Antiferromagnetic spin waves have been predicted to offer substantial functionalities for magnonic applications due to the existence of two distinct polarizations, the right-handed and left-handed modes, as well as their ultrafast dynamics. However, experimental investigations have been hampered by the field-immunity of antiferromagnets. Ferrimagnets have been shown to be an alternative platform to study antiferromagnetic spin dynamics. Here we investigate thermally excited spin waves in ferrimagnets across the magnetization compensation and angular momentum compensation temperatures using Brillouin light scattering. Our results show that right-handed and left-handed modes intersect at the angular momentum compensation temperature where pure antiferromagnetic spin waves are expected. A field-induced shift of the mode-crossing point from the angular momentum compensation temperature and the gyromagnetic reversal reveal hitherto unrecognized properties of ferrimagnetic dynamics. We also provide a theoretical understanding of our experimental results. Our work demonstrates important aspects of the physics of ferrimagnetic spin waves and opens up the attractive possibility of ferrimagnet-based magnonic devices.

由于存在两种不同的极化，即右手模式和左手模式，以及它们的超快动力学特性，反铁磁自旋波已被预测可为镁磁应用提供实质性功能。但是，反铁磁体的场免疫性阻碍了实验研究。铁蛋白已被证明是研究反铁磁自旋动力学的替代平台。在这里，我们使用布里渊光散射研究了铁磁环中磁化补偿和角动量补偿温度下的热激发自旋波。我们的结果表明，右手模式和左手模式在期望纯反铁磁自旋波的角动量补偿温度下相交。场致模式交叉点相对于角动量补偿温度和旋磁反转的位移揭示了铁磁动力学迄今无法识别的特性。我们还提供了对实验结果的理论理解。我们的工作展示了铁磁自旋波物理学的重要方面，并开辟了基于铁磁的磁悬浮装置的诱人可能性。