Colossal magnetoresistance (CMR) has been observed across many ${\mathrm{Eu}}^{2+}$-based materials; however, its origin is not completely understood. Here we investigate the antiferromagnetic insulator ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ through single crystal x-ray diffraction, transmission electron microscopy, electrical transport, magnetization, dilatometry, and electron spin resonance measurements complemented by density functional theory calculations. Our electrical resistivity data reveal a large negative magnetoresistance, $\text{MR}=\left[R\right(H)-R(0\left)\right]/R\left(0\right)$, that reaches $\text{MR}=-99.7$% at 9 T near the antiferromagnetic ordering temperature $T_N=23\mathrm{K}$. Dilatometry measurements show an accompanying field-induced lattice strain. Additionally, ${\mathrm{Eu}}^{2+}$ electron spin resonance reveals a strong ferromagnetic exchange interaction between ${\mathrm{Eu}}^{2+}$ and conduction electrons. Our experimental results in ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ are consistent with a magnetic polaron scenario and suggest magnetic polaron formation as a prevailing explanation of CMR in ${\mathrm{Eu}}^{2+}$-based compounds.

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• Antiferromagnetism
• Colossal magnetoresistance
• Phase separation
• Magnetic semiconductors
• Narrow band gap systems
• Electron spin resonance