Domains and Magnetization Rotation in Exchange Biased Ni/FeF$_2$

Exchange biased Ni/epitaxial-FeF$_{2}$ films have been investigated using vector coil vibrating sample magnetometry as a function of the cooling field strength $H_{FC}$, applied along the FeF$_{2}$ easy axis. At low $H_{FC}$ a single longitudinal hysteresis loop is observed, negatively biased with a large exchange field. With increasing $H_{FC}$, the loop divides into two sub-loops shifted oppositely from zero field by the same amount. The positively biased sub-loop grows in size with $H_{FC}$ until only a single positively shifted loop is found. Throughout this process, the negative/positive (sub)loop shift has maintained the same \textit{discrete} value. This is in sharp contrast to films with twinned FeF$_{2}$ where the exchange field \textit{gradually} changes from negative to positive values with increasing $H_{FC}$.$^{1}$ The transverse magnetization shows clear correlations with the longitudinal sub-loops. Interestingly, over 90{\%} of the Ni reverses its magnetization by rotation, either in one step or through two successive rotations. These results are due to the single crystal nature of the antiferromagnetic FeF$_{2}$, which breaks down into two opposite regions of large domains. $^{1}$ J. Nogu\'{e}s, D. Lederman, T. J. Moran, and I. K. Schuller, Phys. Rev. Lett., \textbf{76}, 4624 (1996). Work supported by NSF, DOE, Cal-IT$^{2}$ and NEAT IGERT.