Microwave Radiation from YIG Magnons

利用高功率微波的非线性激发(参数激发)可以产生特定磁振子的不稳定生长。这些处于非平衡态的磁子被散射到具有不同频率和波矢的其他磁子上。这些磁子偶尔会辐射出微波。通过探测非平衡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。在微波腔中，亚铁磁体YIG(Yttrium Iron Garnet)单晶中激发的非平衡磁振子与微波光子非线性耦合，形成磁振子非线性耦合振子模式。利用附加的弱微波信号对这些耦合模进行了研究。理论计算结果与实验数据吻合较好。YIG球形单晶在平行于[111]轴的静磁场和微波磁场作用下，容易磁化，探测到了微波辐射。圆盘样品也被用来施加平行于样品表面的磁场，该磁场垂直于[111]轴。泵浦微波频率分别为9.5 GHz和15.4 GHz。用绕在样品周围的一个小拾取线圈探测辐射的微波，并用微波频谱分析仪进行分析。在脉冲泵浦结束后，测量了辐射微波强度随时间的变化。结果直接确定磁振子的驰豫时间为19p S，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.