Ferromagnetic/superconducting bilayer structure: A model system for spin diffusion length estimation

We report detailed studies on ferromagnet-superconductor bilayer structures. Epitaxial bilayer structures of half metal-colossal magnetoresistive (HMCMR) ${\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}{\mathrm{MnO}}_3$ and high-$T_c$ superconducting (HTSC) ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ are grown on ${\mathrm{SrTiO}}_3\left(100\right)$ single-crystalline substrates using pulsed laser deposition. Magnetization $M\left(T\right)$ measurements show the coexistence of ferromagnetism and superconductivity in these structures at low temperatures. Using the HMCMR layer as an electrode for spin polarized electrons, we discuss the role of spin polarized self injection into the HTSC layer. The experimental results are in good agreement with a presented theoretical estimation, where the spin diffusion length ${\xi }_{\mathrm{FM}}$ is found to be in the range of ${\xi }_{\mathrm{FM}}\approx 10\mathrm{nm}$.

Figure 1

Sketch of the chosen sample geometry. The ${\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}{\mathrm{MnO}}_3$ (LCMO) and ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) are grown by pulsed laser deposition onto ${\mathrm{SrTiO}}_3\left(100\right)$ single crystalline substrates.

Figure 2

(Color online) X-ray pattern for the bilayer structures with thicknesses of $d_{\mathrm{LCMO}}=50\mathrm{nm}$ and $d_{\mathrm{YBCO}}=20\mathrm{nm}$ (a), $30\mathrm{nm}$ (b), $50\mathrm{nm}$ (c), and $100\mathrm{nm}$ (d), respectively. Only the $\left(00\ell \right)$ peak is found, i.e., both layers show $c$-axis textured growth on the $\mathrm{STO}\left(100\right)$ substrate.

Figure 3

(Color online) Temperature dependence of the magnetization $M\left(T\right)$ after zero-field cooling (a) and field cooling (b) in an in-plane magnetic field $H_{\mathrm{ext}}=10\mathrm{Oe}$. Shown are the results for bilayers of $d_{\mathrm{LCMO}}=50\mathrm{nm}$ and $d_{\mathrm{YBCO}}=20$, 30, and $100\mathrm{nm}$. The $50\mathrm{nm}∕30\mathrm{nm}$ bilayer shows two ferromagnetic transitions in the zero-field-cooled measurement (a). The first at $T_{\mathrm{Curie}}=229\mathrm{K}$ can not be seen in the figure and the second one occurs at $T_{\mathrm{Curie}}=180\mathrm{K}$. We attribute this behavior to a nonhomogeneous magnetic layer in that sample.

Figure 4

(Color online) Temperature dependent in-plane resistance $R\left(\mathrm{T}\right)$ for the same samples as in Fig. 3a. The data are collected from standard four-probe measurements.

Figure 5

(Color online) The superconducting transition temperature of bilayers on STO with varying thickness of the YBCO layer obtained from the diamagnetic onset in the zero-field-cooling magnetization measurements.

Figure 6

(Color online) Ferromagnetic ordering temperature $T_{\mathrm{Curie}}$ and saturation magnetization $M_s$ of bilayers with varying thickness of the YBCO layer. $T_{\mathrm{Curie}}$ is determined from the onset of the ferromagnetic signal in Fig. 3b. $M_s$ is determined from the saturation signal $M\left(T\right)$ in Fig. 3b divided by the volume of the LCMO layer. $T_{\mathrm{Curie}}$ and $M_s$ strongly decrease with increasing YBCO thickness.

Figure 7

(Color online) The normalized superconducting transition temperature of bilayers with varying thickness of the YBCO layer on $\mathrm{STO}(◻)$ and $\mathrm{LSGO}(◇)$ substrates obtained from the diamagnetic onset in the zero-field-cooling magnetization measurement and model according to Eq. (8) (solid line). The model gives a good description of the experimental data.