Pressure effect on multiferroic CuBr2

The quasi-1D spin chain compound CuBr2 has been found to be multiferroic below TN (73.5K) under ambient pressure, in which the spontaneous electric polarization is induced by emerging spin spiral ordering propagating along b-axis. Herein we studied the hydrostatic pressure effect on the magnetic, dielectric and structural properties of CuBr2. The multiferroic transition temperature is greatly enhanced under hydrostatic pressure. From ambient to about 1 GPa (the limit of our homemade apparatus), TN increases unprecedentedly by more than 20K, and no sign of saturation is observed in our experiments. Meanwhile the corresponding dielectric loss keeps rather low (<<0.1). Further synchrotron-based high pressure X-ray diffraction measurements reveal that there is no pressure-induced structural phase transition in CuBr2 up to 10.2 GPa. Upon increasing pressure, the b-axis (along the spin chain) just changes slowly while transverse a- and c- axis parameters shrink much more greatly. Pressure greatly reduces the separation between spin chains and enhance the inter-chain coupling interactions in CuBr2, which results in the giant increase of multiferroic critical temperate. Our finding suggests a new effective way to improve the known multiferroic systems towards practical high temperature multiferroicity.