Magnetic Coupling At Antiferromagnetic/Ferromagnetic Interfaces - A Spectromicroscopy Study
Abstract
Today's magnetic storage devices consist of magnetic multilayers that are magnetically coupled across their interface. While the interface itself is supposed to dominate the magnetic behavior of the entire system, the identification and characterization of its magnetic properties remains an experimental challenge. A prominent example is the loop shift (unidirectional anisotropy, exchange bias) and the coercivity increase (uniaxial anisotropy) found if a ferromagnet is coupled to an antiferromagnet. The exchange bias effect is utilized in magnetic data storage device to form a pinned magnetic reference layer. Although exchange bias was discovered over 40 years ago by Meiklejohn 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 antiferromagnetic NiO(001) and ferromagnetic Co across its interface. We observe large (1-20 μm) antiferromagnetic domains at the surface of bare NiO(001) single crystals. Upon in situ deposition of thin ferromagnetic Co layers (1.5 nm) a reorientation of the antiferromagnetic axes takes place. The uniaxial anisotropy axes of the ferromagnet and the antiferromagnet 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. An imbalance between free and pinned interfacial moments as origin of the unidirectional anisotropy will be discussed. 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. [1] H. Ohldag, A. Scholl et al., Phys. Rev. Lett. 86(13), pp. 2878, 2001. [2] F. U. Hillebrecht, H. Ohldag et al., Phys. Rev. Lett. 86(15), pp. 3419, 2001. [3] H. Ohldag, A. Scholl et al., Phys. Rev. Lett. to appear in 86(24), (2001).
- Publication:
-
APS March Meeting Abstracts
- Pub Date:
- March 2002
- Bibcode:
- 2002APS..MARG15008O