State tomography is an essential tool for analyzing physical states in quantum science. In magnets, elementary excitation called a magnon, or a spin wave, dominates magnetization dynamics and various magnetic properties. Here, we propose and demonstrate state tomography for magnetization dynamics, enabling us to obtain a density matrix and Wigner function of the magnetization dynamics. Using the technique, we found that parametrically excited magnons can form a mixed state composed of two coherent states. Magnetization state tomography will pave a way to explore a wide range of states of magnons, such as a squeezed state in condensed matter.
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