Switching on antiferromagnetic coupled superparamagnetism by annealing ferromagnetic Mn/CdS nanoparticles

Abstract

Mixed phases of metastable cubic and stable hexagonal manganese assisted CdS nanoparticles were synthesized by a colloidal route. These phases were quantified by the Short and Steward method of X-ray diffraction and differential scanning calorimetry analysis. The photoluminescence (PL) emission observed around ∼ 550 nm for undoped CdS is found to be due to cadmium vacancy defects; on increased addition of Mn²⁺ both the intensity and the line width of this emission continuously but disproportionately decrease and a new emission around ∼ 585 nm, interpreted to be due to manganese d–d emission, emerges with increased intensity but with no change in line width. The disproportionate decrease in the area and bandwidth of the ∼ 550 nm emission reveals that the intensity decrease is not a manifestation of the line width change. This decrease in intensity, i.e., the transition probability, is attributed to both the quenching effect by manganese and reduction in Cd vacancy defects due to substitution by Mn²⁺. The enriched 5% Mn/CdS was subjected to annealing in order to find out any possible changes or transformation during the annealing proces. An increase in the electron paramagnetic resonance intensity of preannealed sample on lowering temperature, much more than suggested by the Boltzmann population difference, indicates it to be ferromagnetic. On the other hand, postannealed sample exhibits antiferromagnetic coupled superparamagnetism as revealed by a drastic reduction in intensity down to the Néel temperature (T_N) with unaltered line width and a substantial decrease in line width below T_N. This conversion from ferromagnetism to antiferromagnetic coupled superparamagnetism after annealing is strongly supported by magnetic measurements, in which the preannealed sample exhibits increased coercivity on lowering the temperature, a case of ferromagnetism, and the postannealed material, however, exhibits low coercivity along with exchange bias below T_N, representing superparamagnetism. This conversion is due to ionic migration of Mn²⁺ as supported by magnetic and PL measurements. High-resolution transmission electron microscopy confirms the morphology and size of nanoparticles.