Glassiness and canted antiferromagnetism in three geometrically frustrated triangular 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 from the TQHAF predictions suggest a canted antiferromagnetic type of ordering, 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 irreversibility in the field-cooled/zero-field-cooled magnetization reveal spin-glass-like behavior. Glassy behavior also is suggested by the specific heat data, which show only a weak broad feature at the transition. We propose that, instead of choosing between the resonant valence bond or noncollinear Neel ground states expected for the ideal TQHAF, these systems undergo, due to the additional Dzyaloshinskii-Moriya interaction, a finite temperature phase transition to a state with both Ising-like canted antiferromagnetic and glassy characteristics. The interplay of Heisenberg exchange, causing frustration, and Dzyaloshinskii-Moriya interaction, determining spin canting, leads to an unusual state in which order and disorder appear to coexist.