Magnetic Quantum Tunneling: Insights from Simple Molecule-Based Magnets

This article takes a broad view of the 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 Ni4, Mn(III)3 (S = 2 and 6) and Mn(III)6 (S = 4 and 12). The Mn(III) complexes are related by the fact that they contain triangular Mn3 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. 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 Mn3 and Ni4). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc..) of the molecules highlighted in this study proves to be of crucial importance.Comment: 32 pages, incl. 6 figure

Attempting to understand (and control) the relationship between structure and magnetism in an extended family of Mn-6 single-molecule magnets

International audienceThe synthesis and characterisation of a large family of hexametallic [Mn-6(III)] Single-Molecule Magnets of general formula [(Mn6O2)-O-III(R-sao)(6)(X)(2)(Sol)(4-6)] (where R = H, Me, Et; X = -O2CR'(R' = H, Me, Ph etc) or Hal(-); sol = EtOH, MeOH and/or H2O) are presented. We show how deliberate structural distortions of the [Mn3O] trinuclear moieties within the [Mn-6] complexes are used to tune their magnetic properties. These findings highlight a qualitative magneto-structural correlation whereby the type (anti- or ferromagnetic) of each Mn-2 pairwise magnetic exchange is dominated by the magnitude of each individual Mn-N-O-Mn torsion angle. The observation of magneto-structural correlations on Such large polymetallic complexes is rare and represents one of the largest studies of this kind

Transverse anisotropy in the mixed-valent (Mn2Mn4Mn3IV)-Mn-II-Mn-III single-molecule magnet

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Magnetic Quantum Tunneling: Insights From Simple Molecule-Based Magnets

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 NiII4, Mn III3 (S = 2 and 6) and MnIII6 (S = 4 and 12). The MnIII complexes are related by the fact that they contain triangular MnIII3 units in which the exchange may be switched from antiferromagnetic to ferromagnetic without significantly altering the coordination around the MnIII 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 Mn6) has barely increased in the 15 years since the first studies of Mn12-acetate, and why the tiny Mn3 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 Mn3 and Ni4). 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: Mn6 possesses the record anisotropy barrier, and Mn3 is the first SMM to exhibit quantum tunneling selection rules that reflect the intrinsic symmetry of the molecule. © The Royal Society of Chemistry 2010

Studies of magnetic properties and HFEPR of octanuclear manganese single-molecule magnets

International audienceA new octanuclear manganese cluster [Mn(8)(Hpmide)(4)O(4)(EtCOO)(6)](ClO(4))(2) (1) is achieved by employing Hpmide as the ligand, and this paper examines the synthesis, X-ray structure, high-field electron paramagnetic resonance (HFEPR), magnetization hysteresis loops and magnetic susceptibilities. Complex 1 was prepared by two different methods, and hence, was crystallized in two space groups: P3(2)21 for 1a and P3(1)21 for 1b. Each molecule possesses four Mn(II) and four Mn(III) ions. The metal-oxo framework of complex 1 consists of three face-sharing cubes with manganese ions on alternate corners. The four Mn(III) cations have their Jahn-Teller elongation axes roughly parallel to the c axis of the crystal lattice. The dc magnetic susceptibility measurements reveal a spin-frustration effect in this compound. The ac magnetic susceptibilities, as well as the magnetization hysteresis measurements, clearly establish that complex 1a is a single-molecule-magnet (SMM) with a kinetic energy barrier (10.4 cm(-1)) for spin reversal. HFEPR further confirms that complex 1a is a new SMM with a magnetoanisotropy and quantized energy levels. However, interpretation of the complete set of measurements in terms of a well defined spin ground state is not possible due to the spin frustration

Twisting, bending, stretching: strategies for making ferromagnetic [Mn-3(III)] triangles

International audienceThe synthesis and characterisation of a large family of trimetallic [Mn-3(III)] Single-Molecule Magnets is presented. The complexes reported can be divided into three categories with general formulae (type 1) [(Mn3O)-O-III(R-sao)(3)(X)(sol)(3-4)] (where R = H, Me, Bu-t; X = -O2CR (R = H, Me, Ph etc); sol = py and/or H2O), (type 2) [(Mn3O)-O-III(R-sao)(3)(X)(sol)(3-5)] (where R = Me, Et, Ph, Bu-t; X = -O2CR (R = H, Me, Ph etc); sol = MeOH, EtOH and/or H2O), and (type 3) [(Mn3O)-O-III(R-sao)(3)(sol)(3)(XO4)] (where R = H, Et, Ph, naphth; sol = py, MeOH, beta-pic, Et-py, Bu-t-py; X = Cl, Re). We show that deliberate structural distortions of the molecule can be used to tune the observed magnetic properties. In the crystals the ferromagnetic triangles are involved in extensive inter-molecular H-bonding which is clearly manifested in the magnetic behaviour, producing exchange-biased SMMs. These interactions can be removed by ligand replacement to give "simpler" SMMs

Twisting, bending, stretching: strategies for making ferromagnetic [MnIII3] triangles

The synthesis and characterisation of a large family of trimetallic [Mn-3(III)] Single-Molecule Magnets is presented. The complexes reported can be divided into three categories with general formulae (type 1) [(Mn3O)-O-III(R-sao)(3)(X)(sol)(3-4)] (where R = H, Me, Bu-t; X = -O2CR (R = H, Me, Ph etc); sol = py and/or H2O), (type 2) [(Mn3O)-O-III(R-sao)(3)(X)(sol)(3-5)] (where R = Me, Et, Ph, Bu-t; X = -O2CR (R = H, Me, Ph etc); sol = MeOH, EtOH and/or H2O), and (type 3) [(Mn3O)-O-III(R-sao)(3)(sol)(3)(XO4)] (where R = H, Et, Ph, naphth; sol = py, MeOH, beta-pic, Et-py, Bu-t-py; X = Cl, Re). We show that deliberate structural distortions of the molecule can be used to tune the observed magnetic properties. In the crystals the ferromagnetic triangles are involved in extensive inter-molecular H-bonding which is clearly manifested in the magnetic behaviour, producing exchange-biased SMMs. These interactions can be removed by ligand replacement to give “simpler” SMMs