铁磁绝缘体中磁振子输运和弛豫的第一性原理研究

Magnon spintronics is a new field of research that is concerned with information transport and processing by spin waves whose quanta are called magnons. Compared to electrons and spin currents carried by electrons, information propagation by magnons does not need motion of charges and thus gets rid of the drawbacks such as energy dissipation due to Joule heating. With careful choices of materials whose Gilbert damping is low, spin waves can propagate long distances of orders of magnitude larger than those of spin currents. It has already been realized in experiments that charge signals are converted to magnons and transmitted to far distant places and converted back to output charge signals, where ferromagnetic insulator Yttrium iron garnet (Y3Fe5O12, YIG) is chosen for its rather low Gilbert damping. Despite so many intriguing achievements in experiments, theoretical studies of magnon transport are rare. The description of interaction between magnons and phonons still follows the theory established in the 50’s which employs a black-box like relaxation time to characterize the interaction, and the evaluation of this relaxation time has not reached an overall agreement. We propose first-principles calculations to determine this parameter where the electronic structure of a real material is taken into account. Ballistic magnon transport from also first-principles, which has not been realized before, is proposed to be implemented by analogy with electron transport and is expected to play an important role in the theoretical and computational studies of magnon spintronics.

磁振子自旋电子学是一门新兴的研究领域，关注利用自旋波或者说磁振子来完成信息的传播和处理。由磁振子传递信息比起电流和电子承载的自旋流有明显的优势，前者不需要电荷运动，因此克服了焦耳热损失能量这一内禀缺陷。选取弱阻尼材料作为磁振子器件可以保证信号的传播距离比起自旋流高数个量级。实验上广泛选用铁磁绝缘体钇铁石榴石，已经实现了电信号转化为磁振子信号，在室温下长距离传播至另一处再转化为电信号。对比实验研究取得的重要成果，磁振子输运的理论研究还非常少。其中对磁振子和声子相互作用的描述依然沿用50年代提出的弛豫时间的概念，磁振子-声子弛豫时间这一重要参数也没有确定的数值。本课题研究将通过第一性原理计算在真实材料电子结构的基础上确定各种不同的物理机制对该参数的影响，并将移植电子的输运理论来实现的磁振子的弹道输运的第一性原理研究，为磁振子自旋电子学领域的理论和计算研究奠定基础。

铁磁绝缘体钇铁石榴石以极低的吉尔伯特阻尼而备受瞩目，在该材料中磁振子的低耗散传播距离室温下长达10微米，且该传播并不涉及电子的运动，因此没有热耗散的问题，对于目前触及摩尔定律瓶颈的磁存储元件的密度极限将是一个革命性的转机，因此钇铁石榴石被大量用以磁振子传播信号的研究中。在这些传播中，声子对于磁振子的弛豫是不可避免的，我们通过第一性原理计算，确定了室温下钇铁石榴石中磁振子谱的蓝移和展宽，第一次定量给出磁振子和声子的相互作用时间，对于用钇铁石榴石制作自旋波电子元件有重要参考意义。由于铁磁绝缘体体系大，磁振子色散关系复杂，我们研究了铁钴合金，该合金具有目前铁磁合金的最低的吉尔伯特阻尼，我们系统地研究了磁振子及强磁无序对于磁耗散的影响，对于理解合金阻尼有较为重要的理论意义，对于进一步研究铁磁绝缘体的磁振子耗散也有参考价值。在实验测量中，钇铁石榴石及其镥铋取代物的磁光效应显著，因此光学测量也是研究这类材料的常用手段。我们与清华大学朱静院士实验组合作，对钇铁石榴石及其镥铋取代物的物性进行了系统研究，分析了取代物的电子结构的变化以及对应的光吸收行为的特点。以上研究内容对应工作已经发表两篇Physical Review B，第三篇正在准备投稿。