Superparamagnetic blocking, nanoparticle

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)...

At high temperatures, a nanoparticle is in a superparamagnetic state with thermal equilibrium properties as described in the previous section. At low temperatures, the magnetic moment is blocked in one potential well with a small probability to overcome the energy barrier, while at intermediate temperatures, where the relaxation time of a spin is comparable to the observation time, dynamical properties can be observed, including magnetic relaxation and a frequency-dependent ac susceptibility. 

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). 

The loss of coercivity with size, i.e., the superparamagnetic nature of nanoparticles, has been well studied. The discussion earlier deals with the behavior below the blocking temperature. Neel s theory suggests an exponential behavior for the temperature induced relaxation (t) ... 

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