Browsing by Author "Dunstan, MA"

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    Elucidation of the electronic structure in lanthanoid-radical systems by inelastic neutron scattering
    (Australian Nuclear Science and Technology Organisation, 2021-11-24) Dunstan, MA; Calvello, S; Soncini, A; Krause-Heuer, AM; Mole, RA; Boskovic, C
    Single-molecule magnets (SMMs) are metal organic compounds which exhibit magnetic hysteresis and slow magnetic relaxation at low temperature. They have potential applications in high density data storage, quantum computing, and molecular spintronics. Coordination complexes of the trivalent lanthanoid (Ln(III)) ions are the current best performing SMMs, with examples showing hysteresis above liquid nitrogen temperature.[1] The magnetic properties of Ln(III) ions stems from the crystal field (CF) splitting of the ground Russel-Saunders state. These CF states give rise to the energy barrier to reversal of magnetisation, and can be tuned by modification of the ligand environment around the Ln(III) centre. Slow magnetic relaxation in Ln-SMMs can also be modulated by the introduction of magnetic exchange coupling with another magnetic moment, such as that of an organic radical ligand.[2] Quantifying the magnitude of magnetic exchange coupling in many Ln(III) systems is, however, difficult using conventional magnetometric techniques, due to the often large spin-orbit coupling. Inelastic neutron scattering (INS) is an ideal spectroscopic tool to measure both CF splitting and magnetic exchange coupling in Ln(III) systems.[3] We have used INS measurements to elucidate the magnetic exchange coupling and CF splitting in Ln(III)-semiquinonate complexes. Using this information we have rationalised the magnetic properties of these compounds, with the hope that a better understanding of the magnetic exchange in these systems can be used to design SMMs with improved performance. © 2021 The Authors
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    Inelastic neutron scattering of lanthanoid-radical molecular nanomagnets
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Dunstan, MA; Calvello, S; Krause-Heuer, AM; Soncini, A; Mole, RA; Boskovic, C
    SSingle-molecule magnets (SMMs) are materials which exhibit slow relaxation of magnetization and quantum tunneling of molecular origin. These properties make them promising for future applications in high-density data storage, as qubits in quantum computing, and in molecular spintronics.[1] The best performing SMMs are complexes of the late trivalent lanthanoid (Ln(III)) ions. The energy barrier to reversal of magnetization here stems from the crystal field (CF) splitting of the spin-orbit coupled ground state with total angular momentum J. The identity and geometry of the coordinated ligands determines the relative order, energy and composition of these CF states, such that appropriate choice of ligands can tune the CF splitting and therefore the SMM behaviour. Incorporation of organic radicals can be used to improve SMM behaviour, by shifting quantum tunneling of the magnetisation, a through-barrier relaxation pathway, from zero field.[2] The nature of magnetic exchange coupling between a Ln(III) ion and another paramagnetic moiety is, however, hard to determine, and often cannot be determined directly due to the large spin-orbit coupling inherent in many Ln compounds. Inelastic neutron scattering is a powerful experimental technique for directly measuring the CF splitting and exchange coupling in Ln(III) compounds.[3] Our group has been studying a family of compounds with formula [Ln(dbsq)Tp2], Tp– = tris(pyrazolyl)borate, dbsq– = 3,5-di-tert-butyl-semiquinonate, which show exchange coupling between the Ln(III) ion and the dbsq organic radical.[4] We have studied the INS spectra the Ln = Tb, Ho, Er, and Yb analogues on the cold neutron time-of-flight spectrometer PELICAN, as well as their magnetic properties. We observe temperature dependent CF transitions, which are compared to the energy level splitting obtained from electronic structure calculations, as well as exchange transitions in two analogues, which give us both the magnitude of and spatial information about the exchange coupling in this family of compounds.
