Bo Wha Lee

Tunable blue-green-emitting wurtzite ZnS:Mg nanosheet-assembled hierarchical spheres for near-UV white LEDs

Mg-doped ZnS hierarchical spheres have been synthesized via hydrothermal method using mixed solvents of ethylenediamine and DI water without any surface-active agent. The surface morphology and microstructure studies revealed that the hierarchical spheres were consisted of many well-aligned nanosheets with width 10 nm and length about 50 ~ 100 nm. X-Ray diffraction results show that the ZnS:Mg hierarchical spheres have wurtzite structure with high crystallinity. The absorption edge in the diffuse reflection spectra shifts towards lower wavelength with increasing Mg concentration, indicating an expansion in the bandgap energy that is estimated to be in the range of 3.28 to 3.47 eV. Blue-green photoluminescence with tunable intensity and peak position was observed depending on the Mg content. The Mg2+-activated ZnS phosphor can be good candidates for blue-green components in near-UV white light-emitting diodes.

Preparation of Fe-doped ZnO ferromagnetic semiconductor by sol-gel method with hydrogen treatment

Zn/sub 1-x/Fe/sub x/O (x=0.00,0.01,0.03,0.05,0.07, and 0.10) compounds were fabricated by the sol-gel method. The crystal structure and magnetic properties were investigated as a function of doped Fe concentration. Specifically, we have used hydrogen treatment for the control of phase separation. The X-ray diffraction patterns show that the wurzite structure of ZnO does not change for the doping range below x=0.07. Furthermore, we could not find any Fe cluster or phase separation in the X-ray diffraction patterns. The Fe-doped ZnO indicate ferromagnetic behaviors with the Curie temperature higher than room temperature. Then, the magnetic moment per Fe atom increased with increasing Fe concentration.

Investigation of magnetic properties of non-magnetic ion (Al, Ga, In) doped Ba2Mg0.5Co1.5Fe12O22

Ba2Mg0.5Co1.5Fe12O22 (non-doped) and non-magnetic ion (Al, Ga, In) doped Ba2Mg0.5Co1.5(Fe0.99M0.01)12O22 (M-doped) polycrystalline samples were prepared by the solid-state reaction method. The crystal structures and magnetic properties of samples were investigated with x-ray diffractometer (XRD), vibrating sample magnetometer (VSM), and Mossbauer spectroscopy. The crystal structures of non-doped and M-doped samples were determined as hexagonal structures with R-3m space group by the Rietveld refinement. The unit cell volume (Vu) of non-doped sample was Vu = 1298.0 A3, while those of M-doped samples (M = Al, Ga, In) were Vu = 1295.7, 1296.7, and 1299.9 A3, respectively. From the magnetic hysteresis curves at room temperature, the saturation magnetization (Ms) and coercivity (Hc) of non-doped sample were found to be Ms = 28.0 emu/g and Hc = 255.4 Oe, but those of M-doped samples (M = Al, Ga, In) were Ms = 24.7, 28.4, 28.6 emu/g, and Hc = 222.0, 215.9, 190.7 Oe, respectively. From the temperature dependence ...

Mössbauer studies of dynamic Jahn-Teller relaxation on the Cu-substituted sulfur spinel

Samples of Fe1−xCuxCr2S4 (x=0.0, 0.1, 0.3, and 0.5) have been studied with Mossbauer spectroscopy, x-ray diffraction, magnetization, and magnetoresistance. A cusp-like anomaly is observed for the sample x=0.1 in the both field-cooled and zero-field-cooled magnetization curves near 130 K under an applied field H=100 Oe. The charge state of iron ions are ferrous for the samples x=0.0 and x=0.1, whereas they are ferric for the samples x=0.3 and x=0.5. The Mossbauer spectra for the sample x=0.1 show asymmetric line broadening, and it is considered to be dynamic Jahn-Teller relaxation. The unusual reduction of magnetic hyperfine field below 110 K can be interpreted in terms of cancellation effect between the mutually opposite orbital current field HL and Fermi contact field HC.

Growth of Textured CoFe2O4 Thin Films on Platinized Silicon Prepared by a Sol-Gel Method

We fabricated textured polycrystalline CoFe2O4 thin films on Pt(111)/TiO2/SiO2/Si substrate through a sol-gel method. We varied the thickness of the films, by using precursor solutions with different concentrations of 0.1, 0.2, and 0.3 M, and by depositing 5, 8, or 10 layers on the substrate by spin-coating. X-ray diffraction spectra indicated that when the precursor concentration of the solution was higher than 0.1 M, the spin-coated films were preferentially oriented in the direction. Inspection of the surface morphology by scanning electron microscopy revealed that CoFe2O4 thin films prepared with 0.2 M solution and 5-time spin-coatings had smoother surface, as compared to the other conditions. Each coating had an average thickness of about 50 nm. The magnetic properties measured by vibrating sample magnetometer showed magnetic anisotropy, as evidenced from the difference in the in-plane and out-of-plane hysteresis loops, which we attributed to the textured orientation of the CoFe2O4 thin films.

