Slow relaxation in a {Tb2Ba(α-fur)8}n polymer with Ln = Tb(iii) non-Kramers ions

Abstract

We report the synthesis, crystal structure and magnetic properties of a new heteronuclear polymeric complex based on non-Kramers Tb ions and carboxylic α-fur = C₄H₃OCOO ligands: {[Tb₂Ba(α-fur)₈(H₂O)₄]·2H₂O}_n. The α-furoate ligands consolidate 1D zig-zag chains running along the c-axis, formed by Tb₂ dimers separated by Ba ions. Ab initio calculations, in combination with the fit of experimental data, predict that the single-ion magnetic ground state is highly anisotropic () and consists of a quasi-doublet with a Δ_Tb/k_B = 3.22 K gap, well separated from the next excited state, while the gap for the Tb₂ dimer is Δ_2Tb/k_B = 2.58 K. Static magnetization and heat capacity measurements show that, magnetically, the system can be modeled as dimers of non-Kramers Tb ions, coupled by an antiferromagnetic intradimer interaction J′*/k_B = −1.6 K. Dipolar interactions couple the Tb ions in the dimer with their first neighbour ions along the chain, with J′′*/k_B = −0.15 K, and with the surrounding ions out of the chain, with maximum J′′′*/k_B = −0.03 K. Ac susceptibility measurements in H = 0 performed down to 50 mK temperatures have enabled us to observe slow relaxation of magnetization, with an Orbach-like activation energy of U/k_B = 1.1 K. It is assigned to the sluggish response of the 3D spin system due to a short-range ordering, possibly enhanced by the presence of disorder caused by defects in the polymeric chains. Under the application of a magnetic field, the system slowly relaxes by two distinct direct processes, strongly affected by a phonon bottleneck effect. We discuss the different relaxational phenomenology of the new complex in comparison with that of the isostructural {[Dy₂Ba(α-fur)₈(H₂O)₄]·2H₂O}_n, differing only in the Kramers nature of the ions, and the mononuclear {Ln(α-fur)₃(H₂O)₃}_n (Ln = Tb, Dy) complexes, previously reported.