Magnetization dynamics blocking temperature

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. 

The measurements of imaginary part of magnetic susceptibility (losses) x"(a>,T) are necessary to analyze the blocking process dynamics. Such measurements have been carried out at different frequencies co and temperatures T. The results are reported in Fig. 2.47. It follows from Fig. 2.47, that x"(co,70 has maxima at temperatures T which increase with frequency increase. Such behavior is characteristic for relaxation time described by Arrhenius law t(T ) = zoexp(Em/kT). The relaxation time s T=T can be obtained from the expression (i>x(Ta,)= 1. Latter expression and Arrhenius law at T = permit to calculate l/r , as a function... 

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