Probing the magnetic polaron state in the ferromagnetic semiconductor HgCr$_2$Se$_4$ with resistance fluctuation and muon-spin spectroscopy measurements

TL;DR

This study uses resistance fluctuation and muon-spin spectroscopy to investigate magnetic polarons in HgCr$_2$Se$_4$, revealing their formation, percolation transition, and influence on magnetic and electronic dynamics near the ferromagnetic transition.

Contribution

It provides experimental evidence for isolated magnetic polarons and their critical role in the insulator-to-metal transition in HgCr$_2$Se$_4$, highlighting magnetoelectric coupling effects.

Findings

Magnetic polarons form at elevated temperatures.

Percolation transition occurs near 95-98 K.

Unusual two-level fluctuator dynamics observed.

Abstract

Combined resistance noise and muon-spin relaxation ($\mu$SR) measurements of the ferromagnetic semiconductor HgCr$_2$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 $\sim 95 - 98$ K in the vicinity of the magnetic ordering temperature $T_C \sim 105 - 107$ 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 $\mu$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.