Magnetic blocking from exchange interactions: Slow relaxation of the magnetization and hysteresis loop observed in a dysprosium-nitronyl nitroxide chain compound with an antiferromagnetic ground state

摘要

The combination of the anisotropic Dy~(III) ion and organic radicals as spin carriers results in discrete and one-dimensional lanthanide-radical magnetic materials, namely, [Dy(hfac)_3(NITThienPh) _2] (1) and [Dy_2(hfac)_6(NITThienPh) _2]_n (2; hfac=hexafluoroacetylacetonate, NITThienPh=2-(5-phenyl-2-thienyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3- oxide). Linking monomeric 1 with the DyIII ion leads to the formation of polymeric 2, and the transformation between them is chemically controllable and reversible. The characterization of both static and dynamic magnetic properties shows that the dominant intrachain exchange interaction is important to observe magnetic bistability in 2 rather than that in 1. Monomeric 1 exhibits paramagnetic behavior, whereas polymeric 2 shows the unusual coexistence of superparamagnetic and two-step field-induced metamagnetic behaviors. The antiferromagnetic ground state of 2 does not prevent the dynamic relaxation of the magnetization with the finite-sized effect in the lanthanide-radical system. Energy barriers to thermally activated relaxation for 2 are 53 and 98K in the low- and high-temperature regimes, respectively. A hysteresis loop is observed with the coercive field of 99Oe at 2K. Mag-neato! Paramagnetic mononuclear dysprosium-nitronyl nitroxide 1 was successfully assembled to form superparamagnetic one-dimensional dysprosium-nitronyl nitroxide 2. Compound 2 shows the unusual coexistence of superparamagnetic and two-step field-induced metamagnetic behaviors with an antiferromagnetic ground state (see figure)