Magneto-optical study of the spin freezing in Hg1-xMnxTe semimagnetic semiconductors.

The dynamic behavior of the magnetic susceptibility in the region of spin-glass formation is studied in ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te semimagnetic semiconductors. The Faraday rotation at low ac magnetic field is proportional to \ensuremath{\chi}; this has allowed us to measure the ac susceptibility magneto-optically. The temperature dependence of the real component \ensuremath{\chi}' (dispersion) as well as the imaginary component \ensuremath{\chi}'' (absorption) of the complex susceptibility around the freezing temperature (${T}_{f}$\ensuremath{\simeq}9.3 K) is determined within the frequency range 70\char21{}3000 Hz, for the compound ${\mathrm{Hg}}_{0.70}$${\mathrm{Mn}}_{0.30}$Te. Effects of the external static magnetic field (H\ensuremath{\le}1000 G) on the complex susceptibility are also investigated. The susceptibility behavior is consistent with a simple phenomenological model; in particular, the fundamental relation \ensuremath{\chi}''=-(\ensuremath{\pi}/2)\ensuremath{\partial}\ensuremath{\chi}'/\ensuremath{\partial}ln\ensuremath{\omega} 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\ensuremath{\le}x\ensuremath{\le}0.45.