  Apart from being so far the only known binary multiferroic compound, CuO has
a much higher transition temperature into the multiferroic state, 230 K, than
any other known material in which the electric polarization is induced by
spontaneous magnetic order, typically lower than 100 K. Although the
magnetically induced ferroelectricity of CuO is firmly established, no
magnetoelectric effect has been observed so far as direct crosstalk between
bulk magnetization and electric polarization counterparts. Here we demonstrate
that high magnetic fields of about 50 T are able to suppress the helical
modulation of the spins in the multiferroic phase and dramatically affect the
electric polarization. Furthermore, just below the spontaneous transition from
commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213
K, even modest magnetic fields induce a transition into the incommensurate
structure and then suppress it at higher field. Thus, remarkable hidden
magnetoelectric features are uncovered, establishing CuO as prototype
multiferroic with abundance of competitive magnetic interactions.

Materials Science