  The origin of x-ray magnetic circular dichroism (XMCD) at the Cu L2,3 edge in
YBa2Cu3O7/La(1-x)Ca(x)MnO3 superlattices is revealed by performing
first-principle electronic structure calculation using fully-relativistic
spin-polarized linear muffin-tin orbital and projected augmented plane wave
methods. We show that the XMCD spectra at the Cu L2,3 edges are proportional to
the difference of the densities of majority- and minority-spin Cu 3z2-1 states.
Although the Cu 3z2-1 states lie well below the Fermi level, a small number of
majority-spin 3z2-1 holes is created by the Cu 3z2-1 - O p_z - Mn 3z2-1
hybridization across the interface. Even this tiny number of holes is
sufficient to produce appreciable Cu L2,3 XMCD. The robustness of this
conclusion is verified by studying the influence of doping, atomic relaxation,
correlation effects, and antiferromagnetic order in a CuO2 plane on the XMCD
spectra.
