Jung Chul Sur

Synthesis and aging effect of spherical magnetite (Fe3O4) nanoparticles for biosensor applications

The chemical coprecipitation process was used to synthesize about 7 nm, spherical magnetite nanoparticles to study magnetic properties and the aging effect. As-produced spherical magnetite nanoparticles have been aged in the atmosphere for 19 months. Magnetic properties and aging effect were studied by Mossbauer spectroscopy at a temperature ranging from 77 to 300 K, vibrating sample magnetometer, and x-ray diffraction. Saturation magnetization and coercivity were found to be 49 emu/g and nearly 0 Oe at room temperature, respectively. A singlet Mossbauer spectrum was observed at room temperature, implying superparamagnetic behavior of the particles, while a two-sextet spectrum was observed at 77 K. The particle size in this study is about 7 nm, which is smaller than the superparamagnetic size of 26 nm as calculated from Neel’s theory of single domain particles. After having aged these particles for 19 months, all magnetic properties and their original shapes were retained. Superparmagnetic magnetite nanop...

Synthesis of nanosized (Li0.5xFe0.5xZn1−x)Fe2O4 particles and magnetic properties

In an attempt to synthesize nanosized (Li0.5xFe0.5xZn1−x)Fe2O4 (0⩽x⩽1) particles with high magnetic saturation and low coercivity, the energetic ball milling technique was employed. LiCO3, α-Fe2O3, and ZnO powders were used as starting materials. The ball milled, partially crystallized lithium zinc ferrite starts to crystallize at about 600 °C. This is much lower than the temperature of 1000 °C, which is used in conventional methods. Particle size of lithium zinc ferrite was in the range of 20 to 50 nm. Regardless of the annealing temperature, the saturation magnetization increases with increasing x and reaches the maximum (about 80 emu/g) at x=0.7 [(Li0.35Fe0.35Zn0.3)Fe2O4], followed by a decrease to 60 emu/g for x=1 [(Li0.5Fe0.5)Fe2O4]. On the other hand, the coercivity of x=0.7 composition decreases with increasing annealing temperatures. Saturation magnetization and low coercivity for x=0.7 annealed at various temperatures are discussed in terms of site occupation.

Spin orientation of hematite (/spl alpha/-Fe/sub 2/O/sub 3/) nanoparticles during the Morin transition

We have synthesized 40-nm-sized hematite particles by the forced hydrolysis of acid Fe/sup 3+/ solution to investigate the change in spin direction during the Morin transition. Mo/spl uml/ssbauer spectra were obtained at various temperatures ranging from 4.3 to 300 K. The Morin temperature (T/sub M/) was found to be 220 K, which is lower than 263 K of bulk hematite. Fe/sup 3+/ spins in the 40-nm-sized hematite particle flip from 90/spl deg/ to 28/spl deg/ with respect to the c-axis of hexagonal structure during the Morin transition, which corresponds to the [110] direction of rhombohedral structure. This is not in agreement with the spin flip from 90/spl deg/ to 0/spl deg/ observed in bulk hematite or sub- and micron-sized particles at T

The Effect of Manganese Substituted M-type Hexagonal Ba-ferrite

The Mn-substituted M-type Ba-ferrite (BaFe 12-x MnO 19 ; x = 0, 2, 4, 6) powders were prepared by the HTTD (High Temperature Thermal Decomposition) method. The effect of Mn 3+ Jahn-Teller ions on the magnetic properties has been studied by x-ray diffraction, vibrating sample magnetometry, and Mossbauer spectroscopy. With increasing Mn substitution, the lattice parameter a0 increases while c0 decreases. The magnetocrystalline anisotropy constants (K₁) were determined as 2.9, 2.2, 1.8, and, 1.3×10? erg/㎤ for x = 0, 2, 4, and 6, respectively, by the LAS method. We have studied the change of cation distribution by Mossbauer spectroscopy which is closely related to K₁.

