C. Rigaux

Spin glass phase transition in Hg1−kMnkTe semimagnetic semiconductors

Abstract Low field interband Faraday rotation and a.c. magnetic susceptibility are investigated in Hg 1− k Mn k Te and Hg 1− k − x Mn k Te semimagnetic semiconductors ( k ⩽0.5). The spin glass phase transition is experimentally manifested by characteristic cusps in χ ( T ) and kinks in θ ( T ). The transition temperatures obtained from both types of experiments are well correlated which enable us to determine the phase diagram for Hg 1− k Mn k Te system. The spin glass phase transition occurs at low temperature in quaternary alloys than in Hg 1− k Mn k Te alloys with identical Mn composition. This implies that the antiferromagnetic interaction between Mn 2+ ions due to the virtual valence to conduction band transitions plays a significant role in the mechanism responsible for the spin glass formation.

Two-mode resonant polaron in the Hg0.72Cd0.28Te semiconductor

Abstract A two-mode polaron behavior is observed in the mixed crystal Hg 0.72 Cd 0.28 Te at magnetic fields corresponding to ω c = ω L1 and ω c = ω L2 resonances. Consistently including the small-gap structure of energy bands and a multi-mode dielectric formulation we can quantitatively describe our observations.

Magnetooptical evidence of exchange interactions in zero-gap Hg1−xFexTe mixed crystals

Abstract Interband magnetoabsorption is carried out on zero gap Hg 1- x Fe x Te alloys of x ∼ 0.015.Γ 6 → Γ 8 magnetooptical spectra for σ + , σ - , γ ‖ H polarization are quantitatively interpreted within the “quasi Ge” model modified by the inclusion of exchange contributions. The field dependence of the magnetization provides evidence of antiferromagnetic interactions between localized spins.

Magnetic anisotropy in Cd1−xFex Te

Abstract We have investigated the magnetization of Cd1−xFexTe at high magnetic fields B ⩽ 15 T, for B parallel to the crystal orientations [100] and [111]. In the non linear region of the magnetization curves, a significant anisotropy is observed, with a [100] easy axis. A model taking the spin-orbit interaction, Zeeman energy and cubic crystal field only, accounts for the experimental results.

Effect of conduction anisotropy of GIC on ESR line shapes

Abstract The Dyson theory of the line shape of ESR in conducting media has been extended to the case of strongly anisotropic conductors with finite dimensions. In typical ESR experiments for GIC the predominant part of the sample surface exposed to the r.f. magnetic field is perpendicular to the c-axis. It has been shown that in spite of that for highly anisotropic GIC the r.f. magnetic field penetrates into the sample mainly across the sample borders parallel to the c-axis. Thus the interpretation of ESR results for GIC based on the standard Dyson formula may give misleading conclusions concerning the spin diffusion.

High field magnetization step in Zn1−xMnxTe

Abstract Low temperature and high field magnetization experiments (H ⩽ 19 T) were carried out in diluted Zn1−xMnxTe alloys (x = 0.03). At 1.3 K, the first magnetization step, corresponding to the level crossing for pairs of nearest-neighbours (NN) is observed around H ⋍ 15 T . Magnetization data are analyzed including, within the molecular field approximation (MFA), interactions between Mn ions distributed in small clusters (singles, pairs, triangles) with more distant neighbours than NN. Both the saturation value observed at H ⋍ 10 T and the magnitude of the step agree with a random Mn distribution. Theoretical fits of the magnetization step provide a determination of the NN exchange constant J k = −9.25 ± 0.3 K .

EXCHANGE INTERACTIONS IN CDFETE SEMIMAGNETIC SEMICONDUCTORS

The exchange splitting of the free exciton in diluted Cd1-xFexTe (x < 0.04) is studied by magnetoabsorption experiments carried out at T = 1.8 K, up to B = 7T. The four Zeeman-like components of the exciton spectrum are observed in circular polarization for magnetic field oriented along the principal crystallographic directions. The comparison between the energy splittings and the magnetization data, analyzed in terms of the crystal field model, yields accurate values for the conduction and valence exchange parameters: N0α = 0.30 ± 0.04eV, N0β = −1.27 ± 0.08eV.

In situ optical study of H2SO4-graphite intercalation compounds

Abstract Optical reflectance experiments were performed on H2SO4-graphite during electrochemical synthesis. The stage formation was studied from the evolution of intervalence transitions. Quantitative analysis of the plasma reflectance spectra of pure stage compounds led to the determination of the Fermi energy as a function of the electrochemically controlled charge. The results prove the validity of the 2D band structure model and provide the first determination of the band parameter γ0 in acceptor compounds.

Optical determination of the charge transfer in AsF5-graphite intercalation compounds

Abstract Optical reflectance experiments are performed on well-characterized compounds C8nAsF5 (n = 1, 2), in the temperature range 10–300 K. The theoretical analysis of the reflectivity spectra within the 2-D model of graphite independent subsystems [1] provides an optical determination of EF and the charge transfer coefficient which is found independent of temperature.

Magnetooptical study of semimagnetic alloys Cd1−xMnxTe

Abstract The splitting of the exciton state in magnetic field is studied by reflectance experiment on concentrated Cd 1−x Mn x Te alloys. A linear decrease of the splitting is observed as a function of x, inferring a zero magnetization for x 0 ∼ 0.61 corresponding to the antiferromagnetic phase transition. Absorption spectra of concentrated alloys show several absorption bands resulting from the excitation of manganese 3d states.