High transmissivity, ferromagnet-superconductor thin film nanocontacts are studied experimentally. Compared to nonmagnetic metal-superconductor contacts, Andreev reflection is strongly suppressed due to the spin polarization of conduction electrons in the ferromagnet. This effect is used to measure both the transparency of the interface and spin polarization of the direct current in the ferromagnet in contrast to the spin polarization of the tunneling current previously measured in ferromagnet-insulatorsuperconductor systems. [S0031-9007(98)07318-9]

We discuss aspects of Andreev reflection ͑AR͒ measurements in normal metal-superconductor ͑N-S͒ and ferromagnet-superconductor ͑F-S͒ devices. We describe the analytical model used to quantify spin polarization from the conductance measurements and discuss the validity of this simple model using parabolic bands as simple surrogates for real band structures. We present ͑AR͒ measurements of spin polarization in a Cu-Pt-Pb and Co-Pt-Pb lithographically fabricated nanocontact systems where a scattering layer of has been deliberately added to the interface to enable the study of the effect of pair-breaking scattering on AR conductance and spin polarization. We compare these results to the previously published results from clean Cu-Pb and Co-Pb devices and argue that the measurements in devices with the Pt layer can be explained by the presence of inelasticscattering-induced pair-breaking effects. We modify the analytical model to include this effect and show that in some instances, it may be impossible to distinguish between the effects of a finite spin polarization and inelastic scattering. This has implications for AR measurements of spin polarization at disordered or poorly formed F-S interfaces.

The origin of zero-bias anomalies ͑ZBA's͒ in pure metal nanoconstrictions is investigated. It is shown that the ZBA's in titanium nanoconstrictions can be completely understood in terms of electron-assisted tunneling systems. The behavior of such a system is expected to depend on two distinct energy scales, a Kondo energy below which the electrons behave in a non-Fermi-liquid manner, and a splitting energy below which the Fermi-liquid behavior is restored. Titanium nanoconstrictions have been found to exhibit zero-bias anomalies that show both the non-Fermi-liquid behavior and the low-energy restoration of the Fermi liquid.

Certain zero-bias anomalies (ZBAs) in the voltage, temperature and magnetic field dependence of the conductance G(V, T, H) of quenched Cu point contacts have previously been interpreted to be due to non-magnetic 2-channel Kondo (2CK) scattering from near-degenerate atomic two-level tunneling systems (Ralph and Buhrman, 1992;, and hence to represent an experimental realization of the non-Fermi-liquid physics of the T = 0 fixed point of the 2-channel Kondo model. In this, the first in a series of three papers (I,II,III) devoted to 2-channel Kondo physics, we present a comprehensive review of the quenched Cu ZBA experiments and their 2CK interpretation, including new results on ZBAs in constrictions made from Ti or from metallic glasses. We first review the evidence that the ZBAs are due to electron scattering from stuctural defects that are not static, but possess internal dynamics. In order to distinguish between several mechanisms proposed to explain the experiments, we then analyze the scaling properties of the conductance at low temperature and voltage and extract from the data a universal scaling function Γ(v). The theoretical calculation of the corresponding scaling function within the 2CK model is the subject of papers II and III. The main conclusion of our work is that the properties of the ZBAs, and most notably their scaling behavior, are in good agreement with the 2CK model and clearly different from several other proposed mechanisms.

Spin-wave excitation in ultrathin Co and Fe films on Cu(001) by spin-polarized electron energy loss spectroscopy (invited)In situ observation of magnetic domain pattern evolution in applied fields by spin-polarized low energy electron microscopy