Condensed Matter Physics Seminar

Thursday, September 14, 2000, 2 p.m.
Plant Sciences Building, Room 1130

Probing the Spin Polarization of Metals with Andreev Reflection

Boris Nadgorny

(Naval Research Laboratory, Washington, DC)

Abstract:  Spin-dependent transport is playing an increasingly important role due to the rapid advance of a new area of electronics dubbed spintronics, which is based on electron spin imbalance in ferromagnets or semiconductors.  We have developed a  new experimental technique for the spin polarization measurements based on Andreev reflection at the normal metal/superconductor interface [1].  The Andreev process allows a single electron with the energy below the superconducting gap to propagate from the normal metal to the superconductor.   In a non-magnetic normal metal this process is always allowed, because every energy state in a normal metal has both spin-up and spin-down electrons.  However, in a magnetic metal this is no longer true: Andreev reflection is limited by a minority spin population and forbidden in a 100% spin-polarized metal with only one (majority) spin population at the Fermi level.  Using point contact conductance measurements we made the first experimental observation of this effect.  This newly developed technique has many possible applications, since the spin polarization can be studied quickly and directly in magnetic materials of almost any shape or form, such as single crystals, films and foils.  We discuss several systems in detail, such as Ni-Fe alloys [2] and optimally doped perovskite, La0.7SrO3MnO3 [3].  We will also briefly discuss experimental and theoretical issues that have important implications for spin dependent transport and tunneling, as well as for Andreev reflection in magnetic materials.

1. R. Soulen et al, Science 282, 85 (1998).
2. B. Nadgorny et al,  Phys. Rev. B  61, R3788  (2000).
3. B. Nadgorny et al, submitted to Nature

Host: Victor Yakovenko

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