  Colossal magnetoresistance (CMR) emerges from intertwined spin and charge
degrees of freedom in the form of ferromagnetic clusters also known as trapped
magnetic polarons. As a result, CMR is rarely observed in antiferromagnetic
materials. Here we use electron spin resonance (ESR) to reveal microscopic
evidence for the formation of magnetic polarons in antiferromagnetic
Eu$_{5}$In$_{2}$Sb$_{6}$. First, we observe a reduction of the Eu$^{2+}$ ESR
linewidth as a function of the applied magnetic field consistent with
ferromagnetic clusters that are antiferromagnetically coupled. Additionally,
the Eu$^{2+}$ lineshape changes markedly below T' ~ 200 K, a temperature scale
that coincides with the onset of CMR. The combination of these two effects
provide strong evidence that magnetic polarons grow in size below T' and start
influencing the macroscopic properties of the system.
