Superparamagnetic effect

Because the appearance of the superparamagnetic effect depends on the particle size and on the anisotropy constant, it is often displayed at room temperature by iron oxides <10 nm in size, for example, soil iron oxides. Superparamagnetic relaxation may be counteracted by lowering the temperature and thereby increasing x. Superparamagnetic particles will usually be ordered below a blocking temperature,Tb, which is ... 

Muller, K., Skepper, J.N., Posfai, M., Trivedi, R., Howarth, S., Corot, C., Lancelot, E., Thompson, P.W., Brown, A.P., and Gillard, J.H. (2007) Effect of ultrasmall superparamagnetic iron oxide nanoparticles (Ferumoxtran-10) on human monocyte-macrophages invitro. Biomaterials, 28 (9), 1629-1642. 

The ferric oxide, hematite, used in the present work was a high purity powder reagent with a BET surface area of 27 m2/g 30 mg was employed in each run. Some measurements were made on hematite calcined in air to see the effects of sintering the surface on the chemical structure of the adsorbed metal ions. The hematite samples were checked by Mossbauer absorption and powder X-ray diffraction measurements. The Mossbauer absorption spectra consisted of a magnetic sextet with no superparamagnetic component due to fine particles ( ). 

The superparamagnetic properties of -Fe203 have also been studied via the Mossbauer effect. Nakamura et al, (32) have investigated the temperature dependence of the internal field in -Fe203 particles of approximately 50 A. diameter. At 120 °K. they obtain a spectrum which is almost identical to the bulk material, except that no Morin transition has occurred and the spins still lie in the basal plane. At room temperature the magnetic hyperfine spectrum collapses (even though the bulk... 

The height of the energy barrier between the forward and reverse states is the product of the particle volume, V, and the anisotropy constant Kefr (which is, to some extent, a function of particle size). Superparamagnetic relaxation occurs when the thermal energy of the particles exceeds the activation energy barrier between the spin states and so allows rapid, spontaneous fluctuations between these states. The effect of these spin reversals is that the observed magnetic field is reduced or even absent. 

Morris, R.V. Lauer, H.V.Jr. (1990) Matrix effects for reflectivity spectra of dispersed nanophase (superparamagnetic) hematite with application to Martian spectral data. J. Geophys. Res. 95 5101-5109... 

The interaction of CO and C02 with the surface of Fe203 (for 8-nm particles) was also studied by Suzdalev et al. (247) using the Mossbauer effect. Due to the small size of these particles, more than 50% of the iron atoms appear superparamagnetic in the Mdssbauer spectrum at room temperature, and the effects of chemisorption were investigated through analysis... 

Many cells, such as lymphocytes, do however lack efficient internalizing receptor systems. The development of a new and effective internalizing pathway thus appears as a very important goal. Weissleder et al. [83, 96] have tried to develop such a system by attaching a superparamagnetic iron oxide to a membrane translocating signal (MTS) peptide. Several MTSs have been described... 

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