Pinned Spin Depth Profile of an Oxidized-Mn/Ga $_{1 - x}$ Mn $_{x}$ As Exchange Bias Bilayer—The

Mn/Ga_1-xMn_xAs bilayers can exhibit large exchange bias if annealed for an optimal period of time to transform the Mn cap into antiferromagnetic MnO. Annealing for too long tends to drastically increase the number of spins that become pinned along the direction of a cooling field, but it also results in samples that exhibit little or no exchange bias. To investigate why the increase in pinned spins does not enhance the exchange coupling, we used polarized neutron reflectometry to examine the magnetic depth profile of a typical overannealed Mn/Ga_1-xMn_xAs bilayer. We observe a large magnetization with a pinnable component distributed throughout the cap. This shows that overannealing results in a cap that is not entirely antiferromagnetic, and that the additional pinned spins do not originate from uncompensated moments directly at the antiferromagnet/ferromagnet interface-explaining why they do not contribute to the exchange bias coupling