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Temperature dependence of in-plane correlation lengths in exchange biased Co/FeF2

We have measured resonant soft x-ray diffuse magnetic scattering as a function of temperature in a positively exchange biased Co/FeF2 bilayer and analyzed the data in the distorted wave Born approximation to obtain in-plane charge and magnetic correlation lengths associated with the Co and FeF2 layers and estimate interfacial roughness. Tuning to the Fe and Co L3 edges reveals significantly different temperature trends in these quantities in the antiferromagnetic and ferromagnetic layers, respectively. While the magnetic correlation length of the uncompensated interfacial spins in FeF2 layer increase as temperature decreases, these quantities remain unchanged in the Co layer. Our results indicate that uncompensated Fe spins order within a range of few hundred nanometers in otherwise randomly distributed uncompensated magnetic moments, giving rise to spin clusters in the antiferromagnet whose size increase as the temperature decrease.