Evidence and evolution of magnetic polaron in HgCr2Se4 investigated by electron spin resonance

The evidence and evolution of magnetic polarons (MPs) in HgCr2Se4 have been studied by electron spin resonance (ESR), magnetism and conductivity measurements in a temperature range of 5–300 K. A single paramagnetic resonance line is observed in the high-temperature range while multiple resonance lines appear in the low-temperature range. As temperature decreases, the peak-to-peak linewidth ΔH pp shows a minimum at T min ≈ 210 K, with the activation energy fitted by small polaron hopping model consistent with the bottleneck mechanism, providing an evidence for existence of small MPs above T min. The analysis of the temperature dependence of ΔH pp, double integrated intensity I, and g factor of ESR signals, combined with the temperature dependence of magnetization and conductivity, reveals an evolution process from small MPs at zone I (T > T min) to correlated MPs at zone II (T c < T * ≤ T ≤ T min) in the paramagnetic regime. Three critical temperatures, T min (≈210 K), T th (≈175 K), and T * (≈121 K), which determine the evolution characteristics of MPs, are distinguished. The magnetic correlation length ξ of Cr3+–Se2−–Cr3+ should account for the evolution of MPs.