High-Resolution Magneto-Optical Kerr-Effect Spectroscopy of Magnon Bose-Einstein Condensate

Using a novel heterodyne magneto-optical Kerr-effect technique, we have studied the spectral characteristics of a room-temperature magnon Bose-Einstein condensate (BEC) in yttrium-iron-garnet (YIG) films. Thanks to the ultimate frequency resolution of the technique of better than 100 Hz, we were able to directly determine the spectral linewidth of the spontaneously formed condensate of microwave-frequency magons. The value of 1.7 MHz provides direct evidence of high temporal coherence of the magnon condensate, and shows that the condensation phenomenon can be used for implementation of tunable microwave oscillators with a quality factor Q = f/Delta f = 3x10(3). The measured linewidth is close to, but larger than, the lower limit estimated from previous indirect experiments based on the measurements of the condensate lifetime. Additionally, the technique enables a determination of the wavevector of the condensate k(BEC) = 5x10(4) cm(-1), which is in agreement with previous studies.