AVS 49th International Symposium
    Magnetic Interfaces and Nanostructures Thursday Sessions
       Session MI+SS-ThM

Invited Paper MI+SS-ThM3
Polarized X-Rays and Magnetic Interfaces

Thursday, November 7, 2002, 9:00 am, Room C-205

Session: Magnetic Spectroscopies
Presenter: H. Ohldag, Stanford Synchrotron Radiation Laboratory
Authors: H. Ohldag, Stanford Synchrotron Radiation Laboratory
A. Scholl, Advanced Light Source
E. Arenholz, Advanced Light Source
F. Nolting, Swiss Light Source
Y. Acremann, Stanford Synchrotron Radiation Laboratory
J. Stohr, Stanford Synchrotron Radiation Laboratory
F.U. Hillebrecht, Forschungszentrum Karlsruhe, Germany
S. Maat, IBM Almaden Research Center
M.J. Carey, IBM Almaden Research Center
Correspondent: Click to Email
While interfaces are supposed to dominate the behavior of magnetic multilayer their identification and characterization remains an experimental challenge. A prominent example is the loop shift (exchange bias) and the coercivity increase found if a ferromagnet (FM) is coupled to an antiferromagnet (AFM). Although exchange bias was discovered over 40 years ago our understanding of its origin is still poor. We use dichroism x-ray absorption spectromicroscopy in a photoemission electron microscope to study the magnetic coupling between AFM NiO(001) and FM Co. We observe large (1-20mm) AFM domains at the surface of bare NiO(001) single crystals. Upon in situ deposition of thin FM Co layers (1.5nm) a reorientation of the AFM axes takes place. The uniaxial anisotropy axes of the FM and the AFM are then aligned parallel domain by domain. Spectroscopy data show that the Co deposition causes a chemical reaction and formation of an interfacial CoNiOx layer. Microscopy images reveal its polarization to be aligned parallel to the Co layer. Upon annealing both, the uniaxial anisotropy and the amount of interfacial spins increases indicating the direct link between interfacial polarization and parallel exchange coupling. A small fraction of interfacial spins does not follow the external field. These so called pinned moments lead to an additional vertical shift in the hysteresis loop of the interfacial spins. The number of pinned spins can be diretly correlated to the size of the exchange bias field. Our findings clearly show that a proper description of magnetic coupling in Co/NiO as well as in other AFM/FM systems needs to consider the properties of a distinct interfacial layer that can deviate significantly from the bulk properties of each material. @FootnoteText@ @footnote 1@H. Ohldag, A. Scholl et al., PRL 86(13), pp. 2878, 2001. @footnote 2@F. U. Hillebrecht, H. Ohldag et al., PRL 86(15), pp. 3419, 2001. @footnote 3@H. Ohldag, A. Scholl et al., PRL 87 art. no 247201, 2001.