Magnetic field dependence of the proximity-induced triplet superconductivity at ferromagnet/superconductor interfaces

Long-ranged superconductor proximity effects recently found in superconductor-ferromagnetic (S-F) systems are generally attributed to the formation of triplet-pairing correlations due to various forms of magnetic inhomogeneities at the S-F interface. In order to investigate this conjecture within a single F layer coupled to a superconductor, we performed scanning tunneling spectroscopy on bilayers of La${}_{2/3}$Ca${}_{1/3}$MnO${}_3$ (LCMO) ferromagnetic thin films grown on high-temperature superconducting films of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ or Pr${}_{1.85}$Ce${}_{0.15}$CuO${}_4$ under various magnetic fields. We find a strong correlation between the magnitude of superconductor-related spectral features measured on the LCMO layer and the degree of magnetic inhomogeneity controlled by the external magnetic field. This corroborates theoretical predictions regarding the role played by magnetic inhomogeneities in inducing triplet pairing at S-F interfaces.

Figure 1

Normalized (to the gap edge) tunneling dI/dV vs. $V$ spectra at 4.2 K acquired on a 17-nm LCMO/(100)YBCO sample in different magnetic fields applied perpendicular to the interface. Left inset: The normalized zero-bias conductance of the superconductor proximity gap as a function of the magnetic field. Note the nonmonotonic dependence of the ZBC on the magnetic field. Right inset: Scheme of the bilayer film and measurement configuration. The error bars are smaller than the dot size.

Figure 2

Normalized (to the gap edge) tunneling spectra at 4.2 K in different magnetic fields as indicated on a 14-nm LCMO/(100)YBCO bilayer, typical of those measured over a 50 × 50 nm${}^2$ region shown by the topographic image (left inset). All spectra exhibit proximity gaps and, in some cases, a signature of a ZBCP. The right inset shows two spectra, one measured after reducing the field to zero (from 180 mT) and the other after subsequently ramping the field from zero to 150 mT.

Figure 3

Normalized (to the right dip) tunneling dI/dV vs. $V$ spectra at 4.2 K acquired on a 17-nm LCMO/(100)YBCO sample showing the evolution of the ZBCP with magnetic fields in the regimes of (a) 0–175 mT and (b) 225–585 mT. By subtracting the asymmetric background conductance as explained in the main text, the bias shift of the ZBCP decreases practically to zero as seen in the inset of (a) for a spectrum acquired at 490 mT. The magnitude of the ZBCPs shows a complex dependence on the magnetic field as portrayed by the inset of (b) where the peak height is plotted as a function of the magnetic field; the black line is a guide to the eye.

Figure 4

Normalized tunneling dI/dV vs. $V$ spectra at 4.2 K acquired from a 15-nm LCMO/(001)PCCO bilayer at zero field. The blue and green curves were measured right after zero-field cooling, whereas, the reddish curves were measured right after ramping the field to 180 mT and back to zero. It is evident that the superconductor-related features, either (a) the gap or (b) the ZBCP, are enhanced due to the field cycling.