Synthesis and properties of cadmium mangnaese sulfide diluted magnetic semiconductor nanoparticles

Cd_1−xMn_xS diluted magnetic semiconductor nanoparticles have been synthesized by using both aqueous and nonaqueous precipitation methods with and without a coating of organic thiols. The crystal structure and magnetic properties of the Cd_1−xMn_xS particles were studied in correlation to the surface and size. The noncoated particles with a size smaller than 50 Å crystallized into a zincblende structure; their magnetic behavior was predominately antiferromagnetic (AF) with a Neel temperature of 32K if the size was smaller than 30 Å, and 37K if the size was between 30 Å and 50 Å. With a size larger than 50 Å, the particles crystallized into the wurtzite structure, and their Neel temperature was 42K. The strength of the AF phase transition peak decreased in proportion to the increase in particle volume and disappeared when the diameter was larger than 110 Å. For particles coated with thiophenol or dodecanethiol, no antiferromagnetic phase was observed. These results imply that this AF phase is due to surface ions. Since the bulk materials is a spin glass of frustrated, antiferromagnetically coupled manganese ions, we propose that the coordination reduction results in stronger superexchange for spins on the surface, and the broken symmetry at the surface relieves the frustration leaving a surface antiferromagnetic phase. The size dependence of the strength of the transition implies that the transition is site specific in that a constant number of surface manganese ions, independent of particle size, participate in the transition. Also, the dynamics of the antiferromagnetic phase displayed a plateau and a linearly increasing range for the temperature dependence of the viscosity. The plateau indicates that the macroscopic quantum tunneling is the dominant relaxation process at low temperature for the magnetization of the anti ferromagnetic phase in Cd_1−xMn_xS nanoparticles.; The size effects on the spin glass phase of the Cd_1−xMn _xS materials from the bulk to nanoparticles were also studied. The freezing temperature T_f of the spin glass decreased with the composition x value and the spin glass transition persisted to small x. T_f decreased also with decreasing size and a critical size existed for the spin glass formation.