Probing magnetic order in insulators with small magnetization is particularly challenging in the ultrathin limit, where magnetometry and diffraction lose sensitivity. Here we show that topological spin order with vanishingly small magnetization can be electrically detected through an interfacial topological Hall effect in heavy-metal/magnetic-insulator heterostructures. Using Pt coupled to the canted antiferromagnetic insulator hexagonal $\mathrm{LuFe}{\mathrm{O}}_3$ (h-$\mathrm{LuFe}{\mathrm{O}}_3$) we observe an unusually large and robust Hall response arising from the transfer of real-space spin topology across the interface via magnetic proximity effect (MPE). This interfacial signal enables detection of magnetic order in h-$\mathrm{LuFe}{\mathrm{O}}_3$ with an extremely small net magnetization ($0.025{\mu }_{\mathrm{B}}/\mathrm{Fe}$), corresponding to Hall-conductivity-magnetization ratio $\approx 2{\mathrm{V}}^{-1}$, which is one to two orders of magnitude large than other MPE-induced Hall effects and anomalous Hall effects of conducting magnets. This high sensitivity allows detection of magnetic order in h-$\mathrm{LuFe}{\mathrm{O}}_3$ down to a thickness of only 1.5 unit cells. Our results establish an interfacial Hall-based approach for electrically probing topological spin structures in ultrathin insulators and the related interfaces, enabling access to magnetic order in regimes where conventional probes fail.

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• Spin texture
• Topological Hall effect
• Ferrites
• Interfaces
• Magnetic insulators
• Thin films
• Magnetotransport