Proton NMR study of spin dynamics in the magnetic organic chains M (hfac)3 NITEt (M=Eu3+,Gd3+)

In this work, we present a nuclear magnetic resonance (NMR) study of the spin dynamics in the rare-earth-based low-dimensional molecular magnetic chains Eu(hfac)3NITEt and Gd(hfac)3NITEt (in short, Eu-Et and Gd-Et). Although both samples are based on the same chemical building block, [(hfac)3NITEt], their magnetic properties change dramatically when the Eu3+ ion, which is nonmagnetic at low temperatures, is substituted by the magnetic Gd3+ ion. The present proton NMR investigation shows that, down to the lowest investigated temperature (T=1.5 K for Gd-Et and T=3 K for Eu-Et), the Eu-Et chain behaves as a one-dimensional Heisenberg model with antiferromagnetic exchange coupling (J=-20 K) between s=1/2 organic radicals, and has a T-independent exchange frequency (ωe=2.6×1012 rad/s). In the Gd-Et chain, in contrast, a competition arises between nearest-neighbor ferromagnetic coupling and next-nearest-neighbor antiferromagnetic coupling; moreover, two phase transitions have previously been found, in agreement with Villain's conjecture: a first transition, at T0=2.2 K, from a high temperature paramagnetic phase to a chiral spin liquid phase, and a second transition, at TN=1.9 K, to a three-dimensional helical spin solid phase. Contrary to the Eu-Et chain (whose three-dimensional ordering temperature is estimated to insurge at very low, TN≈0.3 K), critical spin dynamics effects have been measured in the Gd-Et chain on approaching TN=1.9 K: namely, a divergence of the proton nuclear spin-lattice relaxation rate 1/T1, which in turn produces a sudden wipe-out of the NMR signal in a very narrow (ΔT∼0.04 K) temperature range above TN. Below TN, an inhomogeneous broadening of the NMR line indicates a complete spin freezing. At T0=2.2 K, instead, such critical effects are not observed because NMR measurements probe the two-spin correlation function, while the chiral spin liquid phase transition is associated with a divergence of the four-spin correlation function.