Low-temperature thermodynamic properties of molecular magnetic chains

We studied the low-temperature specific heat $C(T)$ and the magnetic susceptibility χ(T) of Gd(hfac)₃NITiPr, a quasi-one-dimensional (1D) molecular magnetic chain with competing nearest-neighbor (nn) and next-nearest-neighbor (nnn) intrachain exchange interactions. The anomaly observed in $C(T)$ was reproducibly found at T₀=2.09 K, although the $C(T)$ curves measured on different samples showed clear evidence for aging effects. For fresh, high-quality samples, the λ shape of the anomaly and the negligible latent heat (less than 7×10^-3 J/mol) strongly suggest the presence of a phase transition of order higher than the first. In the best sample at temperatures well below T₀ (0.2--1 K), a linear C-vs-T dependence was found, which we interpreted in the framework of 1D quantum free spin-wave theory. Strong aging effects were also found in the magnetic susceptibility χ(T) which, for the best and freshest sample, exhibits a very small anomaly close to T₀ (at T_χ=2.1 K), superimposed to a diverging paramagnetic behavior at lower temperatures. The anomaly at T₀(~T_χ) could be due to a transition to three-dimensional (3D) helical long-range order, but this is inconsistent with the observed linear C-vs-T behavior, typical of a 1D magnet with strong intrachain antiferromagnetic interactions. An alternative interpretation, capable of explaining the $C(T)$ data both in the low-temperature and in the critical region, calls for the onset, below T₀, of nonconventional chiral long-range order due to interchain interactions. The chiral phase can be described as a collection of parallel corkscrews, all turning clockwise (or all counterclockwise), whereas their phases are random. An estimation of T₀ according to this model and using the exchange constants evaluated from the linear C-vs-T slope supports this interpretation. The chiral order is compatible with the breaking of the chain into finite segments, due to aging effects, while the phase coherence necessary for the onset of 3D helical long-range order is destroyed. In this framework, a qualitative account for the very small anomaly observed in the magnetic susceptibility χ(T) can also be given.