| Summary: | 碩士 === 國立交通大學 === 物理研究所 === 94 === Magnetic properties of a series of Cd1-xMnxSe quantum dots with different sizes and Mn concentrations (x) were measured by using superconducting quantum interference device (SQUID) magnetometer. The field dependent magnetization of the Cd1-xMnxSe quantum dots at 2 K was obtained and it was identified not to be dominated by CdSe quantum dots but Mn ions. We fitted the field dependent magnetization to get the total number of Mn ions in the sample. We assumed that the Mn ions still follow the Curie law and obey the Brillouin function up to room temperature so we can remove their contributions in magnetization of Cd1-xMnxSe quantum dots. In consequence, we can obtain the temperature dependent and field dependent magnetization of pure CdSe quantum dots and we can discuss the size and Mn-ion effects on magnetic properties of CdSe quantum dots.
Before subtracting Mn effects, We found that our CdSe quantum dots display diamagnetism in high fields and at high temperatures, while they exhibit paramagnetic properties in low fields and at low temperatures. The results are consistent with previous studies. After removing Mn effects, with the same Mn-concentration we obtained enlarged paramagnetic and diamagnetic susceptibility for the smaller-size CdSe quantum dots in the lower and higher fields. As for CdSe quantum dots with the same size, we found that the inclusion of Mn ions in the dots will magnify their paramagnetism at low temperature, and the size effect will be quenched.
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