Extended Data Fig. 1: Correlation measurement procedure.

The input microwave is generated by combining two channels (Ch.1 and Ch.2) into the pump channel (Ch.3) at the pump frequency \({f}_{{\rm{p}}}\) using a multichannel signal generator (Holtzworth 8990B). Ch.1 is used to finely tune the input microwave power with small increments, while Ch.2 is used for harmonic locking at frequency \(2{f}_{2}\). The signals from Ch.2 and Ch.3 are amplified ( + 35 dB) and combined with the output from Ch.1. The resulting microwave signal is delivered to the sample via the main coplanar waveguide (CPW; orange antenna). The AC signal generated from the YIG/Pt disc (orange circle) is detected by a secondary CPW (blue antenna) and routed, via an RF switch, to either the lock-in amplifier circuit or a spectrum analyser (SA). Before entering the lock-in amplifier, the signal is filtered (providing −80 dB attenuation at \(2f\)) to suppress crosstalk, amplified ( + 35 dB), and mixed with a local oscillator signal from Ch.4 of the signal generator. The mixed signal is then fed into a lock-in amplifier (Zurich UHFLI). FMR absorption spectra and spectra during the non-degenerate process are measured by routing the microwave signal to a vector network analyser (VNA) or spectrum analyser (SA) using an additional RF switch.