No abstract

Temperature variations of the electron magnetic resonance (EMR) spectra and magnetization measurements are used to show that Ce3+ ions in concentration ≃18% are present in 3 nm CeO2 nanoparticles supported on silica aerogel. It is argued that the presence of Ce3+ implies the defect structure CeO2- x for ceria nanoparticles due to oxygen vacancies. This transformation of Ce4+ to Ce3+ driven by oxygen vacancies may be the key to understanding the catalytic properties of ceria.

A comparative study of the magnetic and electron paramagnetic resonance (EPR) parameters of bulk and Co 3 O 4 nanoparticles (NP), synthesized by a sol-gel process, is presented. Both samples possess the cubic phase with a slightly lower (by 0.34%) lattice parameter for the Co 3 O 4 NP. The average crystallite size D = 17 nm determined by x-ray diffraction (XRD) for the Co 3 O 4 NP is quite consistent with the electron microscopic observations. The bulk Co 3 O 4 has particle size in the 1-2 μm range. A Néel temperature of T N = 30 K (lower than the 40 K usually quoted in the literature) is determined from the analysis of the magnetic susceptibility versus temperature data for bulk Co 3 O 4. This T N = 30 K is in excellent agreement with the T N = 29.92 K reported from specific heat measurements. The Co 3 O 4 NP powder exhibits a still lower T N = 26 K, possibly due to the associated finite size effects. The values of coercivity, H c = 250 Oe, and exchange bias, H e = −350 Oe, together with the training effect have been observed in the Co 3 O 4 NP sample (cooled in 20 kOe). Both H c and H e approach zero as T → T − N. For T > T N , the χ versus T data for both samples fit the modified Curie-Weiss law (χ = χ 0 + C/(T + θ)). The magnitudes of C, θ and T N are used to determine the following: exchange constants J 1ex = 11.7 K, J 2ex = 2.3 K, and magnetic moment per Co 2+ ion μ = 4.27 μ B for bulk Co 3 O 4 ; and J 1ex = 11.5 K, J 2ex = 2.3 K and μ = 4.09 μ B for Co 3 O 4 NP. EPR yields a single Lorentzian line near g = 2.18 in both samples but with a linewidth H that is larger for the Co 3 O 4 NP. Details of the temperature dependence of H , line intensity I 0 , and disappearance of the EPR on approach to T N are different for the two samples. These effects are discussed in terms of spin-phonon interaction and additional surface anisotropy present in Co 3 O 4 NP.

No abstract

Magnetic properties of oleic acid/oleylamine coated magnetite nanoparticles of average diameter D=4, 6, 8, 10, and 12nm are reported. The samples were characterized by transmission electron microscopy and x-ray diffraction (XRD) with XRD showing increasing disorder with decreasing D. Magnetization M vs temperature T data show the blocking temperature TB decreasing with decreasing D from TB=38K for 12nm to TB=6.5K for D=4nm. The saturation magnetization Ms at 2K also decreases from Ms=62emu∕g for 12nm to Ms=17emu∕g for 4nm but the coercivity at 2K increases with a decrease in D. It is shown that Ms fits Ms=92(1−2d∕D)3 with d=0.68nm for D>4nm and d=0.86 for D=4nm. This equation is derived assuming a core-shell model with shell of thickness d consisting of disordered spins not contributing to Ms.