Superparamagnets

Below a critical size the particle becomes superparamagnetic in other words the thermal activation energy kTexceeds the particle anisotropy energy barrier. A typical length of such a particle is smaller than 10 nm and is of course strongly dependent on the material and its shape. The reversal of the magnetization in this type of particle is the result of thermal motion. 

The crystal stmcture of the intermediate is not well understood. The final iron phase is termed superparamagnetic because the particle size is too small to support ferromagnetic domains. At low rates, the discharge occurs in two steps separated by a small voltage difference. At high rates, however, the two steps become one, indicating that the first step is rate limiting, ie, the second step (eq. 34) occurs immediately after formation of the intermediate (eq. 33). 

Products of decomposition may be of such small particle size that superparamagnetism is exhibited [329] (e.g. by Fe203 [324,326] where the characteristic six-line spectrum of antiferromagnetic Fe203 is replaced by a doublet with an isomeric shift corresponding to Fe3+). 

Mew York (1967) see also a paper by L.M. Mulay et al. this describes "Superparamagnetism" in Proc. Am. Inst. Chem. Engrs. Conf. (Philadelphia, 1978), "Microfische No. 60" available from Am. Inst. Chem. Engrs., New York, NY. 

MOssbauer Spectroscopy. Small, single domain, ferro- or ferri-magnetic particles can show both collective magnetic excitation (precession of the magnetic moment) and superparamagnetic (relaxa-... 

In studies of superparamagnetic relaxation the blocking temperature is defined as the temperature at which the relaxation time equals the time scale of the experimental technique. Thus, the blocking temperature is not uniquely defined, but depends on the experimental technique that is used for the study of superparamagnetic relaxation. In Mossbauer spectroscopy studies of samples with a broad distribution of relaxation times, the average blocking temperature is commonly defined as the temperature where half of the spectral area is in a sextet and half of it is in a singlet or a doublet form. 

Figure 6.13 shows the Mossbauer spectra of ferritin [51], which is an iron-storage protein consisting of an iron-rich core with a diameter around 8 nm with a structure similar to that of ferrihydrite and which is surrounded by a shell of organic material. At 4.2 K essentially all particles contribute to a magnetically split component, but at higher temperatures the spectra show the typical superposition of a doublet and a sextet with a temperature dependent area ratio. At 70 K the sextet has disappeared since all particles have fast superparamagnetic relaxation at this temperature. 

Magnetic separation is the most documented and one of the most useful applications of superparamagnetic NPs. The unique feature of magnetic NPs in the... 

Cai, W. and Wan, J.Q. (2007) Facile synthesis of superparamagnetic magnetite nanoparticles in liquid polyols. Journal of Colloid and Interface Science, 305 (2), 366-370. 

Liu, C., Zou, B.S., Rondinone, A.J. and Zhang, Z.J. (2000) Reverse micelle synthesis and characterization of superparamagnetic MnFe204 spinel ferrite nanocrystallites. Journal of Physical Chemistry B, 104 (6), 1141-1145. 

Tartaj, P. and Serna, C.J. (2002) Microemulsion-assisted synthesis of tunable superparamagnetic composites. Chemistry of Materials, 14 (10), 4396-4402. 

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