Superconducting STM tips in high magnetic fields:
A new atomic scale probe of the spin polarization of tunneling electrons
Markus Etzkorn
Max-Planck-Institute for Solid State Research, Stuttgart, Germany
Reducing dimensionality of objects leads to a multitude of interesting phenomena. For example
superconductors can remain in their ordered state in magnetic fields much larger than their critical
field when their dimensions are reduced. This leads to the interesting situation that the Cooper
pairs are exposed to high magnetic fields. For superconductors with small spin orbit interaction
this condition lead to a Zeeman splitting of the quasi-particle density of states that has been
observed in planar tunnel junctions [1]. Under such circumstances a tunnel junction between a
superconductor and a ferromagnet is an ideal probe to access the absolute value of the spin
polarization of the ferromagnet at the Fermi energy [2]. We have transferred this concept from
planar tunnel junctions to scanning tunneling microscopy using a superconducting tip. For
vanadium tips, we indeed find that the tip apex, being confined, remains superconducting in
external magnetic fields as high as 4 Tesla! We observe a clear splitting of the quasi-particle
density of states well described by a Zeeman term. We analyze the measured spectra in the
framework of Maki´s theory [3]. I will show first experimental results obtained using such tips for
spin polarized scanning tunneling microcopy on ferromagnetic structures. Since such
measurements provide direct access to the spin polarization of the tunneling electrons in absolute
scales, they can certainly complement established spin polarized STM techniques. For example
the dependence of the measured spin polarization on the tip to sample distance reflects the decay
of different electronic states into vacuum.
[1] R. Meservey, P.M. Tedrow and P. Fulde, Phys. Rev. Lett. 25 (1970) 1270.
[2] P.M. Tednow and R. Meservey, Phys. Rev. Lett. 26 (1971) 192.
[3] K. Maki, Prog. Theor. Phys. 32 (1964) 29.