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We report detailed studies of magnetization (M) as a function of magnetic field H (up to 50 kOe) and temperature T (5-380 K) in x% (Co/TiO[SUB2] (rutile) films for x=1, 4, 6, 10, and 12. These films, prepared by sputtering, show room temperature ferromagnetism (RTFM). Measurements of M vs T at H=50 Oe reported here in zero-field-cooled (ZFC) and field-cooled (FC) cases show M peaking at T[SUBp] ≃ 30 K for 1% Co to T[SUBp] ≃ 125 K for 12% Co doped films whereas bifurcation of the FC and ZFC data is observed near 300 K. For T

Results of the characterization of six Co-based Fischer–Tropsch (FT) catalysts, with 15% Co loading and supported on SiO2 and Al2O3, are presented. Room temperature X-ray diffraction (XRD), temperature and magnetic field (H) variation of the magnetization (M), and low-temperature (5 K) electron magnetic resonance (EMR) are used for determining the electronic states (Co0, CoO, Co3O4, Co2+) of cobalt. Performance of these catalysts for FT synthesis is tested at reaction temperature of 240 °C and pressure of 20 bars. Under these conditions, 15% Co/SiO2 catalysts yield higher CO and syngas conversions with higher methane selectivity than 15% Co/Al2O3 catalysts. Conversely the Al2O3 supported catalysts gave much higher selectivity towards olefins than Co/SiO2. These results yield the correlation that the presence of Co3O4 yield higher methane selectivity whereas the presence of Co2+ species yields lower methane selectivity but higher olefin selectivity. The activities and selectivities are found to be stable for 55 h on-stream. [ABSTRACT FROM AUTHOR]

In this paper, we compare the magnetic relaxation of DNA-coated γFe2 O3 with that of parent 7-nm γFe2 O3 particles by measuring the temperature dependence of AC susceptibilities (χ′ and χ″) at frequency f = 0.1, 1, 10, 100, and 1000 Hz and electron magnetic resonance (EMR) at 9.28 GHz. Changes in the blocking temperature TB (where χ′ peaks) with frequency are analyzed using the relation: TB = Ta + [T0/ln(f0/f)] derived from Vogel-Fulcher relaxation, where Ta(T0) measures the effective energy barrier (interparticle interaction). The fit to the data yields f0 = 1.3 × 1016 Hz, Ta = 2760 K(2960 K), and T0 = 40 K(20 K) for the uncoated (DNA-coated) γFe2 O3. From the relative shift ΔTm of Tm (the temperature where χ″ peaks) with change Δ log f in frequency, the parameterΦ = ΔTm/(TmΔ log f) is determined to be 0.04(0.06) for the uncoated (coated) ΔFe2 O3. These results show only minimal changes in the magnetic dynamics of γFe2 O3 upon DNA coating. [ABSTRACT FROM AUTHOR]

SQUID magnetometry and electron magnetic resonance (EMR) spectroscopy at 9.28 GHz were employed to probe magnetism and surface spin disorder in Fe3 O4 nanoparticles. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed magnetite particles with size ≃8 nm. Magnetization (M) versus temperature studies yielded blocking temperature TB ≃ 170 K in H = 50 Oe. For T > TB, M scales approximately as H/T with μP ≃ 5400 μB as the magnetic moment/particle. At 5 K, the magnitudes of hysteresis loop parameters viz. coercivity Hc ≃ 37 Oe and saturation magnetization M8 = 62 emu/g are considerably smaller than the reported values for larger 150 nm Fe3 O4 particles. In EMR studies, a single symmetric line at g ≃ 2.11 with linewidth ΔH = 850 Oe is observed at 300 K with the g-value and ΔH increasing with decreasing T. These results are interpreted in terms of a disordered surface spin layer of 0.5 nm thickness. [ABSTRACT FROM AUTHOR]

In gold nanoparticles (Au NPs) capped with dodecanethiol (DT), the authors report the observation of superparamagnetic blocking temperature TB≃50 K in D≃5 nm NPs but only diamagnetism in 12 nm NPs. For TB=50 K, the strong temperature dependence of coercivity Hc, saturation magnetization Ms, and exchange bias He (in the field-cooled sample) confirm the blocked state resembling ferromagnetism with Hc≃250 Oe, He≃-40 Oe, and Ms≃10-2 emu/g at 5 K. The observed electron magnetic resonance line shows expected shift, broadening, and reduced intensity below TB. A magnetic moment μ≃0.006μB per Au atom attached to DT is determined using a model which yields Ms varying as 1/D, with its source being holes in the 5d band of Au produced by charge transfer from Au to S atoms in DT [ABSTRACT FROM AUTHOR]

Temperature dependence (5 K to 300 K) of the magnetic properties of two Fe/SiO2 (15/85) nanocomposites with Fe particle size D ≃ 17 nm and 30 nm is reported using magnetometry and electron magnetic resonance (EMR) spectroscopy. Structural characterization of the samples, done by X-ray diffraction, TEM, and Mössbauer spectroscopy shows the presence of α-Fe, amorphous SiO2, and some nanosize Fe3+ oxides. Both samples are ferromagnets in the 5 K to 370 K range with coercivity HC(30 nm) > HC(17 nm). In EMR studies, a single resonance line near g = 2.2 characteristic of α-Fe is observed in both samples at 300 K, with linewidth ΔH(30 nm) > ΔH(17 nm). With decreasing temperature, the EMR line shifts to lower fields and broadens, which is similar to observations in other nanoparticle systems. [ABSTRACT FROM AUTHOR]