State tomography for magnetization dynamics and coherence anomaly in Y3Fe5O12

Precessional motion of magnetization around the magnetic field decays to thermal equilibrium due to magnetization damping after cutting of the excitation force. After the decay, the phase coherence of the precession motion of magnetization was thought to be completely lost. Here we developed state tomography technique for time-resolved magnetization dynamics in a thin disk of yttrium iron garnet (YIG). We revealed that the temporal evolution of the coherence after cutting off the driving ac magnetic field exhibits different time constant from the one evaluated by linewidth measurement of ferromagnetic resonance, which is supported by theoretical modeling.