Microscopic probe of magnetic polarons in antiferromagnetic Eu5In2Sb6

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 Eu5In2Sb6. First, we observe a reduction of the Eu2+ ESR linewidth as a function of the applied magnetic field consistent with ferromagnetic clusters that are antiferromagnetically coupled. Additionally, the Eu2+ line shape changes markedly below T' similar to 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' and start influencing the macroscopic properties of the system. (AU)