硒化鎘奈米顆粒尺寸及摻雜Mn效應之磁性研究

Abstract

我們利用超導量子干涉儀(SQUID)量測不同尺度(D = 5 nm, 8 nm)及不同Mn濃度(x = 0.375%, 0.75%, 1.5%)的Cd1-xMnxSe奈米顆粒磁性；我們推測在溫度2 K下，所量測的磁化強度是由Mn原子貢獻而非來自奈米顆粒，擬合其磁化強度對磁場曲線圖而得出樣品中Mn原子個數，同時假設Mn離子的磁性行為高達室溫仍滿足Brillouin function和Curie順磁定律，所以可從高溫的數據扣除Mn的順磁貢獻。進而觀察歸納出以下: 首先，CdSe奈米顆粒在低溫(<30 K)弱場(<2000 Oe)下為自旋電子貢獻的順磁現象，隨著外加磁場增強及溫度升高，磁化率會有從順磁到抗磁變化的現象；就尺寸效應而言，尺寸較小的奈米顆粒，在較弱的外加磁場下其電子軌道順磁磁化率較大，而在較強的外加場下軌道抗磁磁化率則較強，以上兩點吻合先前量子點磁性的研究。 對於磁性雜質摻雜在奈米顆粒的影響，就相同尺寸、摻雜Mn濃度百分比較高的奈米顆粒會增強奈米顆粒本身低溫下的居禮順磁行為，亦指Mn的d軌域電子將會增強量子點上電子自旋磁矩貢獻。 Mn的摻雜行為同時會破壞電子軌道運動貢獻的順磁矩與抗磁矩，使得磁化率在不同溫度下由小磁場到大磁場的偏移變小，量子點的磁性尺寸效應將被減弱。

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.