Condensed States of Nonequlibrium Magnons in Ferromagnets

在铁磁体中施加平行于磁矩方向的强微波磁场，可以通过参量激发激发出具有特定波矢的磁振子非平衡态。这些磁振子通过磁振子-磁振子散射过程释放能量，导致磁振子带底部的凝聚态。本研究的目的是通过探测凝聚态磁振子的微波辐射来阐明凝聚态。通过施加平行于直径为1 mm的钇铁石榴石（YIG）单晶球的易磁化轴[111]的磁场进行非线性激发实验，由合成器输出的9.6 GHz微波经行波管放大器（TWTA）放大到最大16 W。微波信号由缠绕在样品周围的小型拾波线圈检测。用低通滤波器和低噪声放大器组成的微波频谱分析仪对微波信号进行观测。检测微波的频率与激发微波的频率不同。检测到的发射具有低于自旋波段底部的频率。还观察到第二和第三高次谐波。发射的线形和频移取决于温度。微波腔中光子与非平衡磁振子对之间的非线性相互作用变得十分重要，目前正从实验和理论两方面对这种耦合系统进行研究。

Nonequilibrium states of magnon having the specific wave vector can be excited by parametric exciation, which is achieved by applying intense microwave magnetic field parallel to the magnetic moments in a ferromagnet. These magnons loose energy by magnon-magnon scattering processes, resulting to condensed states at the bottom of the magnon band. The purpose of this research is to clarify the condensed states by detecting microwave emission from the condensed magnon. The nonlinear excitation experiments were performed by applying magnetic field parallel to the easy magnetization axis [111] of a sphere of Yttrium-Iron-Garnet (YIG) single crystal in diameter of 1 mm. Microwave of 9.6 GHz from a synthesizer was amplified to the maximum 16 W by a Traveling Wave Tube Amplifier (TWTA). Microwave signal was detected by a small pick-up coil wounded around the sample. The microwave signal was observed by a microwave spectrum analyzer with using low-pass filter and low-noise amplifier. Frequency of the detected microwave was different from that of exciation microwave. The detected emission has a frequency of lower than the bottom of the spin-wave band. The second and third higher harmonics are also observed. Line shapes and frequency shift of the emission depend on temperature. Nonlinear interaction between photons in a microwave cavity and monequilibrium magnon pairs becomes important and this coupled system is under investigation by both experimentally and theoretically.