High relaxation barrier in neodymium furoate-based field-induced SMMs.

Two new neodymium molecular magnets of formula {[Nd(alpha-fur)(3)(H2O)(2)]center dot DMF} n (1) and {[Nd0.065La0.935(alpha-fur)(3)(H2O)(2)]} n (2), alpha-fur = C4H3OCOO, have been synthesized. In (1) the furoate ligands, in bidentate bridging mode, consolidate zig-zag chains running along the a-direction. Compound (2) is a magnetically diluted complex of a polymeric chain along the b-axis. Heat capacity, dc magnetization and ac susceptibility measurements have been performed from 1.8 K up to room temperature. Ab initio calculations yielded the gyromagnetic factors g(x)* = 0.52, g(y)* = 1.03, g(z)* = 4.41 for (1) and g(x)* = 1.35, g(y)* = 1.98, g(z)* = 3.88 for (2), and predicted energy gaps of Delta/k(B) = 125.5 K (1) and Delta/k(B) = 58.8 K (2). Heat capacity and magnetometry measurements agree with these predictions, and confirm the non-negligible transversal anisotropy of the Kramers doublet ground state. A weak intrachain antiferromagnetic inter-action J'/k(B) = -3.15 x 10(-3) K was found for (1). No slow relaxation is observed at H = 0, attributed to the sizable transverse anisotropy component, and/or dipolar or exchange interactions enhancing the quantum tunnelling probability. Under an external applied field as small as 80 Oe, two slow relaxation processes appear: above 3 K the first relaxation mechanism is associated to a combination of Orbach process, with a sizeable activation energy U/k(B) = 121 K at 1.2 kOe for (1), Raman and direct processes; the second, slowest relaxation mechanism is associated to a direct process, affected by phonon-bottleneck effect. For complex (2) a smaller U/k(B) = 61 K at 1.2 kOe is found, together with larger g*-transversal terms, and the low-frequency process is quenched. The reported complexes represent rare polymeric Nd single-ion magnets exhibiting high activation energies among the scarce Nd(III) family.