Analysis of point-contact Andreev reflection spectra in spin polarization measurements

Abstract

We present a systematic analysis of point-contact Andreev reflection (PCAR) spectra for ferromagnetic materials, using both modeling and experimental data. We emphasize the importance of consistent data analysis to avoid possible misinterpretation of the data. We consider the relationship between ballistic and diffusive transport, the effect of different transport regimes on spin polarization measurements, and the importance of unambiguous identification of the type of transport regime. We find that in a realistic parameter range, the analysis of PCAR spectra of purely diffusive character by a ballistic model yield approximately the same (within ~3%) values of the spin polarization and the barrier strength Z larger by ~ 0.5-0.6. We also consider the dependence of polarization values on Z, and have shown by simple modeling that letting the superconducting gap vary as an adjustable parameter can result in a spurious dependence of the spinpolarization P c on Z. At the same time we analyzed the effects of finite Z on the apparent value of P c measured by the PCAR technique, using a large number of examples from both our own measurements and from the literature. We conclude that there is a systemdependent variation in P c (Z), presumably due to spin-flip scattering at the interface.However, the exact type of this dependence is hard to determine with any statistical certainty.

Evidence for spin mixing in holmium thin film and crystal samples

“…3(a) and 3(b) respectively. The P-Z relationship of an S/F interface always takes a similar form in that high P is associated with low Z and vice versa and the origin of this dependence has been widely discussed in the literature [28][29][30][31][32] . The 'intrinsic' polarization is usually obtained by extrapolating a P-Z plot to low Z -although not usually to Z = 0 29 because Z also includes the effect of Fermi velocity mismatch between the S/F materials 33 .…”

Influence of Barrier Width on Spin-Polarisation Measured by Point Contact Andreev Reflection

“…Typically, all these models assume that the ferromagnetsuperconductor (FM/SC) interfaces are described by the Dirac's delta potential with a finite transparency that is related to the microscopic properties in a non-trivial way [3]. The importance of this assumption when the spin polarisation is extracted in different transport regimes was discussed in detail by Woods et al [4]. Nonetheless, it should be noted that the Dirac's delta in the BTK model is used to preserve analytical simplicity, and it cannot be regarded as a consequence of various physical phenomena at the real interface, in the general case.…”

“…Although the idea of this method is relatively simple with no restriction on the sample geometry, the interpretation of experimental data requires careful analysis [3,4].…”

Fermi level spin polarization of polycrystalline thulium by point contact Andreev reflection spectroscopy

“…8) theory, taking into account superconducting proximity, electronic heating, modulation broadening and series resistance, is used for least squares fitting of the experimental data. The algorithm is also based on the extensions by Mazin et al 9 for the case diffusive transport, and the work of Woods et al 10 on the ballistic and diffusive regimes. Fixed closed interval difference norm minimization is used, rather than guess value generalized gradient minimization, for its robustness and only marginally higher computational cost.…”