Coexistence of antiferromagnetism and ferromagnetism in Mn2+/CdS nanocrystals and their photophysical properties

Abstract

Manganous ion doped CdS nanocrystals with different concentrations of Mn²⁺ have been synthesized by a chemical synthesis route. The as-synthesized Mn²⁺/CdS nanocrystals evince interesting optical emission properties; the intensity of cadmium vacancy emissions V_Cd decreases on the addition of the dopant ion, proving the displacement of cadmium by manganese which is accompanied by the appearance and enhancement of the Mn²⁺ d–d emission, ⁴T₁ → ⁶A_1. However, the intensity of the latter emission decreases on further increase in [Mn²⁺] due to strong cluster formation, such as Mn²⁺–Mn²⁺. The low temperature electron paramagnetic resonance measurements suggest strong antiferromagnetism due to strong exchange coupling between the spins of nearby Mn²⁺ centres both in pre- and post-annealed 5% Mn²⁺/CdS samples. These magnetic properties for pre- and post-annealed 5% Mn²⁺/CdS samples are explained by a core/shell (antiferromagnetism/ferromagnetism) model. The pre-annealed sample, on one hand, shows antiferromagnetic behavior with weak ferromagnetism below the Néel temperature, the latter due to uncompensated surface spins. On the other hand, annealing leads to the formation of more Mn²⁺–Mn²⁺ dimers or clusters due to ionic migration. This will result in increased antiferromagnetic character as compared to the pre-annealed sample. Both are confirmed by magnetization hysteresis curves at various temperatures. The electron microscopy images confirm the morphology prediction by XRD.