from first principles

Type-II multiferroics, in which ferroelectric polarization is induced by inversion-nonsymmetric magnetic order, promise new and efficient applications based on the mutual control of magnetic and electronic properties. Recent experiments show a dramatic increase of the multiferroic T_c in CuBr₂ from 73.5K to 162K under a pressure of 4.5GPa, with no evidence that the effect saturates. We provide a quantitative explanation by two-step density-functional-theory calculations that include correlations by GGA+U. First we fix the lattice parameters from XRD experiments performed up to 15GPa and optimize the full atomic structure at each pressure using VASP. Then we estimate 9 key magnetic interactions in each structure by a total-energy mapping analysis within the FPLO framework. Our results show how rising pressure increases the interactions between CuBr₂ chains by a factor of 4 up to 4.5GPa, strongly reinforcing their incommensurate spiral magnetism, whose ordering temperature sets T_c. Our results create a microscopic basis for the possibility of engineering room-temperature multiferroicity in suitably strained CuBr₂.
This work was performed in collaboration with J S Zhang, Y Xie, X Q Liu, C Wang, J P Sun, Y Cui, J C Wang, X Ren, H Deng, X Yin, Y Ding, Y Li, J G Cheng, J Feng, and W Yu.