Direct measurement of charge transfer phenomena at ferromagnetic/superconducting oxide interfaces

YBa$_{2}$Cu$_{3}$O$_{7-x}$/La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ ferromagnetic/superconducting interfaces have been analyzed by scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) with monolayer resolution. 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. Since CMR and HTCS oxides are extremely sensitive to doping these charge transfer processes at the interfaces will directly affect the superconducting and/or magnetic properties of the individual layers. This behavior has not been observed with insulating PrBa$_{2}$Cu$_{3}$O$_{7}$ layers in YBa$_{2}$Cu$_{3}$O$_{7-x}$/ PrBa$_{2}$Cu$_{3}$O$_{7 }$superlattices. EELS in these samples provides direct confirmation that holes in the YBa$_{2}$Cu$_{3}$O$_{7-x}$ are located on the CuO$_{2}$ planes.