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.
@article{arxiv.1101.4284,
title = {Unconventional Multiferroicity in Cupric Oxide},
author = {Pierre Toledano and Naemi Leo and Dmitry D. Khalyavin and Laurent C. Chapon and Tim Hoffmann and Dennis Meier and Manfred Fiebig},
journal= {arXiv preprint arXiv:1101.4284},
year = {2011}
}