Dependence of Energy Product to the Shell Thickness in FePt/Fe3O4 Core/Shell Nanoparticles

Abstract

A new model is presented to calculate energy product variation for FePt/Fe3O4 core/shell in the terms of Fe3O4 shell thickness. The spins at the core/shell interface and some parts of the core volume have a random orientation due to the presence of chemical disorder created by the experimental procedures. The mentioned random effects based on the chemical disorder and intermixing of core and shell produces random interactions, which according to the presented model can behave similar to the spin glass system. Due to the existence of a competition between exchange and anisotropy energies, the coercivity field contain two parts: In the first part, the effects of exchange interactions will dominate the effects of anisotropy. As a result, this section will include low anisotropy and frustrated spins, which is coupled to neighbor particles due to the strong exchange interactions. In the second part, which is the volume part of matter, the anisotropy is higher; in this case, the spins are frozen and remain fixed so that the neighbor particles do not have much effect on them. Since the nanoscale particles have a significant value of surface to volume ratio, there will be a competition between these two parts. Energy product variations for FePt/Fe3O4 core/shell increases when coherent rotation is dominant in the core/shell system. By increasing shell thickness, coherent rotation of spins disappears and the magnetization rotation will not be uniform, which leads to a decrease in energy product. To increase the maximum product energy, we need to increase the value of coercivity which is achieved by increasing the core thickness. If we can increase the magnetocrystalline anisotropy constant of the core, then the coercivity and thus the maximum energy product will increase.