Origin of spin-polarization decay in point-contact Andreev reflection

Abstract

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Andreev reflection spectroscopy of niobium point contacts in a magnetic field

“…However P a and Z a are generally strongly dependent on the field. Reporting P(Z) dependence is a usual way experimental PCAR results are presented for magnetic samples [10]; therefore in Fig. 4 we plot P a versus Z a with h as an implicit parameter.…”

“…Earlier experimental works [24][25][26][27][28] on PCAR reported the enhancement of spectral broadening [29] with magnetic field, although without a consistent explanation of the mechanism. The influence of the magnetic field on the tip has not been addressed at all in the context of the spin polarisation measurements previously, despite the significant number of experimental studies on point-contact with ferromagnetic materials [4][5][6][7][8][9][10][11][12][13][14][15][16] and magnetic semiconductors [17,18].Here we present the analysis of the behaviour of niobium tips in contact with an epitaxial copper film, in presence of an external magnetic field H, applied parallel or perpendicular to the tip. To describe the field dependence of PCAR spectra, we introduce an extension to the Blonder-Tinkham-Klapwijk (BTK) model [24] that takes into account the effect of normal vortex cores in the SC tip explicitly.…”

“…These results have important implications for interpretation of transport spin polarisation measurements using PCAR technique. We demonstrate that stray magnetic fields can be responsible for the experimentally observed dependence of the spin polarisation on the strength of the interface barrier, and potential misassignment of the inferred spin polarisation.PACS Numbers: 74.45.+c; 75.50.Cc; The recent theoretical [1][2][3] and experimental [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] development of point-contact Andreev reflection (PCAR) spectroscopy is driven by the interest to find materials with high degree of spin polarization P. Such materials can be used for integration in spinbased microelectronics [19][20][21]. The technique may be used to determine the spatial variation of spinpolarised current in semiconductors [22,23].…”

Evidence for spin mixing in holmium thin film and crystal samples

“…In the case of Ni 0.19 Pd 0.81 , hardly a change in P or Z is found. Note that the difference in behaviour of Z in the Ho cannot be explained purely by an increase in the smearing parameter with decreasing contact resistance as both the Ho crystal and the Cu sample show the same trend of decreased smearing as the contact resistance is reduced 28 .…”

“…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 .…”