Tetranuclear lanthanide complexes display significant slow magnetic relaxation with an open hysteresis loop and the magnetocaloric effect

Abstract

Two tetranuclear lanthanide complexes with the formulas [Dy₄(HL)₄(OAc)₂(H₂O)₂]·2Et₃NH·2CH₃CN (1) and [Gd₄(HL)₄(OAc)₂(H₂O)₂]·2Et₃NH·2CH₃OH·4CH₃CN (2) (H₄L = N,N′-bis(3-hydroxylsalicylidene)benzene-1,2-diamine) were structurally and magnetically characterized. Dynamic magnetic studies revealed that complex 1 exhibits two distinct slow magnetic relaxation processes under a zero direct current (dc) field and the extracted energy barriers (U_eff) of the low-temperature and high-temperature relaxation processes reach 207(2) K and 353(3) K, respectively, which are among the highest U_eff values of reported tetranuclear dysprosium molecular nanomagnets. More strikingly, as for complex 1, the open hysteresis loop behavior is detected until 5 K (6.5 K for the diluted sample), and a coercive field of 453 Oe is observed at 1.8 K, which is unusually seen in tetranuclear lanthanide single-molecule magnets. Further magnetic studies of diluted samples combined with ab initio calculation indicate that the Dy^III–Dy^III interactions play a crucial role in suppressing quantum tunneling of magnetization and consequently resulted in the bulk magnet-like hysteresis behavior. Additionally, the Gd-congener 2 displays the magnetocaloric effect (MCE) with a magnetic entropy change (−ΔS_m) of 20.8 J kg⁻¹ K⁻¹ at 2 K for ΔH = 7 T.