  Magnetoelectric multiferroics in which ferroelectricity and magnetism coexist
have attracted extensive attention because they provide great opportunities for
the mutual control of electric polarization by magnetic fields and
magnetization by electric fields. From a practical point view, the main
challenge in this field is to find proper multiferroic materials with a high
operating temperature and great magnetoelectric sensitivity. Here we report on
the magnetically tunable ferroelectricity and the giant magnetoelectric
sensitivity up to 250 K in a Y-type hexaferrite, BaSrCoZnFe11AlO22. Not only
the magnitude but also the sign of electric polarization can be effectively
controlled by applying low magnetic fields (a few hundreds of Oe) that modifies
the spiral magnetic structures. The magnetically induced ferroelectricity is
stabilized even in zero magnetic field. Decayless reproducible flipping of
electric polarization by oscillating low magnetic fields is shown. The maximum
linear magnetoelectric coefficient reaches a high value of ~ 3.0\times10^3 ps/m
at 200 K.
