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

Merlin Mitschek; Thomas J. Hicken; Shuai Yang; Murray N. Wilson; Francis L. Pratt; Chennan Wang; Stephen J. Blundell; Zhilin Li; Yongqing Li; Tom Lancaster; Jens Müller

arXiv · 2111.08990

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

Merlin Mitschek·Thomas J. Hicken·Shuai Yang·Murray N. Wilson·Francis L. Pratt·Chennan Wang·Stephen J. Blundell·Zhilin Li·Yongqing Li·Tom Lancaster·Jens Müller

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Abstract

Combined resistance noise and muon-spin relaxation ($μ$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 $μ$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.

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Merlin Mitschek, Thomas J. Hicken, Shuai Yang, Murray N. Wilson, Francis L. Pratt, Chennan Wang, Stephen J. Blundell, Zhilin Li, Yongqing Li, Tom Lancaster, Jens Müller. 2021-11-17. Probing the magnetic polaron state in the ferromagnetic semiconductor HgCr$_2$Se$_4$ with resistance fluctuation and muon-spin spectroscopy measurements. https://arxiv.org/abs/2111.08990

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