Anisotropy energy barrier

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 Mossbauer spectra do, however, indicate that the anisotropy energy barrier for the particle magnetization flipping is quite large, 10-20 J (215), for the 1.5-nm iron particles. As discussed in Section III, A, 3, this barrier was estimated from Mossbauer spectra at various temperatures by measuring the fraction of the spectral area that appears paramagnetic. At the temperature for which this ratio is 0.5, the relaxation time can be estimated... 

Fig. 29. Magnetic anisotropy energy barriers for small particles. and K,S are the barriers for shape and magnetosurface anisotropy, respectively, for metallic iron. Figure according to Boudart et al. (215).

Mossbauer spectra were then taken of the small iron particles after various pretreatments, with the catalyst under reaction conditions (765). For increased sensitivity the velocity-offset mode was used (Section II, B, 1), and the magnetically split spectral area versus temperature curves after the various pretreatments are shown in Fig. 30. It is therein seen that the ammonia treatment, which increases the catalytic activity, decreases the magnetically split spectral area at a given temperature this is the result of a decrease in the magnetosurface anisotropy energy barrier. While the effects of these pretreatments are in themselves interesting, the important point for surface... 

Ev magneto-crystalline anisotropy energy barrier function... 

Figure 26.10 Top) Neel loss The magnetic moment rotation, overcoming the anisotropy energy barrier, occurs under the influence of an alternating magnetic field. Bottom) Brown...

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