  Magnon transistors that can effectively regulate magnon transport by an
electric field are desired for magnonics which aims to provide a Joule-heating
free alternative to the conventional electronics owing to the electric
neutrality of magnons (the key carriers of spin-angular momenta in the
magnonics). However, also due to their electric neutrality, magnons have no
access to directly interact with an electric field and it is thus difficult to
manipulate magnon transport by voltages straightforwardly. Here, we
demonstrated a gate voltage ($V_{\rm g}$) applied on a nonmagnetic
metal/magnetic insulator (NM/MI) interface that bended the energy band of the
MI and then modulated the possibility for conduction electrons in the NM to
tunnel into the MI can consequently enhance or weaken the spin-magnon
conversion efficiency at the interface. A voltage-controlled magnon transistor
based on the magnon-mediated electric current drag (MECD) effect in a
Pt/Y$_{\rm 3}$Fe$_{\rm 5}$O$_{\rm 12}$ (YIG)/Pt sandwich was then
experimentally realized with $V_{\rm g}$ modulating the magnitude of the MECD
signal. The obtained efficiency (the change ratio between the MECD voltage at
$\pm V_{\rm g}$) reached 10%/(MV/cm) at 300 K. This prototype of magnon
transistor offers an effective scheme to control magnon transport by a gate
voltage.
