碳、氮摻雜二氧化鈦粉末之室溫鐵磁性研究

Abstract

本研究中，我們利用固態燒結法製備摻雜碳、氮的二氧化鈦粉末，探討藉由非過渡金屬雜質所引發室溫鐵磁性的機制。我們發現摻雜碳、氮的二氧化鈦有很明顯的磁滯曲線，代表碳、氮可引發二氧化鈦在室溫時具有鐵磁性的現象。其鐵磁性的來源，推測是由氧空缺與雜質間相互作用所產生的。從O-1s的XPS可以看出，摻雜碳、氮可使氧空缺變多，而氧空缺產生的多餘電子。同時從Ti-2p XPS的變化發現一部分的電子與Ti4+產生電荷平衡變成Ti3+，另一部分的電子以自由電子的形式存在結構中以提供自旋極化的載子源。另外，二氧化鈦中同時存在碳(氮)雜質以及氧空缺，會發生類似p-d混成的p-p作用。進一步使能階分裂，在費米能階附近產生新的空能態，當電荷轉移時就具有不成對的電子，成為自旋極化的來源。此一現象符合Stoner criterion，亦即當費米能階附近的能態密度增加，因產生Stoner splitting而引發鐵磁性。總而言之，當二氧化鈦摻雜碳、氮使能階產生Stoner splitting，提供起始的自旋極化，接著與氧空缺所產生的自由載子發生exchange interaction，進而形成Bound magnetic polarons (BMP)。當BMP有部分疊合透過自由載子產生進一步的exchange interaction，使全部載子自旋方向排列成同方向而產生鐵磁性。

In this study, we experimentally demonstrate that it is possible to induce room temperature ferromagnetism in carbon, nitrogen doped TiO2 prepared simply by conventional solid state reaction method. From the magnetization hysteresis loop measurements, we can observe that N(C)-TiO2 is ferromagnetic at room temperature. The origin of the RT-ferromagnetism is believed to intimately relate to the interaction of oxygen vacancies and impurities. From XPS spectra, nitrogen doping promotes the increasing loss of oxygen. Due to the charge balance requirement, the existence of oxygen vacancies partly transforms Ti4+ to Ti3+ and provides excessive electrons. When the impurities and oxygen vacancies are present simultaneously, it is anticipated to induce new states appearing near the Fermi level by p-p interaction. It not only satisfies the Stoner criterion but also creates unpaired electrons. These unpaired electrons, in turn, act as the sources of spin-polarized carriers the eventually lead to room temperature ferromagnetism. Therefore, the p-p coupling interaction and charge transfer giving rise to the spin density near each impurity tends to align spin parallel to form moment at impurity ion. The moment-carrying impurities then strongly couples with carrier spins, generated by the existence of oxygen vacancies to form the bound magnetic polarons (BMP). If the concentration of BMPs is sufficiently high, it is able to effectively mediate the indirect ferromagnetic coupling among impurities by carries and result in ferromagnetism observed in the present system.