  We present magnetic stray field measurements performed on a single
micro-crystal of the half metallic ferromagnet CrO$_2$, covered by a naturally
grown 2\,-\,5\,nm surface layer of antiferromagnetic (AFM) Cr$_2$O$_3$. The
temperature variation of the stray field of the micro-crystal measured by
micro-Hall magnetometry shows an anomalous increase below $\sim$\,60\,K. We
find clear evidence that this behavior is due to the influence of the AFM
surface layer, which could not be isolated in the corresponding bulk
magnetization data measured using SQUID magnetometry. The distribution of
pinning potentials, analyzed from Barkhausen jumps, exhibits a similar
temperature dependence. Overall, the results indicate that the surface layer
plays a role in defining the potential landscape seen by the domain
configuration in the ferromagnetic grain.
