Dilution-Induced Slow Magnetic Relaxation and Anomalous Hysteresis in Trigonal Prismatic Dysprosium(III) and Uranium(III) Complexes

Abstract

Magnetically dilute samples of complexes Dy(H(2)BPz(Me2)(2))(3) (1) and U(H(2)BPz(2))(3) (3) were prepared through cocrystallization with diamagnetic Y(H(2)BPz(Me2)(2))(3) (2) and Y(H(2)BPz(2))(3). Alternating current (ac) susceptibility measurements performed on these samples reveal magnetic relaxation behavior drastically different from their concentrated counterparts. For concentrated 1, slow magnetic relaxation is not observed under zero or applied dc fields of several hundred Oersteds. However, a 1:65 (Dy:Y) molar dilution results in a nonzero out-of-phase component to the magnetic susceptibility under zero applied dc field, characteristic of a single-molecule magnet. The highest dilution of 3 (1:90, U:Y) yields a relaxation barrier U(eff) = 16 cm(-1), double that of the concentrated sample. These combined results highlight the impact of intermolecular interactions in mononuclear single-molecule magnets possessing a highly anisotropic metal center. Finally, dilution elucidates the previously observed secondary relaxation process for concentrated 3. This process is slowed down drastically upon a 1:1 molar dilution, leading to butterfly magnetic hysteresis at temperatures as high as 3 K. The disappearance of this process for higher dilutions reveals it to be relaxation dictated by short-range intermolecular interactions, and it stands as the first direct example of an intermolecular relaxation process competing with single-molecule-based slow magnetic relaxation.

Cycloheptatrienyl trianion: an elusive bridge in the search of exchange coupled dinuclear organolanthanide single-molecule magnets

“…However, we cannot rule out the possibility of intermolecular interactions, as application of large static fields has been shown to propagate spin–spin interactions. 33,34 These types of interactions may lead to the formation of magnetic domains, consequently precluding the analysis from a molecular perspective. Due to these phenomena, an effective energy barrier for this process could not be extracted from this data set.…”

Direct measurement of dysprosium(III)˙˙˙dysprosium(III) interactions in a single-molecule magnet

“…In other compounds weak Ln Á Á Á Ln interactions can shift the zero-field quantum tunnelling step to a finite field, known as exchange biasing 15 , with different effects on cryogenic magnetization curves depending on the sign of the exchange 16,17 or even mask single-ion slow relaxation modes 18 . More frequently, however, Ln Á Á Á Ln interactions increase quantum tunnelling rates, leading to apparently lower U eff values when compared with paramagnetic Ln ions doped into a diamagnetic lattice 8,17,19 . Understanding how Ln Á Á Á Ln interactions can have such different and seemingly contradictory influences is important for improving the performance of Ln SMMs.…”

Theoretical Understanding of Anisotropy in Molecular Nanomagnets

“…As an example, the actinide complex [Np IV (COT) 2 ], being isostructural to [Er(COT) 2 ] À and having the D 8h geometry, is a much weaker SIM compared to the latter [92]. Blocking behavior was observed in pentavalent [93] and trivalent [94,95] uranium complexes but with significantly shorter relaxation times and lower heights of activation barriers compared to most lanthanide-based SIMs. Mononuclear transition metal complexes also can exhibit magnetization blocking; that was demonstrated for the first time on Fe (II) complexes [96].…”