Condensed States of Nonequlibrium Magnons in Ferromagnets

具有特定波矢的磁振子的非平衡态可以通过参数激发来实现，这是通过施加平行于铁磁体中的磁矩的强微波磁场来实现的。这些磁子通过磁振子-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。对直径为1 mm的YIG单晶球体施加平行于易磁化轴[111]的磁场进行了非线性激发实验。利用行波管放大器将合成器输出的9.6 GHz微波放大到最大16W。微波信号是通过绕在样品周围的一个小拾取线圈来检测的。微波信号由低通滤波和低噪声放大器组成的微波频谱分析仪进行观测。被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带底部的频率。还观察到二次和三次高次谐波。发射的线形和频移取决于温度。微波腔中光子与单平衡磁振子对之间的非线性相互作用变得非常重要，这种耦合系统正在从实验和理论两个方面进行研究。

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.