Ferromagnetic Semiconductor CdCr2Se4 Research Articles

Terahertz modulation of the Cotton-Mouton effect

The ultrafast dynamics of the magneto-optical response triggered in the ferromagnetic semiconductor CdCr2Se4 by an intense nearly single cycle THz pulse is investigated. The dependence on the strength and the polarity of the external magnetic field, and the polarization of the THz pulse can be explained by a phenomenological expression that attributes the observed dynamics to the Cotton-Mouton effect at THz frequencies.

Non-Linearities in the Faraday Rotation and Absorption Due to Excitons in the Ferromagnetic Semiconductor CdCr2Se4

In CdCr2Se4 single crystals the influence of incident radiation intensity on the spectral dependences of Faraday rotation (FR) and circularly polarized light absorption near the fundamental absorption edge was investigated. A strong growth of the resonant structure at the absorption edge and the Faraday rotation in the band associated with this resonance takes place as the laser radiation intensity increases. The non-linear Faraday rotation is a non-monotonic function of the radiation intensity. The maximum increase of the Faraday rotation approximates 100% of the linear effect value. At the highest radiation intensity a decrease of the non-linear FR effect is observed. The non-linear effects in absorption and FR are due to the influence of photoexcited charge carriers on an excitonic resonance. A non-linear FR of lesser value was observed also in the ferromagnetic semiconductor CdCr2Se2.3S1.7.

Non-Linearities in the Faraday Rotation and Absorption Due to Excitons in the Ferromagnetic Semiconductor CdCr2Se4

Non-Linearities in the Faraday Rotation and Absorption Due to Excitons in the Ferromagnetic Semiconductor CdCr2Se4

Mechanism for strong nonlinearities in the Faraday rotation and absorption in the ferromagnetic semiconductor CdCr2Se4

At high laser radiation intensities a strong nonlinear Faraday rotation (FR) may be observed in a relatively narrow energy band near the absorption edge of CdCr2Se4. To study the nature of this effect, we investigated the influence of the incident radiation intensity on the spectral properties of the FR and the circularly polarized light absorption, and studied the dynamics of the observed nonlinearities. As the laser radiation intensity increases, we find a strong growth of the resonance structure at the absorption edge and an enhancement of the FR in the band associated with the resonance. The nonlinear FR is a non-monotonic function of the radiation intensity and reaches a local maximum, which is about twice the value of the rotation angle for the linear effect. The dynamics of the observed nonlinearities show that the nonlinearities are caused by photo-excited charge carriers. These carriers screen the internal electrical fields and lead to a narrowing and growth of the excitonic resonance in the absorption spectra and FR.

Nonlinear Faraday effect near the fundamental absorption edge in the ferromagnetic semiconductor CdCr2 Se4

A sharp increase of the Faraday rotation with increasing incident radiation power is observed in a band near the fundamental absorption edge of the ferromagnetic semiconductor CdCr2 Se4. The magnitude of the effect is a nonmonotonic function of the radiation intensity. The effect is explained by narrowing of an excitonic resonance as a result of screening of the internal electric fields by photoexcited carriers.

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 μm excitations using a mode locked frequency-doubled YAG laser.

Ultrafast optoelectronic ferromagnetic semiconductor CdCr2Se4 switch

A new ultrafast optoelectronic switch that utilizes the ferromagnetic semiconductor CdCr2Se4 is demonstrated to have a scope-limited sampled response time of approximately 90 ps.