Synthesis of nano-sized spherical barium-strontium ferrite particles

Magnetic recording media requires good particle dispersion, a smooth surface, and small interparticle interaction to make an adequate signal-to-noise ratio (SNR). Well dispersed 50-60 nm sized spherical barium-strontium ferrite (S-Ba/Sr-Fe) nanoparticles were successfully prepared with 40 nm sized hematite precursor particles and BaCO/sub 3//SrCO/sub 3/ colloid. The coercivity and saturation magnetizations of S-Ba/Sr-Fe nano-particles were 1568 Oe and 48.6 emu/g, respectively. In order to evaluate magnetic interaction, magnetic tape was prepared using an Eiger mill with binder and organic solvent. /spl Delta/M measurement showed the S-Ba/Sr-Fe nanoparticles in the tape had negative magnetic particle-to-particle interaction.

Raman studies of spin-phonon coupling in hexagonal BaFe12O19

Phonon softening in hexagonal BaFe12O19 single crystal is observed in temperature dependent polarized Raman spectroscopy. The phonon softening is significantly enhanced below ∼80 K, at which the magnetization in a magnetic field along c-axis shows a clear change in the temperature dependence, indicating a magnetic phase transition near 80 K. Possible interpretations of the anomalous temperature dependence of the phonon frequencies–phonon softening behaviors are discussed. The phonon softening would be correlated to local structural and magnetic phase transition in hexagonal BaFe12O19, in which the spin-phonon coupling would have major contribution for the phonon softening. We also discuss that the spin-phonon coupling would be affected by both the spin ordering and the individual phonon vibration in magnetic material.

Ba

A combination of energetic shake-milling and a subsequent double sintering process was employed to synthesize Co_0.8Zn_1.2Z (Ba₃Co_0.8Zn_1.2Fe₂₄O ₄₁) hexaferrite nanoparticles with a high-saturation magnetization and a low coercivity. A homogeneous mixture of BaCO₃, CoO, ZnO, 40-nm sized alpha-Fe₂O₃, and heat-treating in an oxygen environment were important factors for synthesizing single-phase Co₂Z-type ferrite nanoparticles. In addition to an X-ray diffraction pattern, Mossbauer spectra confirmed that only Fe³⁺ cations are present in the synthesized Co_0.8Zn_1.2Z particles, implying single phase of the particles. Low-temperature sintering processing, 900degC, was then successfully applied to the single-phase Co_0.8Zn_1.2Z particles. The coercivities of the Co _0.8Zn_1.2Z powder and the low-temperature sintered disk were 9 and 20 Oe, respectively, while maintaining the saturation magnetization of 50 emu/g

_3

The mixed series CoxFe1−xCr2S4 has been studied by x‐ray, Mossbauer spectroscopy, and superconducting quantum‐interference device (SQUID) magnetometry. The crystal structure is found to be a cubic spinel, and the lattice constant a0 decreases linearly with increasing cobalt concentration. Mossbauer spectra of CoxFe1−xCr2S4 have been taken at various temperatures ranging from 4.2 to 300 K. The iron ions are ferrous and occupy the tetrahedral sites. The Curie temperature increases linearly with cobalt concentration, suggesting that the superexchange interaction for the Co‐S‐Cr link is stronger than that for the Fe‐S‐Cr link. Magnetic hyperfine and quadrupole interactions in Co0.9Fe0.1Cr2S4 at 4.2 K have been studied, yielding the following results: Hhf=80.8 kOe, 1/2e2qQ(1+1/3η2)1/2=2.65 mm/s, θ=15°, φ=75°, and η=1.0. It is notable that, as the temperature decreases below the Curie temperature, quadrupole splitting increases with decreasing temperature, suggesting the presence of an electric field gradient a...

Co

In this study, we prepared biocompatible, water-soluble, superparamagnetic, manganese-doped, magnetism-engineered iron oxide (MnMEIO) nanocrystals with reactive moieties. This was achieved via a one-pot synthesis protocol by thermally decomposing metal acetylacetonate precursors in 2-pyrrolidone in the presence of polyethylene glycol diacid. Using carbodiimide, we achieved nanocrystals coupled with 9-aminoacridine and folic acid through 2,2′-(ethylenedioxy)-bis-ethylamine. The results suggest that the nanocrystals provide carboxyl functional groups as binding sites. Their surface modification and composition were analyzed by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Using Mossbauer spectroscopy and a magnetometer, we have shown that these nanocrystals exhibit superparamagnetic behaviors. In addition, we have demonstrated that the nanocrystals were not significantly toxic, and hence are biocompatible.

_0.8

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.