Combined resistance noise and muon-spin relaxation ($\mu \mathrm{SR}$) measurements of the ferromagnetic semiconductor ${\mathrm{HgCr}}_2{\mathrm{Se}}_4$ suggest a degree of magnetoelectric coupling and provide evidence for the existence of isolated magnetic polarons. These form at elevated temperatures and undergo a percolation transition with a drastic enhancement of the low-frequency $1/f$-type charge fluctuations at the insulator-to-metal transition at $\sim 95–98\mathrm{K}$ in the vicinity of the magnetic ordering temperature $T_{\mathrm{C}}\sim 105–107\mathrm{K}$. Upon approaching the percolation threshold from above, the strikingly unusual dynamics of a distinct two-level fluctuator superimposed on the $1/f$ noise can be described by a slowing down of the dynamics of a nanoscale magnetic cluster, a magnetic polaron, when taking into account an effective radius of the polaron depending on the spin correlation length. Coinciding temperature scales found in $\mu \mathrm{SR}$ and noise measurements suggest changes in the magnetic dynamics over a wide range of frequencies and are consistent with the existence of large polarized and domain-wall-like regions at low temperatures, that result from the freezing of spin dynamics at the magnetic polaron percolation transition.

• Received 17 November 2021
• Revised 13 January 2022
• Accepted 13 January 2022

DOI:https://doi.org/10.1103/PhysRevB.105.064404