Pressure-induced magnetic phase transition and structural change in the quantum spin liquid system Cu2(C5H12N2)2Br4

The effect of pressure on the quantum spin liquid system Cu-2(C5H12N2)(2)Br-4, which is isostructural with Cu-2(C5H12N2)(2)Cl-4 and has a spin gap Delta/k(B) = 21.3 K, was investigated through magnetic measurements and crystal structural analyses. The susceptibility and magnetization begin to behave gaplessly in a drastic manner above P = 4 kbar, and the antiferromagnetic phase transition is clearly observed at T = 2.8 K. The spin liquid state survives even above P = 4 kbar and coexists with the antiferromagnetic component. The above magnetic transition might be involved in the pressure-induced structural change, which is observed between P = 3 and 4 kbar. Above P = 4 kbar, the compound enters the commensurate phase with a 10a x b x 2c-type superstructure as the parent structure. These results suggest that the pressure in the frustrated three-dimensional quantum anti ferromagnets gives rise to an anti ferromagnetic phase via the crystal structural change, which coexists with the spin liquid phase in the considered pressure region.