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 of the formation of magnetic polarons in antiferromagnetic ${\mathrm{Eu}}_5{\mathrm{In}}_2{\mathrm{Sb}}_6$. First, we observe a reduction of the ${\mathrm{Eu}}^{2+}$ ESR linewidth as a function of the applied magnetic field consistent with ferromagnetic clusters that are antiferromagnetically coupled. Additionally, the ${\mathrm{Eu}}^{2+}$ line shape changes markedly below $T^{\prime }\sim 200$ K, a temperature scale that coincides with the onset of CMR. The combination of these two effects provides strong evidence that magnetic polarons grow in size below $T^{\prime }$ and start influencing the macroscopic properties of the system.

• Received 5 May 2021
• Revised 11 January 2022
• Accepted 11 January 2022

DOI:https://doi.org/10.1103/PhysRevB.105.035135