Unconventional spin dynamics in the noncollinear phase of a ferrimagnet

Ferrimagnets possess collinear and noncollinear magnetic phases. Here we identify a crucial influence of the magnetic phase on the excited spin modes. The spin dynamics is launched in a rare-earth iron garnet film by femtosecond laser pulses. The noncollinearity cardinally changes the behavior of spin mode frequencies: the high-frequency one becomes highly sensitive to the external magnetic field while the low-frequency one gets almost field insensitive and even drops with the field growth near the transition to the collinear phase. Near the magnetization compensation point the frequencies of these modes get close to equality.