Glassiness and canted antiferromagnetism in three geometrically frustratedtriangular quantum Heisenberg antiferromagnets with additional Dzyaloshinskii-Moriya interaction

We report results of extensive magnetic studies of three triangular quantum Heisenberg antiferromagnets (TQHAF's) with weak additional Dzyaloshinskii- Moriya interaction, Cu-2(OH)(3)(CmH2m+1COO), m = 7, 9, and 11. Fits of the de susceptibility data to high-temperature series expansions are consistent with high-temperature TQHAF behavior. At low temperatures the deviations f rom the TQHAF predictions suggest a canted antiferromagnetic type of orderi ng, consistent with the strong peak in the second harmonic of the nonlinear ac susceptibility, which indicates the development of a spontaneous moment . The frequency dependence of the linear ac susceptibility and the irrevers ibility in the field-cooled/zero-field-cooled magnetization reveal spin-gla ss-like behavior. Glassy behavior also is suggested by the specific heat da ta, which show only a weak broad feature at the transition. We propose that , instead of choosing between the resonant valence bond or noncollinear Nee l ground states expected for the ideal TQHAF, these systems undergo, due to the additional Dzyaloshinskii-Moriya interaction, a finite temperature pha se transition to a state with both Ising-like canted antiferromagnetic and glassy characteristics. The interplay of Heisenberg exchange, causing frust ration, and Dzyaloshinskii-Moriya interaction, determining spin canting, le ads to an unusual state in which order and disorder appear to coexist.