Magneto-optical study of the spin freezing in ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te semimagnetic semiconductors

The dynamic behavior of the magnetic susceptibility in the region of spin-glass formation is studied in ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$Te semimagnetic semiconductors. The Faraday rotation at low ac magnetic field is proportional to χ; this has allowed us to measure the ac susceptibility magneto-optically. The temperature dependence of the real component χ’ (dispersion) as well as the imaginary component χ’ ’ (absorption) of the complex susceptibility around the freezing temperature ($T_f$≃9.3 K) is determined within the frequency range 70–3000 Hz, for the compound ${\mathrm{Hg}}_{0.70}$ ${\mathrm{Mn}}_{0.30}$Te. Effects of the external static magnetic field (H≤1000 G) on the complex susceptibility are also investigated. The susceptibility behavior is consistent with a simple phenomenological model; in particular, the fundamental relation χ’ ’=-(π/2)∂χ’/∂lnω derived within the general assumption of a wide distribution of relaxation times, is quantitatively verified in the vicinity of $T_f$. The freezing temperature, obtained from low-field and low-frequency Faraday-rotation experiments, exhibits a linear composition dependence for 0.25≤x≤0.45.