Size and chemical order dependence of magnetic-ordering temperature and spin structure in Fe@Ni and Ni@Fe core-shell nanoparticles.

The effect of particle size and chemical order on the temperature-dependent magnetic properties of Fe@Ni and Ni@Fe core-shell nanoparticles is studied in the framework of a classical spin Hamiltonian and Monte Carlo simulations. We found that the mean temperature-dependent magnetization and magnetic-ordering temperature are strongly affected by both the particle size (in size range of 4 to 16 nm) and core-shell chemical order. As a main result, we report the depression of the magnetic ordering-temperature with decreasing size of the elemental Fe and Ni nanoparticles. More specifically, in the case of Fe and Ni nanoparticles, the magnetic-ordering temperature is lowered by 40 (195 K) to 300 (175 K) compared to the bulk value for nanoparticle diameters ranging from 16 to 4 nm, respectively, consistent with previous theoretical data. We further provide a comprehensive insight into the magnetic properties of Fe@Ni and Ni@Fe nanoparticles, unveiling a rich and distinct magnetic-ordering temperature and spin structure that emphatically depends on the core/shell ratio.