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We report a giant Voigt effect in diluted magnetic semiconductors ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te and ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se and present an excitonic model that describes its dispersion and ${\mathit{M}}^{2}$ dependence, M being the magnetization. We discuss the Voigt effect in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te (0.10x0.45), a zinc-blende diluted magnetic semiconductor, in the context of this model. In addition, we present experimental results for the Voigt effect in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se (x=0.26 and 0.31), a diluted magnetic semiconductor with wurtzite structure, where the effect is considerably more complicated due to its lower symmetry. Specifically, we present a striking anisotropy of the Voigt effect in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se, which depends on the relative orientation of the applied magnetic field with respect to the optic (c^) axis of the crystal.

La birefringence augmente lentement avec l'augmentation de l'energie des photons bien en dessous de la limite d'absorption, mais decroit brutalement lorsque l'energie des photons approche cette limite. La variation en temperature de la birefringence est dominee par la variation en temperature de la bande interdite. La birefringence depend de la concentration en Mn. Sous champ magnetique, les polarisations sont 2 vibrations elliptiques tournant dans des directions opposees. A la limite d'absorption, la difference de phase entre ces 2 vibrations augmente avec le champ, due a la rotation Faraday geante

Mesure du premier et du second moment (cinetique) de la bande de photoluminescence 2.15 et Mn 2+ dans Zn 0.5 Mn 0.5 Se, dans l'intervalle de temperatures 4-300 K. Le premier moment de la bande d'emission de Mn 2+ presente un deplacement vers le rouge puis vers le bleu. La position de la bande caracterisee par le premier moment se trouve etre sensible a un champ magnetique externe de 6T sur tout l'intervalle de temperatures. Interpretation des ces observations en termes de la transition optique 4T→ 6 A 1 accompagnee d'une paire de basculement de spins des ions Mn 2+ voisins

Bond lengths, Debye-Waller factors, and site occupancy in the diluted magnetic semiconductor ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se have been measured using extended x-ray-absorption fine structure. The nearest-neighbor bond lengths at both room temperature and low temperature (77 K) are found to be constant as a function of alloy composition within the experimental uncertainty of 0.01 \AA{}. Because the average cation-cation distance changes with Mn content, these results necessarily imply distortion of the tetrahedral bond angles. The anion sublattice is shown to suffer the largest distortion, but the cation sublattice also exhibits some relaxation. The repercussions of these results are discussed, in terms of the amount of cation and anion sublattice distortion at low temperature and its connection to the superexchange mechanism occurring between the ${\mathrm{Mn}}^{2+}$ ions and mediated by the intervening anion in ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se.

The electrical resistivity, electron concentration, and mobility of ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Fe}}_{\mathrm{x}}$Se are reported for 4.2lTl300 K and for 0.0001lxl0.12. The data are interpreted within an electronic band-structure model that assumes the existence of resonant donors (due to the presence of Fe ions) whose ground-state energy coincides with the conduction-band continuum. The electron-concentration data enable determination of the donor energy as a function of the temperature and crystal composition. Unexpectedly high values of the low-temperature electron mobility for \ensuremath{\sim}0.0003\ensuremath{\le}x\ensuremath{\le}0.01 indicate a reduction of the charged-impurity scattering rate at these temperatures. This effect is interpreted in terms of a spatial ordering of charges within the Fe-related impurity system.

The Mn 3d contribution to the electronic density of states was analyzed for the wide-band-gap material ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se for 0\ensuremath{\le}x\ensuremath{\le}0.20. Resonant photoemission at the Mn 3p-3d transition energy shows a new Mn-derived density-of-states feature 3.4\ifmmode\pm\else\textpm\fi{}0.1 eV below the valence-band maximum. Core photoemission data indicate that the presence of Mn atoms in the lattice yields a relatively small perturbation to the stability of the Cd---Se bond.

We have measured ultrasonic transit times to determine the elastic constants of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te crystals with 0\ensuremath{\le}x\ensuremath{\le}0.52 and of ${\mathrm{Cd}}_{0.52}$${\mathrm{Zn}}_{0.48}$Te at 296 K using hydrostatic pressures up to 4 kbar. It is found that Mn, but not Zn, weakens the zinc-blende crystal structure and makes it less stable under pressure. Applying a modified Born criterion to our data, we deduce the pressure expected to cause the transition to the rocksalt structure in each compound. The influence of Mn we attribute to Mn 3d orbitals hybridizing into the tetrahedral bonds.

Photoluminescence and reflectivity measurements have been carried out on ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se solid solutions in the complete range of crystal compositions, 0\ensuremath{\le}x\ensuremath{\le}0.55. The features of the photoluminescence and reflectivity data near the band edge enabled us to determine values of the fundamental band gap as a function of Mn molar fraction x and temperature (8 K\ensuremath{\le}T\ensuremath{\le}300 K). The energy gap in crystals with compositions in the vicinity of the zinc-blende--wurtzite structural transition (0.2\ensuremath{\lesssim}x\ensuremath{\lesssim}0.3) exhibited a scatter of values, probably associated with the presence of various polytypes (as evidenced by transmission-electron-microscopy measurements). The dependence of ${E}_{g}$ on x is anomalous for 0\ensuremath{\le}x\ensuremath{\le}0.2, showing a minimum in the ${E}_{g}$ vs x curve. A simple model (in terms of second-order perturbation theory in s-d and p-d interactions) relates this minimum to a maximum observed in the magnetic susceptibility as a function of x. For samples with x\ensuremath{\gtrsim}0.35, the ${E}_{g}$ dependence on T shows an onset of an additional blue shift as the temperature is lowered below \ensuremath{\sim}150 K. This onset, though not reproduced by our simple calculation, appears to be also related to the presence of s-d and p-d interactions.

Optical-absorption results on single-crystal samples of ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se are presented. Dichroism associated with the fundamental edge is established for x\ensuremath{\ge}0.35, consistent with the onset of the wurtzite phase. Up to four absorption bands associated with ${\mathrm{Mn}}^{2+}$ intraion transitions are seen at x\ensuremath{\sim}0.50; the shifts of the three lowest transitions with x are analyzed with use of ligand-field theory applied to localized ${\mathrm{MnSe}}_{4}$ clusters. Consistent polarization in the Mn absorption spectrum occurs in the wurtzite region. Possible mechanisms leading to such polarizations are considered.

Ultrasonic transit-time measurements have been made down to 1.5 K on hexagonal, wurtzite-structured ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se, with x=0.37 and 0.53. The velocities of longitudinal waves increased monotonically with decreasing temperature, as expected from the anharmonicity of crystal bonding forces. However, the shear-wave velocities fell below that expected from ordinary anharmonic effects starting at about 150 K and went through a broad, shallow minimum with its lowest point near the spin-glass transition temperature, ${T}_{\mathrm{sg}}$. The depression was smaller at x=0.37 than at x=0.53 and smaller at 145 MHz than at 30 MHz. We attribute it to ultrasonic strain-induced modulation of the Mn-Mn superexchange interaction and the concomitant changes in Mn spin-spin correlations involving some kind of anisotropy. Comparison with results on ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te indicates anion dependence. An analysis is given of our results which employs an empirical expression involving the magnetic susceptibility. The expression was constructed by combining theoretical relations developed by others for ordinary antiferromagnetic materials and for spin glasses. It is inferred that some type of ordering begins near 160 K (associated with isolated spin clusters) in addition to the ordering which begins at the much lower spin-glass temperature.