| A point-contact spectroscopy technique has been used to study (1) the spin polarization of several ferromagnetic materials, including candidates of half-metallic ferromagnets, and (2) spin-transfer torque effects (more specifically current-induced spin-wave excitations) in single ferromagnetic layers.; The values of spin polarization of common ferromagnetic materials (Ni, Co and Fe) have been measured by the point-contact Andreev reflection (PCAR) technique. PCAR uses a sharp superconducting tip to create a small-area interface between the superconductor and the material to be measured. Conductance vs. bias voltage curves have been recorded at helium temperature for specific contacts. Strong nonlinearity exists in the G - V curves due to the Andreev reflection, the process of normal-currentto-supercurrent conversion. The probability of Andreev reflection will be suppressed if the metal is a ferromagnet with certain degree of spin polarization at the Fermi level. Proper quantitative analyses of a conductance curve yield the value of spin polarization P and the strength of interfacial scattering Z. A systematic dependence of P upon Z has been demonstrated and attributed to spin-flip scattering. Most importantly, the intrinsic value of spin polarization can be uniquely determined in the limit of Z = 0, an ideally clean interface.; A few candidates of half-metallic ferromagnets, materials with 100% spin polarization, have been studied using the PCAR technique. Single-crystal CrO 2 has been demonstrated to be indeed half-metallic with a polarization no less than 0.96. Strontium doped Lanthenum manganites, La0.7Sr 0.3MnO3 and La0.6Sr0.4MnO3, are highly spin-polarized with P ∼ 0.80, but are unlikely to be half-metallic. Certain compositions of Heusler alloys, NiMnSb and Co 2MnSi, have also been found to have significant P but with no evidence of half-metallicity. The spin-transfer torque effect, the fact that a spin-polarized current is able to impart a torque upon a ferromagnetic entity and thereby alters its magnetic configuration, has been demonstrated in a single ferromagnetic layer, without resorting to multilayer structures. (Abstract shortened by UMI.)... |
