  In superconductors spin-split by an exchange field, thermal effects are
coupled to spin transport. We show how an oscillating electromagnetic field in
such systems creates spin imbalance, that can be detected with a spin-polarized
probe. The sign and magnitude of the probe signal result from a competition
between processes converting field induced spin energy imbalance to spin
imbalance, dominant at low frequencies, and microwave-driven pair breaking at
high frequencies. In the presence of spin-flip scattering, we show that ac
excitation also leads to multistabilities in the superconducting state.
