Younghun Hwang

Temperature dependence of Vickers hardness for Cd1−xMnxTe (0⩽x⩽0.82) single crystals

The Cd1−xMnxTe (0⩽x⩽0.82) single crystals were grown by the vertical Bridgman method. The Vickers hardnesses of the grown crystals were measured as a function of Mn composition and temperature for the first time. The activation energy was determined for the dislocation motion from the relation between temperature and hardness, and it was decreased as increasing x in Cd1−xMnxTe single crystals. Also, the Vickers hardness HV was decreased as increasing temperature and increased as Mn composition x.

Temperature dependence of the band-edge photoluminescence of Zn1?xMnxSe films

We report a systematic investigation of band-edge photoluminescence of the diluted magnetic semiconductor alloy Zn1−xMnxSe for a series of compositions (0 0.20 the effect of inhomogeneous broadening Γinh on Γ(T) was found to be stronger than that of the longitudinal optical phonon term ΓLO. Furthermore, the activation energies of thermal quenching were obtained for both the I2 and the I1 peaks from the temperature dependence of the bound exciton peaks, and were found to decrease with increasing Mn concentration. Finally, we show that at high temperature the broadening of the FWHM is dominated by the LO-phonon interaction.

Temperature dependence of the interband transition in a V2O5 film

The temperature dependence of the interband transition in an α-V2O5 film was investigated using absorption and photoluminescence spectral measurements at a temperature range of 10–300 K. Transmission measurements in the experimental temperature range indicate that the α-V2O5 film has two distinct interband transitions, implying indirect and direct transitions. This result was confirmed by spectroscopic ellipsometry. The blue shift of both the transitions in the α-V2O5 film with decreasing temperature was explained by a reduction in the lattice-dilatation effect and the electron-phonon interaction. The PL measurements in the experimental temperature range showed that the emission near 530 nm is due to the indirect transition in the α-V2O5 film.

Magneto-optical properties for bulk Cd0.63−yMn0.37HgyTe single crystals

We investigated the magneto-optical properties of diluted magnetic semiconductor Cd1−x−yMnxHgyTe (x∼0.37) single crystals grown using a vertical Bridgman method. This material crystallizes in the zinc-blende structure for values of y⩽0.07. The band gap energy increases with increasing y (⩽0.03), while decreases with increasing y (⩾0.03). An anomalous behavior was also observed in the magnetic susceptibility and electrical resistivity. Faraday rotation as a function of photon energy with various Hg compositions for CdMnHgTe in the temperature range of 10–300K has been measured. The Verdet dispersion curves were enhanced near the fundamental band gap energy. The Verdet constant becomes larger since the effect of the sp-d exchange interaction is enhanced as the temperature is lowered. At 10K, the Verdet constants of the Faraday rotation for the samples with y=0, 0.01, 0.03, 0.05, and 0.07 were observed to be −1.09, −2.57, −3.94, −0.97, and −0.87deg∕Gcm at 613nm, respectively. The Verdet constant showed a max...

Ferrimagnetism in strained Fe2As thin films on Si(001)

We present the structural and magnetic properties of Fe2As thin films grown on Si(100) by molecular beam epitaxy. From the reflection high-energy electron diffraction and x-ray diffraction patterns, the orientation of Fe2As film on Si(100) was found to be a c-axis in the tetragonal crystal structure. The tetragonal Fe2As thin film exhibited ferrimagnetic (FIM) ordering at room temperature. The magnetic moment of Fe2As was 0.1 μB/unit cell determined from the saturated magnetic moment. The observed FIM in Fe2As films was attributed to the strain in the film.

Temperature dependence of the fundamental band gap energy of Cd1−xCoxTe single crystals

Abstract Cd1−xCoxTe single crystals were grown by the vertical Bridgman method. The optical band gap energy of Cd1−xCoxTe(x=0, 0.001, 0.004, 0.008) were obtained as a function of temperature (12 ∼ 300K) and cobalt composition x. The temperature variation of the energy gap could be well fitted by the empirical Varshni relation. The temperature coefficient at low temperature decreased with x, but was nearly constant at high temperature. The band gap energy decreased linearly with composition.

Formation and ferromagnetic properties of FeSi thin films

In this work, the growth and ferromagnetic properties of e-FeSi thin film on Si(100) substrate prepared by molecular beam epitaxy are reported. The inter-diffusion of Fe layer on Si(100) substrate at 600 °C results in polycrystalline e-FeSi layer. The determined activation energy was 0.044 eV. The modified magnetism from paramagnetic in bulk to ferromagnetic states in e-FeSi thin films was observed. The saturated magnetization and coercive field of e-FeSi film are 4.6 emu/cm3 and 29 Oe at 300 K, respectively.

Electron mediated/enhanced ferromagnetism in a hydrogen-annealed Mn:Ge magnetic semiconductor

We report on the carrier type changes of the p-type for as-grown Mn:Ge films to n-type for post-annealed samples in a hydrogen ambient. The hydrogen-annealed samples exhibit the increased Curie temperature, from 165 to 198 K, and the enhanced magnetic moment, from 0.78 to 1.10 μB/Mn. The first principles calculation using the all-electron full-potential linearized augmented plane wave method indicates that the addition of an electron carrier strengthens the ferromagnetic coupling between the Mn atoms, while the hole carrier caused it to weaken.

Perpendicular ferrimagnetism in strained Mn2As film

Ferrimagnetic Mn2As thin films with perpendicular magnetic anisotropy were successfully grown on Si(100) by molecular-beam epitaxy. From the reflection high-energy electron diffraction and X-ray diffraction patterns, the orientation of the Mn2As film on Si was along the c-axis in the tetragonal crystal structure. Mn2As film exhibited ferrimagnetic ordering at temperatures greater than 300 K, which differs from antiferromagnetic or paramagnetic behaviors in the bulk form. The magnetic moment of Mn2As determined by saturated magnetization was 0.51 µB per unit cell.

Ferromagnetic behavior of CdMnCrTe quaternary system

We describe magneto-optical and magnetic properties of quaternary Cd1−x−yMnxCryTe crystals (x=0.37, 0≤y≤0.03) grown by the Bridgman method. The presence of Cr in this alloy is observed to induce ferromagnetic behavior and to enhance magneto-optical effects. Using magnetic circular dichroism and magnetization measurements, we describe the dependence of this behavior on the Cr concentration, and we discuss possible mechanisms for the observed effects.