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R. J. Soulen Jr., J. M. Byers,* M. S. Osofsky, B. Nadgorny, T. Ambrose, S. F. Cheng, P. R. Broussard, C. T. Tanaka, J. Nowak, J. S. Moodera, A. Barry, J. M. D. Coey
A superconducting point contact is used to determine the spin polarization at the Fermi energy of several metals. Because the process of supercurrent conversion at a superconductor-metal interface (Andreev reflection) is limited by the minority spin population near the Fermi surface, the differential conductance of the point contact can reveal the spin polarization of the metal This technique has been applied to a variety of metals where the spin polarization ranges from 35 to 90 percent: Ni^sup 0.8^Fe^sub 0.2^, Ni, Co, Fe, NiMnSb, La^sub 0.7^Sr^sub 0.3^MnO^sub 3^, and CrO^susb 2^.
A new class of electronics is emerging based on the ability of ferromagnetic metals to conduct spin-polarized currents (1). The effectiveness of magnetoelectronics depends on the extent to which a current is spin-polarized. All device designs improve their performance as the spin polarization P - 100%. For both scientific and technological reasons it is important to be able to directly and easily measure the electronic spin polarization at the Fermi energy, E^sub F^, of a candidate material.
Unfortunately, determining P at E^sub F^ of a ferromagnet (FM) is not easy. A typical transition-metal FM has two components to its electronic structure: narrow d bands that may be fully or partially spin-polarized (due to the on-site exchange energy) and broad s bands with a lesser degree of spin polarization (due to hybridization with the d bands). The quantity P can be defined as where N^sub sigma (E) is the spin-dependent density of states. The value of P is controlled by the extent to which these s and d bands cross the Fermi surface. If the orbital character at the Fermi surface of a FM is primarily d-like, then P will be high. If, however, the orbital character is s-like or s-d-hybridized, then P can be low or high depending on the details of the electronic structure. The magnetization of a material may show that all of the electronic spins associated with the d orbitals are aligned but that P at E^sub F^ can be depressed (2). However, metallic oxide FMs,...