Spin transport measurements on a Kitaev quantum spin liquid candidate BaCo₂(AsO₄)₂

We perform spin transport measurements on BaCo₂(AsO₄)₂, including local and nonlocal spin Seebeck effects and Pt wire spin Hall magnetoresistance (sensitive to magnetization of the substrate), using Pt wire thin films lithographically deposited on the surface of bulk BCAO crystals polished to nanometer flatness. We find a strong enhancement to the local spin Seebeck signal at the "critical region" of the B-T phase diagram below 5 K, near 0.6 T, with 3x enhancement if the field is applied in the a-b plane. This strong feature also weakens as temperature is lowered below 3 K, suggesting the gapping out of spin carrying states, likely a magnon mode. The sign of the local spin Seebeck signal changes from the critical region to the high field, fully spin polarized region, implying that the modes exchange-coupled to the Pt in the two regimes are quite different. We comment on the implications of these results for possible spin liquidity in the critical region.