Quenching of impurity spins at Cu/CuO interfaces: An antiferromagnetic proximity effect

It is observed that the magnetoconductance of bilayer films of copper (Cu) and copper monoxide (CuO) has distinct features compared to that of Cu films on conventional band insulator substrates. We analyze the data above 2 K by the theory of weak antilocalization in two-dimensional metals and suggest that spin-flip scattering by magnetic impurities inside Cu is suppressed in Cu/CuO samples. Plausibly the results imply a proximity effect of antiferromagnetism inside the Cu layer, which can be understood in the framework of Ruderman-Kittel-Kasuya-Yoshida interactions. The data below 1 K, which exhibit slow relaxation reminiscent of spin glass, are consistent with this interpretation.

Figure 1

Temperature dependence of sheet resistance of (a) the Cu/CuO and (b) the Cu/MgO films. The deviation from the straight line above 200 K in (a) is due to the resistance of CuO.

Figure 2

Normalized magnetoconductance at 2.5, 3, and 10 K of (a) the Cu/CuO and (b) the Cu/MgO films. $\Delta \sigma \left(H\right)$ is the difference in sheet conductance between the value under a magnetic field $H$ and the zero-field value, while ${\sigma }_0\equiv e^2/\pi h\simeq 1.23\times {10}^{-5}$ S is a constant for the normalization.

Figure 3

Temperature dependence of (a) $H_1$ in a log scale, (b) $H_2$ in a log scale, (c) $H_1$ in a linear scale, and (d) $H_2$ in a linear scale. In each figure, squares represent the Cu/CuO film, while triangles represent the Cu/MgO film.

Figure 4

Schematic picture of the proximity effect of antiferromagnetism implied by our experimental results. The arrows represent electron spins. This figure serves as an intuitive understanding of how the electron spins inside the metal are polarized by the surface spins of the antiferromagnetic insulator, and how the spin of each magnetic impurity, which is represented as a black arrow, is quenched due to the interactions with neighboring spins in the metal. Note that the actual spatial configuration of spin density induced in the Cu is probably much more complicated than depicted in this figure due to the low symmetry of the surface spin configuration of the CuO.