Determining Spin Polarization of Ferromagnets Using Superconducting Spectroscopy

The tremendous interest in using the spin degree of freedom in electronic devices has led to an extensive endeavor to investigate the intrinsic spin polarization of various magnetic materials. The work done here expands upon existing methods to develop a more general technique of precise electrical determination of spin polarization using superconducting spectroscopy with or without the presence of a magnetic field. As part of this effort, the use of Andreev reflection in planar junction configuration was explored on several ferromagnetic materials including the dilute magnetic semiconductor (DMS) Ga1−xMnxAs and the concentrated magnetic semiconductor EuS. This work also led to the exploration of the effects of barrier strength on the measured spin polarization. Traditionally, using superconducting spectroscopy to measure spin polarization (P) was limited to the case of a tunnel junction in a magnetic field or Andreev reflection measurements in point contact structures in zero-field. This project aimed to develop a method that bridged these two regimes to allow for determination of spin polarization in more practical device structures, such as planar junctions, with arbitrary barrier strength. This work led to the first direct electrical determination of P on the representative DMS Ga1−xMnxAs by measuring GaMnAs/Ga structures using Andreev reflection in planar configuration. The analysis of the conductance spectra on highly transparent junctions consistently yielded P values of at least 85%. These experiments also revealed an extreme sensitivity of P to the interfacial properties. Another major part of this work was the measurement of the P of doped EuS using zerofield and Zeeman-split Andreev reflection spectroscopy (ARS) on EuS/Al planar junctions are reported. The zero-field ARS spectra can be fit straightforwardly to a spin-polarized BTK model, which consistently yield P on the order of 80% regardless of the barrier strength. Moreover, we performed ARS in the presence of a Zeeman-splitting of the quasiparticle density of states in Al. The Zeeman-split ARS spectra are well described theoretically by combining the solution to the Maki-Fulde equations with the spin-polarized BTK analysis. The results have provided an independent verification of the validity of the zero-field ARS, and helped demonstrate the utility of field-split superconducting spectroscopy on Andreev junctions of arbitrary barrier strengths. Additionally the effect of barrier thickness on the measured spin polarization was explored in Al/Al2O3/Ni79Fe21 tunnel junctions. Planar tunnel junction structures were formed by natural oxidation of Al; by varying the oxidation time the barrier thickness could be controlled. The measured spin polarization increased with increasing barrier thickness which is attributed to the interplay of both the sp and d electronic states of the NiFe.