THz spontaneous magnon fluctuations and room-temperature spin switching in the orthoferrite Sm_0.7Er_0.3FeO₃

Ultrafast dynamics of correlated spins in the picosecond to femtosecond timescales have so far been pursued mainly with pump-probe schemes by perturbing the system and measuring the induced changes. Here, we reveal the femtosecond time-resolved autocorrelation function of the sub-THz spontaneous spin fluctuation dynamics in an orthoferrite, existent under the ambient condition due to thermal occupation. The correlation dynamics shows a significant enhancement of the amplitude and coherence time around the spin reorientation transition temperature, which is ascribed to the spontaneous switching of spin moment between the energetically degenerate states.