Experimental detection of magnon noise enhancement near spin reorientation in Sm0.7Er0.3FeO3

Antiferromagnets are one of the most promising candidates for high-speed spintronic applications in the future. The growing interest in thermal spin effects demands for experimental schemes that allow for the investigation of the dynamics of incoherent spin noise at ultrafast time scales. Here, we demonstrate a novel technique enabling time-domain detection of sub-THz-magnon noise dynamics in antiferromagnets with unprecedented time resolution. By measuring the correlation of the pulse-to-pulse polarisation fluctuation of femtosecond probe pulses, we successfully resolved the incoherent magnon noise dynamics in the orthoferrite Sm 0.7 Er 0.3 FeO 3 . Due to the critical effects near the spin reorientation transition, the noise dynamics show strong enhancement. Our concept provides a universal ultrafast means to explore noise properties in a variety of correlated spin systems in the future