Binding of Higher Alcohols onto Mn<sub>12</sub> Single-Molecule Magnets (SMMs): Access to the Highest Barrier Mn<sub>12</sub> SMM

Lampropoulos, Christos; Redler, Gage; Data, Saiti; Abboud, Khalil A.; Hill, Stephen; Christou, George

doi:10.1021/ic901480y

Cited by 53 publications

(29 citation statements)

References 122 publications

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“…high and low symmetry, slow and fast relaxing, integer and odd-integer spin, etc.. This flexibility has, itself, yielded important insights into the magnetization dynamics associated with SMMs and, hence, to many more studies of Mn 12 compounds [8,9,39,40].…”

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confidence: 99%

Magnetic quantum tunneling: insights from simple molecule-based magnets

et al. 2010

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This perspectives article takes a broad view of the current understanding of magnetic bistability and magnetic quantum tunneling in single-molecule magnets (SMMs), focusing on three families of relatively simple, low-nuclearity transition metal clusters: spin S = 4 II 4 Ni , III 3 Mn (S = 2 and 6) and III 6 Mn (S = 4 and 12). The Mn III complexes are related by the fact that they contain triangular III 3 Mn units in which the exchange may be switched from antiferromagnetic to ferromagnetic without significantly altering the coordination around the Mn III centers, thereby leaving the single-ion physics more-or-less unaltered. This allows for a detailed and systematic study of the way in which the individual-ion anisotropies project onto the molecular spin ground state in otherwise identical low-and high-spin molecules, thus providing unique insights into the key factors that control the quantum dynamics of SMMs, namely: (i) the height of the kinetic barrier to magnetization relaxation; and (ii) the transverse interactions that cause tunneling through this barrier. Numerical calculations are supported by an unprecedented experimental data set (17 different compounds), including very detailed spectroscopic information obtained from high-frequency electron paramagnetic resonance and low-temperature hysteresis measurements. Comparisons are made between the giant spin and multi-spin phenomenologies. The giant spin approach assumes the ground state spin, S, to be exact, enabling implementation of simple anisotropy projection techniques. This methodology provides a basic understanding of the concept of anisotropy dilution whereby the cluster anisotropy decreases as the total spin increases, resulting in a barrier that depends weakly on S. This partly explains why the record barrier for a SMM (86 K for Mn 6 ) has barely increased in the 15 years since the first studies of Mn 12 -acetate, and why the tiny Mn 3 molecule can have a barrier approaching 60% of this record. Ultimately, the giant spin approach fails to capture all of the key physics, although it works remarkably well for the purely ferromagnetic cases. Nevertheless, diagonalization of the multi-spin Hamiltonian matrix is necessary in order to fully capture the interplay between exchange and local anisotropy, and the resultant spin-state mixing which ultimately gives rise to the tunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn 3 and Ni 4 ). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc..) of the molecules highlighted in this study proves to be of crucial importance. Not only that, these simple molecules may be considered among the best SMMs: Mn 6 possesses the record anisotropy barrier, and Mn 3 is the first SMM to exhibit quantum tunneling selection rules that reflect the intrinsic symmetry of the molecule.

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Magnetic quantum tunneling: insights from simple molecule-based magnets

et al. 2010

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“…In all cases where the FR species are obtained in single-crystal form, X-ray studies show that one or more of the Jahn-Teller (JT) elongation axes associated with the Mn III atoms are abnormally oriented 18,19,21,24,25,33 in comparison to the usual SR form of Mn 12 . 26,27,[34][35][36][37] This is illustrated in Fig. 1 21,38 these complexes also exhibit fast magnetization relaxation.…”

Section: Introductionmentioning

confidence: 77%

“…The first point to note is that the ZFS is less than for highsymmetry SR Mn 12 , for which the known examples give values in the 300-307 GHz range. 34,35,62 Consequently, one may conclude that the reduction in ZFS is, at most, 10% for FR-Mn 12 tBuAc relative to SR Mn 12 , in spite of the fact that the barrier is ~40% lower.…”

Section: Methodsmentioning

confidence: 98%

“…Subsequent recrystallization of dry FR-Mn 12 tBuAc from a CH 2 Cl 2 /MeCN mixture gives good crystals of the pure SR form (hereon SR-Mn 12 tBuAc) and Xray studies indicate that the JT axes are now in their normal orientations. 25 It should be emphasized that there are relatively few examples of truly high-symmetry Mn 12 , 26,[34][35][36][37] the acetate 9 being the best known. However, when crystallized with acetic acid solvent, the resulting samples are rather disordered: not only are FR molecules randomly distributed throughout the crystal, but the acetic acid is also disordered and this is known to influence the SR species as well; this point has been discussed at length by several authors.…”

Section: Introductionmentioning

confidence: 99%

“…plenty of SR examples exist that possess low symmetry structures. 35 It seems that a very significant structural distortion to the Mn 12 O 12 core is necessary in order to flip JT axes. However, the flipping of a JT axis, by itself, is unlikely to cause a significant enough reduction in the dominant axial anisotropy, D, to explain the observed reduction in U eff .…”

Section: Introductionmentioning

confidence: 99%

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Anisotropy barrier reduction in fast-relaxingMn12single-molecule magnets

2009

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A novel angle-swept high-frequency electron paramagnetic resonance (HFEPR) technique is described that facilitates efficient in-situ alignment of single-crystal samples containing lowsymmetry magnetic species such as single-molecule magnets (SMMs). This cavity-based technique involves recording HFEPR spectra at fixed frequency and field, while sweeping the applied field orientation. The method is applied to the study of a low-symmetry Jahn-Teller variant of the extensively studied spin S = 10 Mn 12 SMMs (e.g. Mn 12 -acetate). The lowsymmetry complex also exhibits SMM behavior, but with a significantly reduced effective

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