Inverse Proximity Effect in Superconductor-Ferromagnet Bilayer Structures

Measurements of the polar Kerr effect using a zero-area-loop Sagnac magnetometer on $\mathrm{Pb}/\mathrm{Ni}$ and $\mathrm{Al}/(\mathrm{Co}\mathrm{\text{−}}\mathrm{Pd})$ proximity-effect bilayers show unambiguous evidence for the “inverse proximity effect,” in which the ferromagnet induces a finite magnetization in the superconducting layer. To avoid probing the magnetic effects in the ferromagnet, the superconducting layer was prepared much thicker than the light’s optical-penetration depth. The sign and size of the effect, as well as its temperature dependence agree with recent predictions by Bergeret et al. [Phys. Rev. B 69, 174504 (2004)].

Figure 1

Cartoon of the measurement scheme. Here each of the two perpendicular linearly polarized lights that come out of the fiber [16] becomes circularly polarized, going through the $\lambda /4$ plate and then focused on the sample using a lens. The electric field of the incident light (represented with a dashed line) penetrates a distance ${\delta }_S\ll d_S$ into the superconductor and thus is not sensitive to the spins in the $F$ layer.

Figure 2

Hysteresis loops for the two systems examined measured through the anomalous Hall effect. Note the sharp transitions, indicative of a moment perpendicular to the plane of current flow.

Figure 3

Kerr effect measurement of the $\mathrm{Pb}/\mathrm{Ni}$ bilayer system. Sample was first cooled in $a+1\mathrm{T}$ field down to 10 K. The field was then turn down to zero, and the sample was cooled further to 0.3 K. Data were taken when the sample warmed up. Also shown is the resistive transition. Note that the Kerr response indicate a magnetization that opposes the magnetization of the ferromagnetic layer.

Figure 4

Kerr effect measurement of the $\mathrm{Al}/(\mathrm{Co}\mathrm{\text{−}}\mathrm{Pd})$ bilayer systems. (a) Sample with 50 nm of Al was first cooled in a field of $+0.8\mathrm{T}$ down to 10 K. The field was then turned off, and the sample was cooled further to 0.3 K. Data was taken when the sample warmed up. (b) Sample with 90 nm of Al was first cooled in a field of $-0.8\mathrm{T}$ down to 10 K. The field was then turn down to zero, and the sample was cooled further to 0.3 K. Data was taken when the sample warmed up. For both samples we also show the resistive transition. Note that the Kerr response indicate a magnetization that opposes the magnetization of the ferromagnetic layer.

Figure 5

Possible fits of the data in Fig. 4a to the theory of Bergeret et al. [15]. Dashed line and dashed-dotted lines are the two limiting cases that the data can extrapolate (see text).