Magnetism and ferroelectricity are essential to many areas of technology and the quest for multiferroic materials, where these two phenomena coexist, is of immense industrial and fundamental importance. Magnetically-induced ferroelectrics (i.e., multiferroics) constitute an exciting new paradigm in the design of functional materials by intimately coupling magnetic and polar orders. Apart from being so far the only known binary multiferroic compound, the cupric oxide 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. However, until now no magneto-electric effect has been observed as direct crosstalk between bulk magnetization and electric polarization counterparts, prompting to label CuO as “material with persistent multiferroicity without magneto-electric effects”.
We have demonstrated that sufficiently high magnetic fields of up to 50 tesla are able to suppress the helical modulation of the magnetic moments in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous magnetic transition from commensurate (paraelectric) to incommensurate (ferroelectric) magnetic structures at 213 K, even modest magnetic fields can induce a transition into magnetic structure compatible with ferroelectricity and then suppress it at higher fields, thus causing remarkable polarization changes. The synergic use of number of experimental techniques at large scale European facilities (the Eur. Magnetic Field Laboratory; Laue Langevin Inst.) allowed tracing the magnetoelectric phase diagram of CuO, identifying new phase transitions and magnetic structures.
This study, conceived and coordinated by ICREA researcher Vassil Skumryev, was carried out in collaboration with scientists from Russia, France, Germany, Brazil and Bulgaria. It adds substantial new knowledge, unveiling important new features of this prominent simple oxide with distinct position among the multiferroic materials and of general interest for material scientists and physicists.