Magnetic proximity effect in perovskite superconductor/ferromagnet multilayers

Multilayers of superconducting/ferromagnetic (SC/FM) ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7∕{\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}{\mathrm{MnO}}_3$ have been studied by neutron reflectometry. The occurrence of a structurally forbidden Bragg peak in the FM state highlights a significant difference between the nuclear and magnetic depth profiles. From comparison with simulated reflectivity curves we identify two possible magnetization profiles: (i) a magnetic moment within the SC layer antiparallel to the one in the FM layer (antiphase magnetic proximity effect) or (ii) a “dead” region in the FM layer with zero net magnetic moment. In addition, we observe an anomalous enhancement of the off-specular reflection in the SC state which signals a strong mutual interaction between SC and FM order parameters.

Figure 1

(Color) (a) Specular reflectivity of sample 1 at 200, 170, 150, 120, 100, 70, 50, and 15 K, for $H_{\mathrm{ext}}=100\mathrm{Oe}$ (field cooled). Curves are offset for clarity. Bragg peaks are marked by arrows. (b) $I(q_x,T)$ map for the first (left) and second (right) Bragg peaks. (c) $T$ dependence of the first (red) and second (blue) Bragg-peak intensities, integrated over $q_x$ (엯) and at $q_x=0$ ($\times$, scaled by 6).

Figure 2

(Color) (a) Polarized specular reflectivity of sample 2 at 200 and 15 K, for $H_{\mathrm{ext}}=100\mathrm{Oe}$ (field cooled). (b) Simulated reflectivity curves (model 1) and (c) model potentials that reproduce the experimental data. Left: antiphase magnetic proximity effect (model 1); Right: “dead layer” (model 2). $\delta \left(z\right)\propto V\left(z\right)$ is the deviation of the refractive index from 1, $\lambda$ is the neutron wavelength.