Theoretical investigation on the magnetization enhancement of Fe₃O₄-reduced graphene oxide nanoparticle system

We present a theoretical study on the enhancement of magnetization of Fe₃O₄ nanoparticle system upon addition of reduced graphene oxide (rGO). Experimental data have shown that the magnetization of Fe₃O₄-rGO nanoparticle system increases with increasing rGO content up to about 5 wt%, but decreases back as the rGO content increases further. We propose that the enhancement is due to spin-flipping of Fe ions at the tetrahedral sites assisted by oxygen vacancies at the Fe₃O₄ particle boundaries. These oxygen vacancies are induced by the presence of rGO flakes that adsorb oxygen atoms from Fe₃O₄ particles around them. To understand the enhancement of the magnetization, we construct a tight-binding based model Hamiltonian for the Fe₃O₄ nanoparticle system with the concentration of oxygen vacancies being controlled by the rGO content. We calculate the magnetization as a function of the applied magnetic field for various values of rGO wt%. We use the method of dynamical mean-field theory and perform the calculations for a room temperature. Our result for rGO wt% dependence of the saturated magnetization shows a very good agreement with the existing experimental data of the Fe₃O₄-rGO nanoparticle system. This result may confirm that our model already carries the most essential idea needed to explain the above phenomenon of magnetization enhancement.