Investigating the electronic charge and magentic spin dynamics in the ferromagnetic semiconductor HgCr₂Se₄ using resistance fluctuation (noise) spectroscopy

The n-type HgCr₂Se₄ has been reported to exhibit a pronounced semiconductor-to-metal transition below and a CMR effect at the ferromagnetic transition at T_C = 107 K. Our recent study of charge carrier dynamics using resistance fluctuation spectroscopy [Phys. Rev B 105, 064404 (2022)] suggests isolated magnetic polarons forming at T > 2T_C which coalesce at T_C. Below this temperature, the trapped carriers in magnetic polarons are unbound in the presence of a magnetic field. The peculiar slow dynamics around this percolation transition has been discussed using a model where the effective radius of the polarons is strongly influenced by the spin correlation length close to T_C. In this talk, we discuss new results highlighting the strong correlation between the magnetic and electronic degrees of freedom that can lead to complex exchange pathways. Likely due to competing AF and FM interactions, we observe a distinctly slow decrease in resistance below the CMR transition. The striking dynamics of distinct two-level fluctuations superimposed on 1/f-type noise corroborates a slowing down of charge carrier and/or magnetic dynamics. Further, below 20 K, a strong upturn in resistance and simultaneously in resistance noise down to 500 mK is observed and is speculated to be linked to the emergence of spiral type magnetic order. Our results demonstrate that the presence of pronounced electron-spin correlations plays a key role in the unconventional temperature dependence of resistance and CMR effect in this spinel.