Superparamagnetic particles asymptotics

With control of particle size and spacing as per annealing conditions, if one considers Eq. 3 it is possible to extract the critical superparamagnetic particle size. Figure 8 shows a plot of Hc as a function of particle size for FePt/C samples. As expected the coercivity asymptotically approaches a maximum value as the particle size increases. A fit to the data with Eq. 3 reveals that the superparamagnetic particle size for FePt is between 2 and 3 nm [6]. The value of the critical superparamagnetic size can be seen in the figure to be the same, within experimental error, for each of the volume fractions of carbon in the samples. 

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