Magneto-thermal and magnetization relaxation dynamics of a family of di-nuclear lanthanide complexes

Abstract

The reaction of Ln(NO3)3·nH2O (where Ln = Gd, Tb, Dy or Yb) in the presence of sodium acetate led us to isolate a family of structurally analogous dinuclear lanthanide complexes with the general molecular formula of [Ln2(CH3COO)4(NO3)2(H2O)4]. 2H2O (where Ln = Gd (1) or Tb (2) or Dy (3) or Yb (4)), which are characterized by single-crystal X-ray diffraction. The dc magnetic susceptibility measurements were performed on 1 reveal the presence of a weak ferromagnetic exchange between the Gd(III) center (J = +0.01 cm−1; −2JS1.S2 Hamiltonian). This weak exchange interaction in 1 is presumably overcome by the external magnetic field, and therefore, the X-band EPR spectral features of 1 were simulated by considering the single-ion spin Hamiltonian parameter (g = 1.98, D = 0.03 cm−1, and ∣E/D∣= 0.004). In contrast to 1, complexes 2–4 exhibit a weak antiferromagnetic exchange between the Ln(III) centers. The magnetization relaxation dynamics studies performed on the anisotropic complexes (2–4) show slow relaxation of magnetization in the presence of an optimum external magnetic field. The spin–lattice relaxation predominantly follows the Direct and Raman mechanism exclusively, and the Orbach process was found to be non-existent. By employing ab initio calculations, the electronic structure and the mechanism of magnetization relaxation dynamics of 2–4 have been investigated. The magnetocaloric effect parameter for 1 shows a change in magnetic entropy (−ΔSm) value of 39 J Kg−1 K−1 at 2.0 K (ΔH = 70 kOe), which is one of the largest −ΔSm value registered among the various discrete molecular coolants reported in the literature..