Spin pumping revisited: towards a non-linear theory

In this work, we revisit the generation of spin current through a normal metal/antiferromagnet interface driven by magnetization precession. Going beyond the linear response framework, we derive a general relation for nonlinear spin pumping. By employing a gauge transformation approach, we demonstrate that the pumped spin current per mode is fundamentally bounded by 2e/T, where e is the electron charge and T is the period of the driving signal. Furthermore, we analyze the crossover between the linear and nonlinear regimes as a function of the exchange interaction and examine the high-frequency behavior of the system.