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An applied magnetic field is known to produce novel effects in the phase behavior of magnetoelastic spin-Peierls systems. Hence we report measurements of the differential susceptibility (χ) and magnetization (M) in fields up to 40 kOe (4 T) on the spin-Peierls compound tetrathiafulvalene (TTF)-AuS4C4(CF3)4 in the temperature region (1.1K <- T <- 4.2K). This range of field and temperature encompasses an interesting phase region, including the zero-field spin-Peierls transition temperature Tc(0)=2.03 K. The measurements of the differential (ac) susceptibility provide a more sensitive probe of the transition behavior than magnetization measurements. The first definitive evidence for significant deviations from mean-field critical behavior appear in these measurements, and the appropriate criteria for determining the precise location of the transitions are thus provided by the thermodynamic theory of λ transitions. Using the new criteria, qualitative and even quantitative agreement is obtained with current theories of the field dependence of spin-Peierls transitions. A novel contour plot of χac in the HT plane is shown to be useful for the delineation of the global phase-transition behavior. An investigation of the role of relaxation effects in χac relative to the nature of the phase boundaries is conducted. A major feature is the observation of a striking degree of "universality" in the phase behavior of three spin-Peierls systems TTF-AuS4C4(CF3)4, TTF-AuS4C4(CF3)4, and methylethylmorpholinium di-tetracyanoquinodimethane [MEM-(TCNQ)2]. hese universal features are preserved through considerable differences in lattice structure and a variation in Tc(0) of a factor of 10.

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©1982 The American Physical Society