Positive exchange bias has been observed in the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}/{\mathrm{Ir}}_{20}{\mathrm{Mn}}_{80}$ bilayer system via soft-x-ray resonant magnetic scattering. After field cooling of the system through the blocking temperature of the antiferromagnet, an initial conventional negative exchange bias is removed after training, i.e., successive magnetization reversals, resulting in a positive exchange bias for a temperature range down to 30 K below the blocking temperature (450 K). This new manifestation of magnetic training is discussed in terms of metastable magnetic disorder at the magnetically frustrated interface during magnetization reversal.

• Received 23 October 2008

DOI:https://doi.org/10.1103/PhysRevLett.102.177208