Microwave Radiation from YIG Magnons

高微波功率的非线性激励（参数激励）可以产生特定磁振子的不稳定生长。处于非平衡态的磁振子被散射到具有不同频率和波矢量的其他磁振子上。这些磁振子偶尔会辐射微波。通过探测非平衡态磁振子系统的微波辐射，可以研究磁振子相互作用或磁振子凝聚态。放置于微波腔内的铁磁体YIG（钇铁石榴石）单晶激发的非平衡磁振子通过配置磁振子-光子非线性耦合振荡器模式与微波光子非线性耦合。利用附加的弱微波信号对这些耦合模式进行了研究。理论计算中考虑了谐振腔模式和磁性样品的耦合系统，结果与实验数据吻合较好。利用平行于易磁化轴[111]的静态磁场和微波磁场，对YIG球形单晶进行微波辐射检测。还使用了圆盘样品，并施加平行于样品表面的磁场，样品表面垂直于[111]轴。抽运微波频率分别为9.5 GHz和15.4 GHz。辐射微波通过在样品周围缠绕一个小的拾取线圈来检测，并通过微波频谱分析仪进行分析。在脉冲抽运结束后，测量了辐射微波强度与时间的关系。结果直接确定了磁振子的弛豫时间为19ps，这与其他实验测得的自旋波弛豫时间一致。

Unstable growth of particular magnon can be generated by nonlinear excitation (parametric excitation) with high microwave power. Those magnons in the nonequilibrium state are scattered to other magnons, which have different frequencies and wave_ vectors. Those magnons occasionally radiate microwave. Magnon interactions or magnon condensation state can be studied by detecting microwave radiation from the nonequilibrium magnon system.Nonequilibrium magnons excited in a single crystal of ferrimagnet YIG (Yttrium Iron Garnet), which was placed in a microwave cavity, nonlinearly couples with microwave photon with configuring magnon-photon nonlinearly coupled oscillator mode. Those coupled modes were studied by means of additional weak microwave signal. Theoretical calculations, which consider coupled systems of resonator mode and magnetic sample, are found to be in good agreement with the experimental data.Microwave radiation was detected from a sphere single crystal of YIG with applying both static and microwave magnetic field parallel to the [111] axis, easy magnetization axis. A disk sample was also used with applying magnetic fields parallel to the sample surface, which is perpendicular to the [111] axis. The frequencies of pumping microwave were 9.5 and 15.4 GHz. Radiated microwave was detected by a small pick-up coil wounded around a sample and analyzed by a microwave spectrum analyzer. The intensity of radiated microwave was measured as a function of time after the end of pulse pumping. As a result, relaxation time of magnon is directly determined as 19 p s which is in agreement with spin-wave relaxation time measured by other experiments.