合金碳化物M_(23)C_(6)的磁性及磁致磁性

Magnetism and magneto-magnetic properties of alloy carbide M_(23)C_(6)

利用第一性原理研究了碳化物M_(23)C_(6)(M=Fe, Cr)的晶胞结构、磁矩和态密度,并结合魏氏分子场理论分析了磁场作用下一些碳化物的磁致磁性。结果显示:Fe_(20)Cr_(3)C_(6)碳化物中的Fe3和Fe4原子连接组成稳定的晶格框架,Cr1、Cr2和C原子位于框架中心;Fe_(20)Cr_(3)C_(6)的磁性主要来源于Fe和Cr原子的3 d轨道贡献,并且贡献的方向相反,Fe_(23)C_(6)和Fe_(20)Cr_(3)C_(6)中存在sp和pd轨道杂化;Fe_(20)Cr_(3)C_(6)中具有负磁矩的Cr原子,削弱了紧邻Fe原子以及碳化物整体的磁矩;磁场作用下,碳化物的铁磁态相区扩大,磁矩增大。最终发现,碳化物M_(23)C_(6)在中温相变时依旧保持着较强的磁性,且磁场诱导磁性增加,可以预测,强磁场能够改变碳化物M_(23)C_(6)的析出稳定性。上述工作揭示了强磁场和温度耦合作用下复杂碳化物的结构、磁性及其稳定性影响的内在关联机制。

The unit cell structure, magnetic moment, and density of states of carbide M_(23)C_(6)(M=Fe, Cr) were studied using first-principles. The magneto-magnetic properties of some carbides under the action of magnetic field were analyzed with Weiss molecular field theory. Firstly, the Fe3 and Fe4 atoms in the Fe_(20)Cr_(3)C_(6) carbide are connected to form a stable lattice framework, and Cr1, Cr2, and C atoms are located in the center of the frame;secondly, the magnetism of Fe_(20)Cr_(3)C_(6) mainly comes from the 3 d orbital contributions of Fe and Cr atoms, and their directions of contribution are opposite, there are sp and pd orbital hybrids in Fe_(23)C_(6) and Fe_(20)Cr_(3)C_(6);thirdly, Cr atoms in Fe_(20)Cr_(3)C_(6) have a negative magnetic moment, it weakens the magnetism of the neighboring Fe atom and the entire carbide;fourthly, under the action of magnetic field, the ferromagnetic phase region of carbide expands and the magnetic moment increases. Ultimately, it was found that the carbide M_(23)C_(6) still maintained strong magnetism during the medium temperature phase transition, and the magnetic field induced magnetism increased. It can be predicted that the strong magnetic field can change the precipitation stability of carbide M_(23)C_(6). The above work revealed the intrinsic correlation mechanism of the structure, magnetic properties and stability of complex carbides under the coupling of strong magnetic field and temperature.