Assessing the exchange coupling in binuclear lanthanide(iii) complexes and the slow relaxation of the magnetization in the antiferromagnetically coupled Dy2 derivative

Abstract

We report here the synthesis and the investigation of the magnetic properties of a series of binuclear lanthanide complexes belonging to the metallacrown family. The isostructural complexes have a core structure with the general formula [Ga₄Ln₂(shi^3-)₄(Hshi^2-)₂(H₂shi^-)₂(C₅H₅N)₄(CH₃OH) _x (H₂O) _x ]·xC₅H₅N·xCH₃OH·xH₂O (where H₃shi = salicylhydroxamic acid and Ln = Gd^III1; Tb^III2; Dy^III3; Er^III4; Y^III5; Y^III_0.9Dy^III_0.16). Apart from the Er-containing complex, all complexes exhibit an antiferromagnetic exchange coupling leading to a diamagnetic ground state. Magnetic studies, below 2 K, on a single crystal of 3 using a micro-squid array reveal an opening of the magnetic hysteresis cycle at zero field. The dynamic susceptibility studies of 3 and of the diluted DyY 6 complexes reveal the presence of two relaxation processes for 3 that are due to the excited ferromagnetic state and to the uncoupled Dy^III ions. The antiferromagnetic coupling in 3 was shown to be mainly due to an exchange mechanism, which accounts for about 2/3 of the energy gap between the antiferro- and the ferromagnetic states. The overlap integrals between the Natural Spin Orbitals (NSOs) of the mononuclear fragments, which are related to the magnitude of the antiferromagnetic exchange, are one order of magnitude larger for the Dy₂ than for the Er₂ complex.

Scheme 1. Synthesis of Ln₂Ga₄ complexes.

Fig. 1. X-ray crystal structure of complex 3. (left) Top view. (right) Side view. Color code: teal spheres = Dy^III; salmon spheres = Ga^III; gray = C; red = O; blue = N. Hydrogen atoms and lattice solvents are omitted for clarity.

Fig. 2. Temperature dependence of the χT product for complexes 1, 2, 3 and 6. The solid lines correspond to the best fit (see text).

Fig. 3. Magnetization vs. applied field at 2 K for complexes 1, 2, 3 and 6. The solid lines correspond to the best fit (see text).

Fig. 4. (left) Orientation of the magnetization axis of the ground Kramers doublet M_J = ±15/2 of the Dy^III ion in 6 where one Dy^III has been replaced by a Lu^III ion. (right) Orientation of the two components of the easy plane of magnetization for the ground Kramers doublet of Er^III ion in 5 where one Er^III has been replaced by a Lu^III ion.

Fig. 5. NSOs for the Dy^III complex determined along direction 1 corresponding to the orientation of the magnetization axis. One Dy^III has been replaced by a Lu^III ion. The isosurfaces are weighted by the corresponding occupation that is indicated below each plot.

Fig. 6. (top) M/M_S = cμ₀H) at 0.03, 0.5 and 1 K for dc field sweep rate of 0.035 T s^–1 for complex 3. (bottom) Field-dependent energy diagram showing the different relaxation processes for the binuclear complex 3.

Fig. 7. Temperature-dependence of the out-of-phase (χ′′) ac magnetic susceptibility for 3 under zero applied dc field (top) and 2000 Oe (middle); and (bottom) ln(τ) vs. 1/T_B where T_B Arrhenius plot with data extracted from the frequency-dependent data at zero applied dc field for the low () and the high () temperature processes. The solid lines are the best linear fit.

Fig. 8. (top) Temperature-dependence of the out-of-phase (χ′′) ac magnetic susceptibility for 6 under zero applied dc field (top) and 750 Oe applied dc field (bottom). Insets: ln(τ) vs. 1/T_B where T_B is the temperature of the maxima of the χ′′ curves.

Fig. 9. Temperature-dependence of the out-of-phase (χ′′) ac susceptibility for 3 in zero dc field (), in 2000 Oe dc field () and for 6 in 750 Oe dc field () at 1284 Hz.