Figure 2.

Variation of coercivity (H_C), spin ordering and the nature of the magnetization curve with MNPs size ‘d’. (i) With a decrease of the size of ferromagnetic particles, the H_C initially increases and then attains a maximum value at the critical single domain size (D_SD) due to coherent rotation. (ii) Further, in the single domain regime (d < D_SD), the H_C decreases as the particle size decreases and (iii) becomes zero at the superparamagnetic regime due to the thermal agitation. (iv) Finally, in ultra-small regimes (d ≤ 3 nm), due to high-spin disorder of elemental spins, the MNPs exhibit a linear relationship between the magnetization and the applied magnetic field [35].