Thermal training of exchange bias in epitaxial Fe/KNiF3 -: art. no. 134425

Shifted hysteresis loops and enhanced coercivities associated with the phenomena of exchange bias were examined experimentally for a model system. Superconducting quantum interference device magnetometry measurements are discussed for bilayer structures, consisting of single-crystal Fe(001) and KNiF3 films grown in ultrahigh vacuum. The structures were characterized using reflection high-energy electron diffraction and x-ray diffraction. The KNiF3 film structure was either single crystal or polycrystalline with a high degree of texture. The interfaces are expected to be fully compensated for this particular growth orientation, and good lattice match between the ferromagnetic and antiferromagnetic layers preserved the cubic structure of both. An exchange bias shift was observed and coercivities were enhanced for temperatures well below the Neel temperature. Features associated with training were exhibited by this epitaxial system and clear evidence of thermally activated processes for single-crystal films were obtained in a thermal pulse experiment. Possible evidence for two types of energy barrier distributions controlling the magnetization process is presented. The existence of training and its correlation with thermal activation processes suggest that exchange bias in this mainly compensated system is controlled by magnetization processes in the antiferromagnet. Spin-flop coupling is very likely in this system, and it is suggested that pinning of antiferromagnet spins near the interface is responsible for the exchange bias shifts.