Ion beam analysis of thin films

The theory of optical absorption due to transitions between a valence band and a hydrogen-like local level associated with a conduction band is modified to permit an arbitrary power-law dependence of energy on the magnitude of the wave-vector of carriers in the valence band. The observed absorption for photon energies below 1.6 eV in the ferromagnetic semiconductor CdCr2Se4 is discussed in terms of a combination of two types of terms. The first type of absorption is due to transitions to a local level from a band with two branches, in each of which there is an energy region with a width of 0.28 eV or more beginning 0.10–0.16 eV from the band edge, in which the energy measured from some origin near but not necessarily equal to the band-edge is approximately proportional to (wave-vector)(13). The second type of absorption has a dependence on photon energy ħω of the form (ħω − E3)2, where E3 is a threshold energy probably connected with indirect transitions between bands as suggested by Sakai, Sugano and Okabe. After constraints on parameters appearing in the theory are imposed by use of results of these authors and of Shepherd, it is found that curves of Harbeke and Lehmann on optical absorption in CdCr2Se4 at 4.2, 78, 130 and 298 K in the photon-energy range 1.14–1.42 eV can be fitted to a mean accuracy of 3%, using an average of 3.75 adjustable parameters for each curve. The strength of the indirect band-to-band absorption does not have the temperature dependence expected for phonon-assisted indirect band-to-band transitions, but can be described by a term independent of temperature plus another term proportional to the square of the deviation of the magnetization from saturation. The fitting of the absorption curves requires that the ratio of the widths of the two branches of the bands varies from about 1.6 at low temperatures to 1.35 at 298 K and that the total width of the bands involved is less than 1 eV.

Reflectance magnetocircular-dichroism spectra and electronic structures of CdCr2Se4

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.

Influence of Nonstoichiometry on the Properties of Undoped CdCr2Se4

The behaviour of the deviation from stoichiometry in the ferromagnetic semiconductor CdCr2Se4 is studied. Specimens of undoped CdCr2Se4 were annealed under variously controlled Se vapor pressures in coexistence with CdSe or Cr2Se3 (or Cr3Se4). The electrical resistivity, Hall coefficient, lattice parameter and X-ray diffraction intensities were measured at room temperature on these specimens. The dependence of the properties on Se vapor pressure and CdSe activity was disclosed. The predominant acceptor-type defect due to the deviation from stoichiometry is found to be a Cd ion substituted for a Cr ion.

Resonant Raman scattering of CdCr2Se4

Abstract The scattering cross section of the Raman-active phonons at 156 cm−1 (Eg) and 169 cm−1 (F2g) in the ferromagnetic semiconductor CdCr2Se4 (Tc=130 K) has been measured as a function of incident photon energy between 1.55 and 2.81 eV, both in the ferromagnetic and paramagnetic phases. The resonance curve peaks sharply near 2 eV and shows a broadening for temperatures below the Curie point. The relative line intensities change significantly with photon energy. The results show that the concept of spin-dependent Raman scattering in the ferromagnetic spinels has to be revised in terms of exchange-splitting-induced resonant Raman scattering.

Magnetic-order-induced band splitting in the thermomodulation spectra of the ferromagnetic semiconductor CdCr2Se4

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Spectral Dependence of the Photoconductance in CdCr2Se4

The spectral response of the photoconductance in In-doped single crystals of the ferromagnetic semiconductor CdCr2Se4 was measured in the range of photon energies from 1 to 3 eV, between 80° and 340°K. The photoresponse is attributable to three distinct transitions: I varies between 1.2 and 1.35 eV, II and III remain constant near 1.45 and 1.8 eV, respectively. Transition I is identified with that which had been previously observed in absorption measurements, and shows the same characteristic ``red shift'' associated with magnetic ordering as the temperature is lowered. Transitions I and II are superimposed near room temperature, and become separated only below 200°K, after I had shifted to lower energies. Transitions II and III are relatively temperature insensitive. The relative photoresponses associated with transitions I and II remain nearly unchanged over a temperature range in which the total photoresponse changes by orders-of-magnitude. This points to the likelihood that the carriers excited by these two transitions move in the same final state. Transition III corresponds to a high-lying one which had been previously inferred from magneto-optical measurements.

HELICON-SPIN WAVE INTERACTION IN THE MAGNETIC SEMICONDUCTOR AgxCd1−xCr2Se4

The microwave transmission characteristics of the ferromagnetic semiconductor CdCr2Se4 doped with Ag have been measured as function of the applied magnetic and electric field. The results are interpreted in terms of active helicon-spin wave interactions; and the agreement between experiment and theory is found to be very good qualitatively. It is believed that the results presented in this Letter represent the first published evidence for active helicon-spin wave interactions.