In superconductor/ferromagnet (S-F) junctions, new quantum phenomena can be expected from the interplay between the superconductivity and the spin polarization of the ferromagnet. This interplay allows us to determine experimentally the spin polarization of carriers P in a ferromagnet using Andreev reflection spectroscopy [1]. However, it is inferred that, in an S-F junction, the pair potential Δ in a superconductor is weakened as a result of the penetration of the exchange field from a ferromagnet into a superconductor, which is called the inverse proximity effect. This interesting deliberation has motivated us to study the influence of the inverse proximity effect on spin-polarized carrier transport across the S-F interface as well as the estimation of the P value.
We fabricated Nb/ferromagnetic semiconductor p-In0.96Mn0.04As junctions as shown in Fig. 1. Below ~10 K, p-In0.96Mn0.04As becomes ferromagnetic, as evidenced by the hysteretic transverse resistance caused by the anomalous Hall effect. Fig. 2 shows the dI/dV-V characteristics of a Nb/p-In0.96Mn0.04As junction at various temperatures. Below the TC of Nb (~8.2 K), a conductance reduction occurs within the bias voltage that is comparable to the Nb superconducting energy gap. A rather moderate slope in the differential conductance curves within the gap region indicates the partial suppression of the Andreev reflection caused by spin-polarized carriers in p-In0.96Mn0.04As. The P value in p-In0.96Mn0.04As has been extracted by fitting the measured differential conductance curves with a newly modified Blonder-Tinkham-Klapwijk model [2] with both spin polarization and the inverse proximity effect as shown in Fig. 3. The extracted P value is P = 0.725 at 0.5 K, and it decreases gradually with increasing temperature.
This work was supported in part by the "Topological Quantum Phenomena" KAKENHI on Innovative Areas from MEXT.
[1] R. J. Soulen Jr. et al., Science 282(1998) 85.
[2] G. E. Blonder et al., Phys. Rev. B 25(1982) 4515.
