Supplemental Material

The supplemental information includes:

1. Additional details about the experimental setup, sample quality and structure, and the power-temperature relation used in the main-text.

2. Results verifying a phase transition to the zigzag order.

3. Results showing helicity dependent scattering in the co-polarization channel.

4. Detailed description of the analytical procedure used to derive the ROA of M1.

5. Detailed description of the analytical procedure used to analyse M0.

6. Calculation of ROA in the paramagnetic regime.

7. Raman spectra measured from 7.5-30 T.