Bistability of magnetic polarons bound to acceptors in a wurtzite semimagnetic semiconductor

The bistable properties of magnetic polarons bound to acceptor impurities ($A^0$-BMP’s) in ${\mathrm{Cd}}_{0.95}$ ${\mathrm{Mn}}_{0.05}$Se have been studied by time-resolved spectroscopy of D-A pair luminescence under two different conditions: (1) action of a magnetic field, in the Faraday configuration and above-band-gap optical excitation; (2) optical orientation of the magnetic moment of $A^0$-BMP’s through a site-selection mechanism, by absorption of circularly polarized light tuned to the tail-of-acceptor–conduction-band transitions. The first experiments give clear indication of two different regimes: (a) At high temperature (T>5 K), the polarization rate $P_c$ of D-A luminescence increases with time delay and the evolution of $P_c$ is faster when T increases. This reflects the thermally activated orientation of $A^0$-BMP’s by jumping over a potential barrier. (b) At low T (T<4 K) $P_c$ decreases with time delay, which reflects the recombination of holes with spin-polarized electrons on donors. In the second kind of experiment, the time evolution of $P_c$ is not influenced by spin-dependent recombination and is related only to the thermal relaxation of $A^0$-BMP orientation. At 1.7 K, $P_c$ is constant for at least 5 μs, which shows that $A^0$-BMP’s are frozen. As T increases, the time-integrated value of $P_c$ decreases because of a progressive unfreezing of the polarons. Both experiments are consistent with a wide distribution of activation energies, presumably related to alloying effects. The distribution is centered around 10 meV, as predicted by a model of anisotropic $A^0$-BMP’s proposed by Bhattacharjee [Phys. Rev. B 35, 9108 (1987)].