自旋波-声子耦合对反铁磁体红外吸收谱的影响

由于交换作用常数受到磁离子间距离变化的影响是不可忽略的,在磁有序物质中存在自旋波和声子的耦合作用。本文探讨了非金属反铁磁体中自旋波-声子耦合在红外吸收谱上可能有的表现,辐射场的电偶极矩作用在激发一个光频支声子的同时,通过自旋波-声子耦合又激发两个自旋波和一个声子,这就在晶格吸收谱的高频边缘以外形成附加吸收带。当反铁磁体的Néel点足够高,并且自施波态密度有尖锐的极大值时,附加吸收带就有超出晶格吸收边缘足够大的能量,且有明显的峯形,上述吸收峯就应在实验上观测到。把这一结果应用于NiO,解释了它的红外光谱中0.24电子伏的附加吸收峯,计算得到与实验相符合的吸收能量和吸收强度,并指出这一吸收机构只能在反铁磁体中而不能在铁磁体中发生。本文最后指出,Mizuno和Koide在企图解释同一现象的工作中,所探讨者相对于本文所考虑的跃迁机构,它只可能产生极其次要的贡献。

INFLUENCE OF PHONON-MAGNON COUPLING ON THE INFRARED ABSORPTION IN ANTIFERROMAGNETS

In ferromagnets and antiferromagnets, the coupling between phonons and magnons cannot be overlooked, because of the exchange interaction changing with the distance between the magnetic ions. In this article, the possible effect of this coupling on the infrared absorption in magnetic insulators has been considered. The transition involved consists of the excitation of an optical phonon by the interaction with radiation field and simultaneously that of two magnons and a phonon due to the perturbation of the phonon-magnon coupling. It produces an absorption band with frequencies higher than the edge of the lattice absorption. Since the total spin component S_z must be conserved, this effect should not be found in ferromagnetic insulators. If the Néel temperature of an antiferromagnet is high enough and the distribution of spin wave states vs. energy has a sharp maximum the predicted absorption band becomes observable. Our treatment is aimed at the 0.24 eV absorption band of NiO. Its peak energy and absorption constant are calculated. The results appear in fair agreement with the experimental findings. Finally, it is pointed out that the transition process adopted by Mizuno and Koide for explaining the same phenomenon can only offer a secondly effect in comparison with ours.