions, often containing poorly characterized amorphous barriers causing higher-order effects in SPT [13–15]. In this letter, we study SPT across the tunable vacuum barrier of a spin-polarized scanning tunneling microscope (Sp-STM) [16–18]. Sp-STM provides an ideal model system to study SPT as it has a simple vacuum barrier, whose width can be controlled in a precise way, and allows as well to use atomically clean surfaces as electrodes. In contrast to a variety of different models for SPT, there is consensus that the tunneling magnetoresitance (TMR) effect is a consequence of the exchange splitting of the band structure of the ferromagnetic electrodes which leads to an unbalanced distribution of majority and minority electrons at the Fermi energy. Assuming conservation of the electron spin during tunneling, this imbalance leads to different tunneling currents I for parallel (p) and antiparallel (ap) magnetic configuration. (∗) Present address: Argonne National Laboratory 9700 S. Cass Avenue, Argonne, IL 60439, USA. (∗∗) E-mail: [email protected]