Measuring spin polarization with superconducting contacts

After the Giant MagnetoResistance (GMR) effect was discovered in magnetic multilayers in 1988, it was commercially applied in the space of less than ten years to the reading of information stored on magnetic hard disk drives. This success stimulated a huge amount of research on spin-dependent electrical transport and refocused attention on how the spin-polarization of a magnetic material is measured.

At an interface between normal and superconducting metals, there is an enhancement of the conductance because a Cooper pair in a superconductor carries double the charge of an electron in the normal material which is "retro-reflected" at the interface - an effect called Andreev reflection (see figure). This enhancement should be suppressed when the normal metal is replaced by a ferromagnetic metal and the degree of suppression should depend on the spin-polarization of the ferromagnet. Determining spin-polarization in this way using "Point Contact Andreev Reflection (PCAR)" has received a great deal of attention. However, the determination is indirect because it requires fitting the measured current-voltage characteristic to a theoretical model - and as such depends on the details of the theoretical model. A very good fit could be achieved without including any spin-dependence of the interface transparency which is especially surprising since such a spin-dependence is the origin of the GMR effect!

By reformulating PCAR in terms of scattering matrices we could make detailed, material-specific calculations taking into account the spin-dependent interface reflection and transmission without introducing any fitting parameters. Comparison with the measured results for Pb|Cu, Pb|Ni and Pb|Co allowed us to conclude that the formalism conventionally used to describe transport through ferromagnet/superconducting interfaces is incomplete and that PCAR measurements do not determine bulk spin-polarization.

K. Xia, P.J. Kelly, G.E.W. Bauer and I. Turek, Spin-dependent transparency of ferromagnet/superconductor interfaces, Phys. Rev. Lett. 89, 166603 (2002).

Macintosh HD:Users:KellyPJ:Documents:Admin:MESA+:Jaarverslag:2002:AndreevReflection.TIF

Figure 1. A spin-up electron incident from the left in a normal metal (N) must pair up with a spin-down electron in order to be able to pass into the superconducting metal (S) on the right where the charge carriers are Cooper pairs. An equivalent way of looking at this is as if the first spin-up electron is "retroreflected" as a spin-down hole.