Mössbauer spectroscopy and neutron diffraction studies of the ferrimagnetic semiconductor on Ga-substituted FeGaxCr2−xS4

The cation distribution and magnetic and transport properties of FeGaxCr2−xS4 are studied. Rietveld refinement of x-ray diffraction and Mossbauer spectroscopy lead to the conclusion that the samples are in inverse spinel type, where Ga ion is present at tetrahedral site (A). The neutron diffraction on FeGaxCr2−xS4(x=0.1) above 10 K shows that there is no crystallographic distortion and reveals an antiferromagnetic ordering, with the magnetic moment of Fe2+(−3.45μB) aligned antiparallel to Cr3+(+2.89μB) at 10 K. The resistance exhibits a strong dependence on Arrhenius model at temperature below 100 K, while it shows a strong correlation on small polaron model at temperature above 200 K. The electric quadrupole splittings of the A and B sites in Mossbauer spectra give a direct evidence that Ga ion stimulate asymmetric charge distribution of Fe ions in the A site.

Anisotropic hyperfine field fluctuation in La/sub 0.67/Ca/sub 0.33/Mn/sub 0.99/Fe/sub 0.01/O/sub 3/

Colossal magnetoresistance (CMR), Mossbauer spectra and neutron diffraction of the iron-doped manganite, La/sub 0.67/Ca/sub 0.33/Mn/sub 0.99//sup 57/Fe/sub 0.01/O/sub 3/, have been studied. The crystal structure is found to be cubic perovskite with the lattice parameter a/sub 0/=3.868 /spl Aring/. It is notable that the Curie temperature, T/sub C/=270 K, line broadening and 1,6 and 3,4 linewidth difference appear to suggest anisotropic hyperfine field fluctuation. The temperature dependence of the effective anisotropy energy is also found to decrease rapidly with increasing temperature.

Neutron and Mössbauer studies of FeCr2Se4

FeCr2Se4 has been studied with x-ray and neutron diffraction techniques, superconducting quantum interference device magnetometer, and Mossbauer spectroscopy. The crystal structure of FeCr2Se4 at 295K is monoclinic (space group I2∕m), with the lattice parameters a=6.267A, b=3.617A, c=11.822A, and β=90.69°, and its magnetic structure is found to be C-type ordering. The Mossbauer spectra were obtained at various temperatures ranging from 4.2to295K. Magnetic hyperfine and electric quadrupole interactions at 4.2K have been fitted, yielding the following results: Hhf=108.8kOe, θ=72°, φ=90°, η=0, ΔEQ=(1∕2)e2qQ[1+(1∕3)η2]1∕2=−1.65mm∕s, and R=−2.25. Our study strongly suggests that the quadrupole interaction of FeCr2Se4 is larger than the magnetic dipole interaction.

Influence of La defect on the magnetoresistance and magnetic properties of La1−xMnO3

Influence of the effect of lanthanum deficiency on the structural, magnetoresistance, and magnetic properties have been studied on a series of La1−xMnO3 (x=0, 0.05, 0.1, 0.2, and 0.33) powder samples. The powder samples were prepared by the sol–gel method. The x-ray diffraction analyses of the samples revealed a single phase of rhombohedral symmetry. Magnetization curves showed a weak ferromagnetic ordering for the sample x=0.0 with its Curie TC=138 K, but for the other samples showed ferromagnetic phase. The Curie temperature TC, with increasing deficiency from x=0.05 to x=0.33, was found to be increased from 272 to 279 K, respectively. The saturation magnetic moments per Mn atom at 80 K for x=0.0 and 0.10 are 2.30, 3.21 μB, respectively. A small increasing La defects resulted in changes of their magnetic structures thoroughly. In all the La defect samples, the temperature dependence of resistance shows that metal to insulator transition occurs near the TC and is closely related to the magnetization. The...

Structural and Magnetic Properties of Ni 0.6 Zn 0.4 Fe₂O₄ Ferrite Prepared by Solid State Reaction and Sol-gel

Ni 0.6 Zn 0.4 Fe₂O₄ prepared using solid state reaction and sol-gel methods were compared for their structural and magnetic properties. Due to the much higher annealing temperature used in solid state reaction, the crystalline size was much larger than that of the nanoparticles prepared by sol-gel. The saturation magnetization of sol-gel nanoparticles was higher, and the coercivity was about 2 times larger, compared to the solid state reaction sample. By analyzing the integration intensity of x-ray diffraction peaks (220) and (222), we proposed that the difference in the saturation magnetization might be due to the inversion of cation distribution caused by the different preparation techniques used.