In two-dimensional (2D) spin systems, the augmentation of spin fluctuations gives rise to quasi-long-range order; however, how they manifest in spin transport remains unclear. Here, we investigate the spin Seebeck effect (SSE) in a quasi-2D antiferromagnet, ${\mathrm{BaNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$, which has been reported to exhibit the Berezinskii-Kosterlitz-Thouless (BKT) transition owing to its very weak interlayer interaction. We found that the SSE in $\mathrm{Pt}/{\mathrm{BaNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ persists well above the Néel temperature, significantly different from the behavior of 3D-ordered magnets. Our numerical analysis for a 2D microscopic spin model confirms that the observed SSE is linked to the gapless magnon excitations and strong magnetic correlations characteristic of BKT magnets.

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• BKT transition
• Spin Seebeck effect
• Spin caloritronics
• Spin dynamics
• Spintronics
• Antiferromagnets