Phase diagram and spin Hamiltonian of weakly-coupled anisotropic S= 1 2 chains in Cu Cl2 2 ((C D3) 2 SO)

Field-dependent specific heat and neutron scattering measurements were used to explore the antiferromagnetic S= 1 2 chain compound Cu Cl2 2 ((C D3) 2 SO). At zero field the system acquires magnetic long-range order below TN =0.93 K with an ordered moment of 0.44 μB. An external field along the b axis strengthens the zero-field magnetic order, while fields along the a and c axes lead to a collapse of the exchange stabilized order at μ0 Hc =6 T and μ0 Hc =4 T (extrapolated to zero temperature) and the formation of an energy gap in the excitation spectrum. We relate the field-induced gap to the presence of a staggered g -tensor and Dzyaloshinskii-Moriya interactions, which lead to effective staggered fields for magnetic fields applied along the a and c axes. Competition between anisotropy, interchain interactions, and staggered fields leads to a succession of three phases as a function of field applied along the c axis. For fields greater than μ0 Hc, we find a magnetic structure that reflects the symmetry of the staggered fields. The critical exponent, β, of the temperature driven phase transitions are indistinguishable from those of the three-dimensional Heisenberg magnet, while measurements for transitions driven by quantum fluctuations produce larger values of β.