In epitaxial (111)-oriented ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕{\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ bilayers, we separate two distinct behaviors: unidirectional anisotropy (exchange bias) in thick ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ and enhanced coercivity in thin ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$. By measuring the magnetization response to a rotating magnetic field, we quantitatively determine the relevant anisotropies and demonstrate that the enhanced coercivity is related to the rotatable uncompensated spins of ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$. We demonstrate the consequences of the variation of anisotropy with temperature and relate our results to other exchange-biased systems.

• Received 6 May 2005

DOI:https://doi.org/10.1103/PhysRevB.71.220410