Unconventional Multiferroicity in Cupric Oxide

TL;DR

This paper provides a theoretical analysis of the magnetic phase transitions and multiferroic behavior in cupric oxide, revealing an unconventional sequence of symmetry-breaking mechanisms and microscopic interactions responsible for polarization.

Contribution

It introduces a new theoretical framework for understanding the multiferroic phases in cupric oxide, highlighting an inverted symmetry-breaking sequence and detailed microscopic interactions.

Findings

Incommensurate and lock-in phases exhibit an inverted symmetry-breaking sequence.

Coupled order-parameters decouple at the lock-in transition.

Microscopic interactions determine the polarization in phases.

Abstract

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.