Anomalous magnetothermal effect in the spin-12 Heisenberg XXZ chain

The low-temperature thermomagnetic power of the spin-12 Heisenberg XXZ chain is exactly calculated by the Bethe ansatz. For finite magnetic fields, the magnetothermal effects arise due to the breakdown of the spin-reversal symmetry. For finite interaction strengths, we find the thermomagnetic coefficient (magnetic Seebeck coefficient) changes sign at certain temperature and magnetic field. This peculiar feature is interpreted as an effect of the competition between the interaction strength and the external magnetic field.