Superparamagnetic transition

This temperature is determined by Fe Mossbauer spectroscopy. The blocking temperature, the spectral superparamagnetic transition temperature, is the temperature at which doublet and sextet signals have equal integrated intensities. The doublet arises from small particles and the sextet from larger particles. 

On the other hand, it has recently been shown that some ferrihydrite species still exhibit a magnetic-superparamagnetic transition at very low temperatures. In particular, this seems to happen when ferrihydrite is closely associated to organic carbon [73]. These so-caUed DOM (dissolved organic matter) ferrihydrites have a lower hyperfine field and are even not completely magnetically ordered at 4 K (see also Sect. 3.5.1). 

Magnetic measurements of hydrogen chemisorption on Fe/Si02 are reversible at 210 torr, 310 °C [410]. For 15 A Fe/MgO, H affects the magnetic moment below the superparamagnetic transition. Above this temperature no effect is found [326] for 80 A Fe/MgO, H does not affect the magnetic moment [326]. 

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... 

AC susceptibility measurements are frequently used to identify thermodynamic phase transitions and to characterize spin-glass behavior and superparamagnets such as the single-molecule magnets discussed below. 

Other Papers.—Various iron species prepared by the vacuum pyrrolysis of acetyl-ferrocene-furfural resins at 400°C have been studied by Mossbauer spectroscopy. These consist of an amorphous glass-like carbon matrix containing free iron atoms, Fe+ ions, iron clusters, superparamagnetic iron, and ferromagnetic iron.333 The effect of pressure of up to 50kbar on the absorption spectra of five iron(m), two iron(n) and one mixed valence compound has been studied. In six of the compounds, but not in basic ferric acetate or soluble Prussian Blue, the observed pressure-induced bands were assigned to d-d transitions of converted iron(n) for the ferric compounds and to spin-forbidden d-d bands for the ferrous compounds. The charge-transfer band from iron(n) to iron(m) in soluble Prussian Blue showed a blue shift at pressures up to 7.2 kbar.334... 

After the advent of MR systems with powerful gradient hardware, dynamic scans with a repetition rate of at least one image or image stack per 2 s have become possible. By use of such sequences it has become feasible to visualize the transit of a bolus of paramagnetic or superparamagnetic contrast agent through the brain (Moseley et al. 1990 Rosen et al. 1990). For DSC, the bolus is injected intravenously. 

Magnetic properties of nanoparticles of transition metals such as Co, Ni show marked variations with size. It is well known that in the nanometric domain, the coercivity of the particles tends to zero. 23 Thus, the nanocrystals behave as superparamagnets with no associated coercivity or retentivity. The blocking temperature which marks the onset of this superparamagnetism also increases with the nanocrystal size. Further, the magnetic moment per atom is seen to increase as the size of a particle decreases 25 (see Figure 7). 

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