Coexistence of a spin-singlet ground state with the 3D magnetic order in the CsC60 polymerized fulleride

The low temperature behavior of the CsC60 polymer was investigated by NMR m easurements on C-13 and Cs-133 nuclei at ambient and under hydrostatic pres sure up to 9 kbar. The existence of a second-order structural phase transit ion to a spin-singlet (nonmagnetic) ground state at T-s = 13.8 K is clearly established at ambient pressure. This state coexists with the magnetic ord er that develops at T-N = 30 K. The application of pressure first suppresse s the magnetic order and a homogeneous nonmagnetic ground state is stabiliz ed at 5 kbar.