Inverse proximity effect at superconductor-ferromagnet interfaces: Evidence for induced triplet pairing in the superconductor

Considerable evidence for proximity-induced triplet superconductivity on the ferromagnetic side of a superconductor-ferromagnet (S-F) interface now exists; however, the corresponding effect on the superconductor side has hardly been addressed. We have performed scanning tunneling spectroscopy measurements on NbN superconducting thin films proximity coupled to the half-metallic ferromagnet $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{C}{\mathrm{a}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LCMO) as a function of magnetic field. We have found that at zero and low applied magnetic fields the tunneling spectra on NbN typically show an anomalous gap structure with suppressed coherence peaks and, in some cases, a zero-bias conductance peak. As the field increases to the magnetic saturation of LCMO where the magnetization is homogeneous, the spectra become more BCS-like and the critical temperature of the NbN increases, implying a reduced proximity effect. Our results therefore suggest that triplet-pairing correlations are also induced in the S side of an S-F bilayer.

Figure 1

Tunneling spectra measured at 4.2 K in different magnetic fields (as labeled) on NbN/50-nm-LCMO bilayers of three different NbN films thicknesses, as indicated. The spectra measured in fields close to the saturation field of LCMO [right panels, (b), (d), and (e)] appear to be more BCS-like compared to those measured at low fields [left panels, (a), (c), and (e)]. The inset to (a) shows the measurement configuration.

Figure 2

Tunneling spectra measured at 4.2 K in different magnetic fields, as indicated, on a 10-nm-NbN/LCMO bilayer. (a) Spectra acquired in zero field and 80 mT showing ZBCP. (b) Spectrum taken in 150 mT exhibiting a wide asymmetric gap with pronounced in-gap structure.

Figure 3

Resistance vs temperature measurements of (a) 10-nm-NbN/LCMO bilayer and (b) 10-nm-NbN/8-nm-MgO/LCMO trilayer. Blue curves: before any field was applied. Green curves: in the presence of a 400 mT field along the LCMO easy axis. Red curves: after the field is turned off.