Honeycomb-lattice antiferromagnets have attracted wide attention for the exploration of exotic heat transport and their interplay with magnetic excitations. In this work, we have revealed a contrasting behavior in the magnetothermal conductivity (MTC) between two Co-based honeycomb-lattice magnets ${\mathrm{Co}}_4{\mathcal{M}}_2{\mathrm{O}}_9$ ($\mathcal{M}=\mathrm{Nb},\mathrm{Ta}$), despite their identical lattice structures and quite similar magnetism. ${\mathrm{Co}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$ exhibits enhanced MTC of about $550\%$ at $9\mathrm{T}$ of an in-plane magnetic field, comparable to other honeycomb magnets, while MTC for ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ reaches only $\sim 30\%$. This marked difference is ascribed to distinct features in the field-induced evolution of magnetic excitations that resonantly scatter phonons. This finding sheds light on the implicit impacts of nonmagnetic ions on thermal transport and hints at the potential for broad heat-transport tunability while preserving magnetism and lattice structures.

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• Heat transfer
• Lattice thermal conductivity
• Magnetism
• Phonons
• Thermal expansion
• Antiferromagnets
• Cobaltates
• Honeycomb lattice
• Magnetic insulators
• Single crystal materials