Giant pressure-enhancement of multiferroicity in CuBr2

Abstract

Type-II multiferroic materials, in which ferroelectric polarization is induced by inversion non-symmetric magnetic order, promise new and highly efficient multifunctional applications based on mutual control of magnetic and electric properties. However, to date this phenomenon is limited to low temperatures. Here we report giant pressure-dependence of the multiferroic critical temperature in CuBr 2 : at 4.5 GPa it is enhanced from 73.5 to 162 K, to our knowledge the highest T C ever reported for non-oxide type-II multiferroics. This growth shows no sign of saturating and the dielectric loss remains small under these high pressures. We establish the structure under pressure and demonstrate a 60% increase in the two-magnon Raman energy scale up to 3.6 GPa. First-principles structural and magnetic energy calculations provide a quantitative explanation in terms of dramatically pressure-enhanced interactions between CuBr 2 chains. These large, pressure-tuned magnetic interactions motivate structural control in cuprous halides as a route to applied high-temperature multiferroicity.The search for application-suitable multiferroics [1-3] has advanced significantly over the last decade in both type-I and type-II materials [4][5][6][7][8][9]. Type-I multiferroics [10] have independent magnetic and ferroelectric transitions [11,12], meaning that even when both transition temperatures are high, the magnetoelectric coupling, and hence the scope for mutual control, is usually weak. The physics of most type-II multiferroics [10,[13][14][15] involves frustrating magnetic interactions that give rise to a spiral magnetic order [16], which immediately generates a ferroelectric polarization by the inverse Dzyaloshinskii-Moriya mechanism [17][18][19][20][21]. However, an intrinsic drawback of magnetic frustration is that it suppresses the onset of long-range order, and hence most currently available type-II multiferroics operate only at low tempera-tures [14].A generic route to higher operating temperatures in type-II multiferroics is to increase the strength of the magnetic interactions. This can, in principle, be achieved through structural alterations, for which perhaps the cleanest method is an applied pressure [22][23][24][25][26]. Pressure, broadly construed to include chemical pressure and substrate pressure, acts to increase electronic hybridization without introducing disorder. In the most minimal model for a magnetic insulator, the antiferromagnetic (AF) exchange interaction is given by J = 4t 2 /U , where t is the orbital hybridization and U the on-site Coulomb repulsion. However, excessive t risks driving the system metallic, thus losing its magnetic and ferroelectric properties. The most scope for achieving large J values is offered by large initial values of both t and U , making the spin-1/2 Cu 2+ ion particularly promising in view of its often strong on-site correlations and significant orbital hybridization with ligands. It is not a coincidence that complex copper oxides become high-temperature superconductors after ...