Ferromagnetism in zinc sulfide nanocrystals: Dependence on manganese concentration

Abstract

Ferromagnetic ordering in nanocrystallites of dilute magnetic semiconductors arises due to a fascinating interplay of carrier concentration, randomness of magnetic impurity sites, and size-induced quantum confinement. We here report results of a magnetization study on ZnS nanoparticles $(\ensuremath{\sim}2.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ carried out by varying the doping concentration of substitutional Mn ions that occupy cationic sites without altering the carrier concentration. Ferromagnetic ordering and giant Zeeman splitting are observed below $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in these nanoparticles for doping above 1.5%. The change in coercive field $\ensuremath{\Delta}{H}_{C}$ exhibited $\sqrt{T}$ temperature dependence expected for noninteracting nanoparticles. The values of blocking temperature ${T}_{B}$ and ${H}_{C}$ are found to be maximized for a doping level of 2.5% that corresponds to around five Mn atoms per ZnS nanocrystal.