The garnet compound ${\mathrm{Yb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ is a fascinating material that is considered highly suitable for low-temperature refrigeration, via the magnetocaloric effect, in addition to enabling the exploration of quantum states with long-range dipolar interactions. It has previously been theorized that the magnetocaloric effect can be enhanced in ${\mathrm{Yb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ via magnetic soft mode excitations, which in the hyperkagome structure would be derived from an emergent magnetic structure formed from nanosized ten-spin loops. We study the magnetic field dependence of bands of magnetic soft mode excitations in the effective spin $S=1/2$ hyperkagome compound ${\mathrm{Yb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ using single-crystal inelastic neutron scattering. We probe the magnetically short-ranged ordered state, in which we determine magnetic nanoscale structures coexisting with a fluctuating state, and the magnetically saturated state, from which we determine the relevant magnetic interactions. We determine that ${\mathrm{Yb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ can be described as a quantum dipolar magnet with perturbative weak near-neighbor and interhyperkagome exchange interaction. The magnetic excitations, under the application of a magnetic field, reveal highly robust soft modes with distinctive signatures of the quantum nature of the ${\mathrm{Yb}}^{3+}$ spins. Our results enhance our understanding of soft modes in topological frustrated magnets that drive both the unusual physics of quantum dipolar systems and future refrigerant material design.

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• Frustrated magnetism
• Magnetism
• Quantum spin models
• Inelastic neutron scattering
• Linear spin wave theory