${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_7/{\text{La}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_3$ bilayers: Interface coupling and electric transport properties

Heteroepitaxially grown bilayers of ferromagnetic ${\text{La}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_3$ (LCMO) on top of superconducting ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_7$ (YBCO) thin films were investigated by focusing on electric transport properties as well as on magnetism and orbital occupation at the interface. Transport measurements on YBCO single layers and on YBCO/LCMO bilayers, with different YBCO thickness $d_Y$ and constant LCMO thickness $d_L=50\text{nm}$, show a significant reduction in the superconducting transition temperature $T_c$ only for $d_Y<10\text{nm}$, with only a slightly stronger $T_c$ suppression in the bilayers, as compared to the single layers. X-ray magnetic circular dichroism measurements confirm recently published data of an induced magnetic moment on the interfacial Cu by the ferromagnetically ordered Mn ions, with antiparallel alignment between Cu and Mn moments. However, we observe a significantly larger Cu moment than previously reported, indicating stronger coupling between Cu and Mn at the interface. This in turn could result in an interface with lower transparency, and hence smaller spin-diffusion length, that would explain our electric transport data, i.e., smaller $T_c$ suppression. Moreover, linear dichroism measurements did not show any evidence for orbital reconstruction at the interface, indicating that a large change in orbital occupancies through hybridization is not necessary to induce a measurable ferromagnetic moment on the Cu atoms.

Figure 1

(Color online) (a) AFM images of LCMO film surface $\left(d_L=50\text{nm}\right)$ with an rms roughness of 0.2 nm and (b) YBCO film surface $\left(d_Y=50\text{nm}\right)$ with an rms roughness of 0.8 nm. Numbers left and right from color bar refer to (a) and (b), respectively.

Figure 2

(Color online) XRD data for a single layer YBCO ($d_Y=10\text{nm}$) and LCMO $\left(d_L=50\text{nm}\right)$ film and for a YBCO/LCMO bilayer ($d_Y=20\text{nm}$ and $d_L=50\text{nm}$). The main graph shows $\Theta -2\Theta$ scans (for YBCO and YBCO/LCMO shifted vertically for clarity). The inset shows a comparison of rocking curves around the YBCO (005) and LCMO (002) peaks for the SLs and the BLs, with full width half maximum $\Delta \omega =0.05°$ and $0.06°$ for LCMO in the SL and BL and $0.10°$ and $0.11°$ for YBCO in the SL and BL, respectively.

Figure 3

(Color online) Resistivity $\rho$ vs temperature $T$ of YBCO and LCMO single-layer films with thicknesses $d_Y$ and $d_L$, respectively.

Figure 4

(Color online) (a) Resistance $R$ vs temperature $T$ of YBCO/LCMO bilayers with $d_L=50\text{nm}$ thick LCMO and different YBCO thickness $d_Y$. (b) Superconducting transition temperature $T_c$ vs YBCO thickness $d_Y$ for YBCO single layers and for YBCO/LCMO bilayers obtained from the $R\left(T\right)$ data shown in (a) and in Fig. 3.

Figure 5

(Color online) XMCD spectra at the $\text{Mn}{L}_3$ edge for different $T$ from the YBCO/LCMO bilayer ($d_Y=20\text{nm}$ and $d_L=5.2\text{nm}$). The maximum dichroic signal is 29% at low $T$ and decreases as $T$ approaches $T_{\text{Curie}}$. The inset shows the XMCD signal at the $\text{Cu}L$ edge with a maximum of 3.0% at $T=46\text{K}$, revealing antiferromagnetic coupling to the Mn magnetic moments. As the Cu XAS signal is one order of magnitude smaller than for Mn, and as Cu dichroism is another order of magnitude smaller, we had to average over many scans and do a careful smoothing of the data. Kinks in the signal arise from switching between different step sizes near the $L_3$ and $L_2$ edges.

Figure 6

(Color online) Evolution of magnetic moments with temperature for YBCO/LCMO bilayer. The comparison of XMCD signals (magnetic moments) at the Cu sites (open triangles) and the Mn sites (open squares) as well as bulk magnetization from SQUID measurements (black dots; field cooled in 10 mT) show the same behavior. The Cu dichroism signal is scaled by a factor of 10.

Figure 7

(Color online) Comparison of $\text{Cu}L$ edge XA spectra taken with in-plane and out-of-plane polarization. Shown are surface-sensitive TEY detection at (a) $T=25\text{K}$ and (b) 200 K and (c) bulk-sensitive FY detection. Data in (a) was taken at ANKA; (b) and (c) at ELETTRA.

Figure 8

(Color online) XLD signal of the YBCO/LCMO bilayer at the $\text{Mn}L$ edge in TEY mode at $T=25\text{K}$. The inset shows the corresponding LD at the $\text{Cu}L$ edge.