Unconventional Multiferroicity in Cupric Oxide
  The magnetic phase transitions reported below 230 K in cupric oxide are analyzed theoretically at the macroscopic and microscopic levels. The incommensurate multiferroic and lock-in commensurate phases are shown to realize an inverted sequence of symmetry-breaking mechanisms with respect to the usual sequence occurring in low temperature multiferroic compounds. The higher temperature spin-spiral phase results from coupled order-parameters which decouple at the lock-in transition to the commensurate ground state phase. Expressing the order-parameters in function of the magnetic spins allows determining the symmetries and magnetic structures of the equilibrium phases and the microscopic interactions which give rise to the polarization. 
Pierre Toledano, Naemi Leo, Dmitry D. Khalyavin, Laurent C. Chapon,   Tim Hoffmann, Dennis Meier, Manfred Fiebig
cond-mat.mtrl-sci cond-mat.str-el