Spin transport from order to disorder

Schwinger boson mean-field theory (SBMFT) is a nonperturbative approach which treats ordered and disor-dered phases of magnetic systems on equal footing. We leverage its versatility to evaluate the spin correlators which determine thermally induced spin transport the spin Seebeck effect (SSE) in Heisenberg ferromagnets (FMs) and antiferromagnets (AFMs), at arbitrary temperatures. In SBMFT, the spin current Js is made up of particle-hole-like excitations which carry integral spin angular momentum. Well below the ordering temperature, Js is dominated by a magnonic contribution, reproducing the behavior of a dilute-magnon gas. Near the transition temperature, an additional, paramagneticlike contribution becomes significant. In the AFM, the two contributions come with opposite signs, resulting in a signature, rapid inversion of the spin Seebeck coefficient as a function of temperature. Ultimately, at high temperatures, the low-field behavior of the paramagnetic SSE reduces to Curie-Weiss physics. An analysis based on our theory confirms that in recent experiments on gadolinium gallium garnet, the low-field spin Seebeck coefficient S(T) oc chi(T), the spin susceptibility, down to the Curie-Weiss temperature. At lower temperatures in the disordered phase, our theory shows a deviation of S(T) relative to chi(T) in both FMs and AFMs, which increases with decreasing temperature and arises due to a paramagnetic liquid phase in our theory. These results demonstrate that the SSE can be a probe of the short-ranged magnetic correlations in disordered correlated spin systems and spin liquids.