by spin Seebeck effect*

Geometrically frustrated magnets have been predicted to host many novel states such as quantum spin liquids and spin ices. A characteristic of these states is the lack of long-range magnetic order, which makes it very difficult to detect them using standard experimental methods. Here, we demonstrate that an application of thermal gradient to the geometrically frustrated spin-liquid Gd₃Ga₅O₁₂ (GGG) gives rise to a pure spin current, a phenomenon known as spin Seebeck effect (SSE). At low temperatures (2 – 5 K), we observe that the spin current is modulated by a field-induced antiferromagnetic ordering in GGG. These magnetic orderings are short range in character and show distinct anisotropies, also confirmed by our model calculations of the spin configuration. Notably, these structures are not observed in neutron scattering measurements at T > 0.9 K. Additionally, previously unexplored spin excitations in the high field regime are revealed in our SSE measurement. Our findings suggest that SSE is a new and sensitive probe for emergent states in geometrically frustrated magnets.