Superparamagnetic blocking temperature

Figure 3.21. Relaxation time t = co versus T(. For the 5 vol% and 17 vol% samples the lines are fits to the critical slowing down relation [Eq. (3.62)] with the parameters given in Table 111.1. The assumptions E = 0 and E = 500 yield exactly the same line. For the 0.06 vol% sample T is the superparamagnetic blocking temperature defined as the maximum of x".

Allen, P.D.,T.G. St. Pierre, R. Street (1998) Magnetic interactions in native horse spleen ferritin below the superparamagnetic blocking temperature. J. Magn. Mag. Mat. 177—181 1459-1460... 

All the samples measured showed characteristic superparamagnetic behavior with a blocking temperature TB. An independent method of determining the parameters of the particle size distribution g(D) is by means of the analysis of magnetic measurements under equilibrium conditions, i.e. at temperatures above the superparamagnetic blocking temperature Tb- For this purpose we performed magnetization measurements as a function of field M(H) at different temperatures [4,5]. 

Figure 8 Temperature-dependent Mossbauer spectra of metallic iron nanoparticles in zeolite NaX (a). The superparamagnetic blocking temperature Ti is aroimd 40 K. The solid lines have been calculated by a relaxation formalism assuming a hyperfine field distribution as shown in (b) with a bimodal size distribution of metallic iron particles. (Reprinted from Schiinemann, Winkler, Butzlaff and Trautwein. With kind permission from Springer Science Business Media)...

Fig. 4.30. M6ssbauer spectra of samples of (a) ferritin and (b) haemosiderin, showing the effects of superparamagnetism. Comparison of the two series of spectra shows that the mean superparamagnetic blocking temperature is greater for haemosiderin than for ferritin. (Bell et at., 1984.)...

In conclusion, the very poor crystallinity and the low superparamagnetic blocking temperature in natural ferrihydrites hamper to some extent the characterization of ferrihydrite with MS at standard measuring temperatures (RT, 80 K). From the point of vie w of identification, ferrihydrite can be recognized by MS as long as it represents the main constituent in soil samples. The broad quadrupole distribution (see Fig. 3.8)... 

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. 

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 ... 

The temperature at the maximum in the zero field dc susceptibility measurement determines the blocking temperature TB (Fig. 3). It was found to increase with the average diameter of the clusters of a given sample. The simplest model to explain superparamagnetism assumes that each particle has a uniaxial anisotropy with a direction independent of that of the other particles. The energy that is needed to reverse the magnetization U, determines the relaxation time of this process,... 

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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