Magnets with isotropic easy-plane symmetry host Goldstone modes that can be leveraged for efficient spin transport. Here, we present a time-resolved optical polarimetry technique that allows us to detect and characterize such low-frequency modes, and use it to observe the Goldstone mode in the multi-$\mathit{Q}$ broken helix phase of ${\mathrm{EuIn}}_2{\mathrm{As}}_2$. The strength of our technique comes from the ability to distinguish between nematic and magnetization dynamics in order to yield information about the mode structure, in addition to its frequency. We find that the nearly uniform spin precession characteristic of a Goldstone mode is realized only when a small magnetic field is used to unpin the broken helix from local strain generated during crystal growth. In this regime, the mode frequency scales linearly with the applied field due to the ground state ${\mathrm{C}}_{2z}$ symmetry of the broken helix. Our work shows how optical polarimetry can be used to study the Goldstone modes of complex magnets.

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• Magnetism
• Magneto-optics
• Magnons
• Spin dynamics
• Noncollinear magnets