  Control of physical property in terms of external fields is essential for
contemporary technologies. The conductance can be controlled by a gate electric
field in a field effect transistor, which is a main component of the integrated
circuit. Optical phenomena induced by an electric field such as
electroluminescence and electrochromism are useful for display and other
technologies. Control of microwave propagation seems also imperative for future
wireless communication technology. Microwave properties in solids are dominated
mostly by magnetic excitations, which cannot be easily controlled by an
electric field. One of the solutions for this problem is utilizing magnetically
induced ferroelectrics (multiferroics). Here we show that microwave
nonreciprocity, which is difference between oppositely propagating microwaves,
can be reversed by the external electric field in a multiferroic helimagnet
Ba$_2$Mg$_2$Fe$_{12}$O$_{22}$. This result offers a new avenue for the
electrical control of microwave properties.
