  The double-exchange (DE) interaction, that is, a ferromagnetic (FM)
interaction due to a combination of electron motion and the Hund coupling, is a
well known source of a wide class of FM orders. Here, we show that the DE
interaction in highly photoexcited states is antiferromagnetic (AFM). Transient
dynamics of quantum electrons coupled with classical spins are analyzed. An ac
field applied to a metallic FM state results in an almost perfect N\'eel state.
A time characterizing the FM-to-AFM conversion is scaled by light amplitude and
frequency. This hidden AFM interaction is attributable to the electron-spin
coupling under nonequilibrium electron distribution.
