The spin contrast due to the spin dependent electronic structures of a sample observed in the spin-polarized scanning tunneling microscopy/spectroscopy (STM/STS) experiments is proportional to the inner product of the polarization vectors of a magnetic sample and a magnetic tip. No reliable calibration method, so far, of a tip polarization vector has been reported.
In this study, photo-excited 50% polarized tunnelling electrons from the conduction band of GaAs(110) sample cleaved in ultra-high vacuum (UHV) are used for in-site measurements of a tip polarization vector. The excited electrons were injected into the tip under tunneling condition.
Standard magnetic Fe-coated W tips were approached to the GaAs surface illuminated by right and left handed circular polarized light and differential conductivity (dI/dV) curves was measured in UHV. Spin contrast was observed in the dI/dV curves. The direction of spin polarization of tunnelling electrons is parallel to the direction of circular polarized excitation light which is reflected by three mirrors in UHV chamber. The mirrors were located at the directions (θ1,φ1), (θ1,-φ1) and (θ2,φ2) in a polar coordinate, where θ=0 is a normal to the sample. The incident light to the mirrors was adjusted to have ellipsoidal polarization which becomes circular polarized (>98%) light after reflection by the mirrors.
Three sets of linear equations with three sets of experimentally obtained spin contrasts give us the direction and quantity of the polarization vector of the tips. If we assume the deflected light beam into the GaAs sample is responsible for the polarization of tunnelling electrons, a tip polarization exceeds 100%, which must not be the case. The tunnelling electrons seem to be excited at the surface of GaAs sample with the spin quantization axis parallel to incident light beam.
We obtained, however, serious results that two tips so far measured showed different polarization vectors: P=42%,θ=42o,φ=121o for one tip and P=98%,θ=45o,φ=305o for another tip. These results may suggest that a tip polarization vector should be calibrated for each spin-polarized STM/STS measurement.
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