Modulation of slow magnetic relaxation by tuning magnetic exchange in {Cr2Dy2} single molecule magnets

Abstract

A new series of heterometallic 3d–4f single molecule magnets (SMMs) of general formula [Cr^III₂Dy^III₂(OMe)₂(RN{(CH₂)₂OH}₂)₂(acac)₄(NO₃)₂] (R = Me, Et, nBu) is reported, displaying slow relaxation of the magnetization and magnetic hysteresis with non-zero coercive fields. Dynamic magnetic susceptibility experiments show that the three complexes possess anisotropy barriers of 34, 37 and 41 K (24, 29 and 26 cm⁻¹); of similar magnitude to their {Co^III₂Dy^III₂} counterparts. The replacement of the diamagnetic Co^III for paramagnetic Cr^III ions results in significantly longer relaxation times, as observed via M(H) hysteresis at low temperatures, absent for the Co^III complexes. The present complexes are also compared to those of a similar Cr^III–Dy^III complex of formula [Cr^III₂Dy^III₂(OMe)₂(O₂CPh)₄(mdea)₂(NO₃)₂] (mdeaH₂ = N-methyldiethanolamine), which displays SMM behaviour with a larger anisotropy barrier of 77 K (∼54 cm⁻¹) and even longer relaxation times. We show that the long relaxation times compared to the Co^III analogues are due to the significant magnetic exchange interactions between the Cr^III and Dy^III ions, resulting in the suppression of quantum tunnelling of the magnetization (QTM) and leading to a multilevel relaxation barrier. The height of the relaxation barrier in these Cr^III systems is then shown to be directly related to the strength of the exchange interactions between the Cr^III and Dy^III ions, showing a clear route towards enhancing the slow magnetic relaxation of coupled Cr^III–Dy^III systems.