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

Abstract

Newly synthesised {Cr ^III ₂Dy ^III ₂} single molecule magnets reveal that the size of the anisotropy barrier is related to the strength of the magnetic exchange interaction in the complex.

Abstract

A new series of heterometallic 3d–4f single molecule magnets (SMMs) of general formula [Cr ^III ₂Dy ^III ₂(OMe) ₂(RN{(CH ₂) ₂OH} ₂) ₂(acac) ₄(NO ₃) 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 ^III ₂Dy ^III ₂} 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 ^III ₂Dy ^III ₂(OMe) ₂(O ₂CPh) ₄(mdea) ₂(NO ₃) ₂] (mdeaH ₂ = 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.