Direct measurement of charge transfer phenomena at   ferromagnetic/superconducting oxide interfaces
  YBa2Cu3O7-x/La0.67Ca0.33MnO3 ferromagnetic/superconducting interfaces are analyzed by scanning transmission electron microscopy and electron energy loss spectroscopy with monolayer resolution. We demonstrate that extensive charge transfer occurs between the manganite and the superconductor, in a manner similar to modulation-doped semiconductors, which explains the reduced critical temperatures of heterostructures. This behavior is not seen with insulating PrBa2Cu3O7 layers. Furthermore, we confirm directly that holes in the YBa2Cu3O7-x are located on the CuO2 planes. 
M. Varela, A.R. Lupini, V. Pena, Z. Sefrioui, I. Arslan, N.D.   Browning, J. Santamaria, S.J. Pennycook