$\mathrm{RbMn}{\mathrm{F}}_3$ is an insulating antiferromagnet with a perovskite crystal structure and Néel temperature $T_N=83\mathrm{K}$. Having magnetic ions with a S electronic ground state in a cubic arrangement, its magnetic anisotropy vanishes so that it is a nearly perfect three-dimensional Heisenberg antiferromagnet. Here, we report measurements of the spin Seebeck effect (SSE) in bulk samples of $\mathrm{RbMn}{\mathrm{F}}_3$ with the shape of a slab onto which a thin Pt film strip is deposited. By applying a thermal gradient perpendicular to the slab plane, we detect an electric voltage along the Pt strip due to the charge current resulting from the conversion of the thermally generated spin current by means of the inverse spin Hall effect. By varying the magnetic field applied on the plane of the slab, we observe the antisymmetric step variation typical of the SSE both in the spin-flop and paramagnetic phases. Surprisingly the SSE increases with temperature in the paramagnetic phase and becomes comparable to the one in the ordered phase. This enhancement is attributed to a combination of the free spin fluctuations of paramagnetic insulators with the thermal conduction of long-lived paramagnons made possible by the short-range order that persists above $T_N$.

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• Magnetism
• Spin Seebeck effect
• Spintronics
• Antiferromagnets
• Heisenberg model