Preparation and basic physical properties of single crystals of cadmium chromium selenide CdCr2Se4

Single crystals of CdCr₂Se₄ were prepared by crystallization of polycrystalline material from the melt of cadmium chloride and by synthesis from anhydrous CrCl₃ and CdSe. Basic optical and transport properties were measured on the crystals. The results are compared with the available literature data and a brief survey of existing interpretation is given.     

On the anisotropy of the FMR linewidth in iron-silicon single crystals

The ferromagnetic resonance linewidthH in in plane configuration was measured in single crystal discs of Fe-2·2% Si at frequency 36·14 GHz. Several methods of electrolytic and chemical polishing were used to minimize the effects of sample surface irregularities. By rotating the external field in the plane of the discs ((110) plane) the linewidth of well polished samples was found to be isotropic within the experimental errors (about 5%). Spurious anisotropy of linewidth was found in some imperfectly polished samples.     

Phonons and magnetic order in ferromagnetic CdCr2Se4 and CdCr2S4

Measurements are reported of the Raman effect on single crystal ferromagnetic CdCr₂Se₄ and CdCr₂S₄ as a function of temperature. The lines observed at 158 cm^?1, 172 cm^?1 and 241 cm^?1 for CdCr₂Se₄ and 257 cm^?1, 281 cm^?1 and 396 cm^?1 for CdCr₂S₄ have been identified as due toГ ₁₂ ⁺ ,Г ₂₅ ⁺ andГ ₁ ⁺ phonons, respectively. Certain lines ofГ ₁ ⁺ orГ ₂₅ ⁺ symmetry, observed in both materials, show a temperature dependence of the intensity which resembles the behaviour of the spin correlation function. Some of these lines also exhibit a symmetry change at the Curie point. A group theoretical analysis of the eigenvectors of the Raman active modes is given. The results are interpreted in terms of a model proposed by Baltensperger which involves the ion position dependence of the magnetic exchange interaction in order to couple spin system and phonons. The weak frequency shift of the lines, which occurs predominantly in the ordered state, is interpreted in terms of the same model.     

Transient photoconductivity in the ferromagnetic semiconductor CdCr2Se4

The transient photoconductive response time of the ferromagnetic semiconductor CdCr₂Se₄ was measured to be smaller than 90 ps, the response time of the scope. The measurements were performed at room temperature (300 K) for 0.53 and 1.06 m excitations using a mode locked frequency-doubled YAG laser.     

Photoluminescence in YbGa2Se4 and YbGa2Se4:Nd3+ single crystals

The excitation and photoluminescence spectra of YbGa₂Se₄ and YbGa₂Se₄:Nd³⁺ single crystals recorded at different temperatures and neodymium concentrations have been investigated. The photoluminescence spectrum exhibits intracenter luminescence lines of the Nd³⁺ ion in wavelength ranges of 0.804?C0.84, 0.88?C0.92, 0.96?C0.996, and 1.06?C1.12 ??m against a broadband emission background. These ranges are related, respectively, to the ⁴ F _5/2-⁴ I _9/2, ⁴ F _3/2-⁴ I _9/2, ⁴ F _5/2-⁴ I _11/2, and ⁴ F _3/2-⁴ I _11/2 transitions.     

Reflectance magnetocircular-dichroism spectra and electronic structures of CdCr2Se4

Summary Reflectance magnetocircular-dichroism spectra of a ferromagnetic semiconductor CdCr₂Se₄ were measured for temperatures between 4.2 K and 157 K. Off-diagonal elements of the dielectric tensor were calculated from the measured spectra. A number of fine structures was resolved, the energy position of which was plotted against temperature. It is found that there are two red-shifting structures: one at 1.1 eV (the well-known absorption edge) and the other at 1.9 eV. Other structures show slight blue-shift. These energy positions were compared with the band structure calculated by means of the self-consistentDV-Xα technique. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Propagation of electromagnetic waves in the magnetic semiconductor CdCr2Se4

Saratovskii State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 32, No. 7, pp. 891–896, July, 1989.     

Energy band structure of CdCr2Se4 by APW method

The results of the first band structure calculation of the CdCr₂Se₄ magnetic semiconductor are reported and discussed in connection with the experiment and band structure results for related nonmagnetic compounds. For the calculation, the unsymmetrized APW method has been chosen due to complexity of the problem as well as physical reasons.     

Magnetization reversal of CuyCr2Se4−zBrx single crystals

Due to a partially inhomogeneous Cu concentration, Cu_yCr₂Se_4?zBr_x single crystal can show a complicated temperature dependence of the wall motion threshold field which is about 0.5 kA/m near the Curie temperature. The crystals can be homogenized by means of a NH₄OH treatment.     

FMR in surface oxidized iron single crystals

Ferromagnetic resonance absorption has been observed at room temperature in iron single crystals of whisker type (grown along [100] direction). The surface of the samples has been oxidized in air at 760 Torr at approx. 350 °C for periods up to 240 min. With the increasing degree of oxidation the resonance linewidth broadens and the resonance field shifts to lower values. Moreover, in oxidized samples an additional resonance peak situated above, the main resonance occurs, with intensity, position and linewidth depending on the period of oxidation. The ferrimagnetic resonance absorption in the iron oxide surface layer could be responsible for the occurence of this peak.     

Angular correlation measurements in (Ag,In)/CdCr2Se4

We report time differential perturbed angular correlation (TDPAC) measurements above and below the Curie temperature of the ferromagnetic semiconductor (Ag, In)/CdCr₂Se₄. The TDPAC spectra for the Ag and In dopants have profound differences which we attribute to distinct site occupations of these impurities in the spinel structure.     

The non-linear paramagnetic effect in CdCr2Se4 near the Curie temperature

Results are presented of measurements made of alternating current susceptibility changes Δχ = χ_B - χ₀ induced by external application of a constant magnetic field using the beat method (f ～ 1MHz). For 1 K < (T ? T_c) < 50 K and taking the appropriate values of magnetic fields the relation Δχ ～ B² was satisfied. Making use of the Debye theory adapted for the non-linear dielectric effect, it was shown that the mean number n of elementary cells in a cluster varies with temperature according to the relation n ～ (T ? T_c)^-1.31.     

Optical energy gaps of undoped and Co-doped ZnIn2Se4 single crystals

Single crystals of undoped and Co-doped ZnIn₂Se₄ were grown by the vertical Bridgman technique. The optical energy gaps of the single crystals were investigated in the temperature range of 10–300 K from the optical absorption measurements. The indirect optical energy gaps of the single crystals were found to be 1.624 eV for undoped ZnIn₂Se₄ and 1.277 eV for Co-doped one at 300 K. Also, the direct optical energy gaps were given by 1.774 and 1.413 eV for undoped ZnIn₂Se₄ and co-doped one, respectively. The temperature dependence of the optical energy gaps was well fitted by the Varshni equation.     

X-Ray investigations of temperature dependence of the lattice constants of HgCr2S4, HgCr2Se4, CdCr2S4, CdCr2Se4, ZnCr2Se4 Polycrystals in a temperature range from 70°K to the decomposition temperature

The temperature dependence of the lattice constants of HgCr₂S₄, HgCr₂Se₄, CdCr₂Se₄, CuCr₂Se₄ and ZnCr₂Se₄ was determined in temperatures ranging from 70°K to the decomposition temperature by the X-ray powder diffraction technique. The abnormal volume exchange caused by the exchange interaction was observed below the temperature Θ.