A quantitative determination of magnetic fluctuations in CuGeO3

Abstract

A polarised neutron scattering investigation has been carried out on a powder sample of CuGeO3 within the temperature range of 1.5 K to 600 K. The magnetic scattering has been separated from all other contributions by using polarised neutrons and polarisation analysis and placed onto an absolute scale. At low temperatures the long wavelength components of the paramagnetic response are suppressed consistent with the formation of Cu dimers in which the magnetic moments are correlated antiferromagnetically. This form of the scattering persists to temperatures well above the dimerisation temperature T sp ∼ 14 K. However as the temperature is raised the intensity of the long wavelength spin fluctuations increases and above ∼150 K they are the dominant feature in the wave vector dependence of the response. At all temperatures the observed scattering extrapolates smoothly to the Q = 0 value given by the uniform susceptibility. Consequently the thermal variation of the uniform susceptibility arises from the evolution of the long wavelength magnetic fluctuations. At large wave vectors the energy dependence of the scattering revealed that the response occurs below 16 meV in agreement with the reported maximum magnetic excitation energy at the zone boundary in the ground state. However the total magnetic scattering is significantly less than that expected for a local moment system suggesting that the spectrum of thermal and quantum fluctuations overlap.