Probing the magnetic polaron state in the ferromagnetic semiconductor HgCr2Se4 with muon-spin spectroscopy and resistance-fluctuation measurements

Abstract

Combined resistance noise and muon-spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) 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 $\ensuremath{\sim}95--98\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in the vicinity of the magnetic ordering temperature ${T}_{\mathrm{C}}\ensuremath{\sim}105--107\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. 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 $\ensuremath{\mu}\mathrm{SR}$ 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.