Polarized neutron reflectivity (PNR) is used to obtain the magnetic depth profile of an antiferromagnetically coupled ferrimagnetic/ferrimagnetic bilayer, ${\mathrm{Gd}}_{40}{\mathrm{Fe}}_{60}/{\mathrm{Tb}}_{12}{\mathrm{Fe}}_{88}$. This system shows a transition from positive to negative exchange bias field $H_E$ as the cooling field $H_{\mathrm{cf}}$ is increased from small to large positive value. It also exhibits training behavior upon field cycling which affects $H_E$ and the coercive field $H_C$. From the PNR measurements at room temperature and at 15 K, we confirm that the magnetic configuration inside the TbFe layer is frozen when the sample is cooled in various $H_{\mathrm{cf}}$. The thickness and pitch of the magnetic twist inside the TbFe layer depend on $H_{\mathrm{cf}}$ and give rise to the observed differences in the bias field. Irreversible reorganization of the TbFe magnetization at the interface occurs upon GdFe magnetization reversal and is found to explain the training effect as well as the overall reduction in coercivity.

• Received 27 September 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.067207