Point contact Andreev reflection (PCAR) spectroscopy is a common technique for determining the spin polarization of a ferromagnetic sample. The polarization is extracted by measuring the bias dependence of the conductance of a metallic/superconducting point contact. Under ideal conditions, the conductance is dominated by Andreev reflection and the Blonder-Tinkham-Klapwijk (BTK) model can be used to extract a value for the polarization. However, PCAR spectra often exhibit unwanted features in the conductance that cannot be appropriately modelled with the BTK theory. In this paper we isolate some of these unwanted features and show that any further extraction of the spin polarization from these non-ideal spectra proves unreliable. Understanding the origin of these features provides an objective criterion for rejection of PCAR spectra unsuitable for fitting with the modified BTK model.

The dependence of threshold current on applied magnetic field for domain wall depinning from a pinning potential in Gd doped permalloy wires is measured using pulsed current measurements. By increasing the Gd concentration we find a marked reduction in threshold currents. This is shown to arise due to the enhanced non-adiabatic spin-transfer torque: we calculate the non-adiabaticity parameter β to be around 0.11 for 10% Gd concentration. On the other hand we show that the adiabatic spin-transfer torque is largely unaffected by Gd concentrations up to 10%.

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A modification of the Blonder-Tinkham-Klapwijk (BTK) model is proposed for the description of the ferromagnet-superconductor interface. Modelling the contact potential with a rectangular barrier, we investigate the influence of the barrier width at the interface on the value of spin polarisation measured by point contact Andreev reflection. Results suggesting that neglecting the width of the barrier at the interface can lead to an overestimation of the spin polarisation using the original BTK model are presented. This effect is particularly strong for low values of polarization and vanishes for high polarisation. The impact on analysis of the experimental data is also discussed.