  By means of magnetic, specific heat and pyroelectric measurements, we report
on magnetic ferroelectricity in the quadruple perovskite \namno, characterized
by a canted antiferromagnetic (AFM) CE structure. Surprisingly,
ferroelectricity is concomitant to a dramatic broadening of the magnetic
hysteresis loop, well below the AFM ordering temperature. This unconventional
behavior shows that the formation of ferroelectric domains is induced by the
symmetric exchange interaction in the local scale, e.g. at magnetic domain
boundaries or defects. The value of electric polarization, $P = 0.027 \mu$C
cm$^{-2}$, measured in polycrystalline samples is comparatively large as
compared to other magnetic multiferroics, suggesting that the above scenario is
promising indeed for the rational design of practical multiferroic materials.