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    Magnetic properties and neutron spectroscopy of lanthanoid-{tetrabromocatecholate/18-crown-6} single-molecule magnets
    (CSIRO Publishing, 2022-03-14) Dunstan, MA; Cagnes, MP; Phonsri, W; Murray, KS; Mole, RA; Boskovic, C
    Lanthanoid single-molecule magnets (Ln-SMMs) exhibit slow magnetic relaxation at low temperatures. This arises from an energy barrier to magnetisation reversal associated with the crystal field (CF) splitting of the Ln(III) ion. The magnetic relaxation is impacted by the interaction of the molecule with the crystal lattice, so factors including particle size and crystal packing can play an important role. In this work, a family of compounds of general formula [Ln(18-c-6)(NO3)(Br4Cat)]·X (Ln = La, Tb, Dy; 18-c-6 = 18-crown-6; Br4Cat2− = tetrabromocatecholate) has been studied by inelastic neutron scattering (INS) and magnetometry to elucidate the effects of crystal packing on the slow magnetic relaxation of the Tb(III) and Dy(III) compounds. The deuterated analogues [Ln(18-c-6-d24)(NO3)(Br4Cat)]·CH3CN-d3 (1-LnD; Ln = La, Tb, Dy) have been synthesised, with 1-TbD and the diamagnetic analogue 1-LaD measured by INS. The dynamic magnetic properties of 1-TbD and 1-DyD have also been measured and compared for two samples with different particle sizes. To probe packing effects on the slow magnetic relaxation, two new solvatomorphs of the hydrogenous compounds [Ln(18-c-6)(NO3)(Br4Cat)]·X (2-Ln: X = CH2Cl2; 3-Ln: X = 0.5 toluene) have been obtained for Ln = Tb and Dy. The CF splitting between the ground and first excited CF pseudo-doublets has been experimentally determined for 1-TbD by INS, and strongly rare earth dependent and anharmonic lattice vibrational modes have also been observed in the INS spectra, with implications for slow magnetic relaxation. Dynamic magnetic measurements reveal significant particle-size dependence for the slow magnetic relaxation for 1-TbD, while a previously reported anomalous phonon bottleneck effect in the 1-DyD analogue does not change with particle size. Further dynamic magnetic measurements of 2-Ln and 3-Ln show that the slow magnetic relaxation in these Ln-SMMs is strongly dependent on lattice effects and crystal packing, which has implications for the future use of Ln-SMMs in devices. © 2022 The Author(s) (or their employer(s)). Published by CSIRO Publishing.
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    Modulation of slow magnetic relaxation in Gd(III)‐tetrahalosemiquinonate complexes
    (Wiley, 2022-07-15) Dunstan, MA; Brown, DS; Sorace, L; Mole, RA; Boskovic, C
    Incorporating lanthanoid(III)‐radical magnetic exchange coupling is a possible route to improving the performance of lanthanoid (Ln) single‐molecule magnets (SMMs), molecular materials that exhibit slow relaxation and low temperature quantum tunnelling of the magnetization. Complexes of Gd(III) can conveniently be used as model systems to study the Ln‐radical exchange coupling, thanks to the absence of the orbital angular momentum that is present for many Ln(III) ions. Two new Gd(III)‐radical compounds of formula [Gd(18‐c‐6)X4SQ(NO3)].I3 (18‐c‐6=18‐crown‐6, X4SQ⋅−=tetrahalo‐1,2‐semiquinonate, 1: X=Cl, 2: X=Br) have been synthesized, and the presence of the dioxolene ligand in its semiquinonate form confirmed by X‐ray crystallography, UV‐Visible‐NIR spectroscopy and voltammetry. Static magnetometry and EPR spectroscopy indicate differences in the low temperature magnetic properties of the two compounds, with antiferromagnetic exchange coupling of JGd‐SQ∼−2.0 cm−1 (Hex=−2JGd‐SQ(SGdSSQ)) determined by data fitting. Interestingly, compound 1 exhibits slow magnetic relaxation in applied magnetic fields while 2 relaxes much faster, pointing to the major role of packing effects in modulating slow relaxation of the magnetization. © 2022 The Authors. Chemistry – An Asian Journal published by Wiley-VCH GmbH - Open Access - CC-BY-NC
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    Tetraoxolene-bridged rare-earth complexes: a radical-bridged dinuclear Dy single-molecule magnet
    (Royal Society of Chemistry, 2019-08-22) Reed, WR; Dunstan, MA; Gable, RW; Phonsri, W; Murray, KS; Mole, RA; Boskovic, C
    Two families of neutral tetraoxolene-bridged dinuclear rare earth complexes of general formula [((HBpz3)2RE)2(μ-tetraoxolene)] (RE = Y and Dy; HBpz3− = hydrotris(pyrazolyl)borate; tetraoxolene = fluoranilate (fa2−; 1-RE) or bromanilate (ba2−; 2-RE)) have been synthesised and characterised. In each case, the bridging tetraoxolene ligand is in the diamagnetic dianionic form and each rare earth metal centre has two HBpz3− ligands completing the coordination. Electrochemical studies on the soluble 2-RE family reveal a tetraoxolene-based reversible one-electron reduction. Bulk chemical reduction with cobaltocene affords the cobaltocenium (CoCp+) salt of the 1e-reduced analogue: [CoCp][((HBpz3)2RE)2(μ-ba˙)] (3-RE) that incorporates a radical trianionic form of the bromanilate bridging ligand. Alternating current (ac) magnetic susceptibility studies of 2-Dy reveal slow magnetic relaxation only in the presence of an applied magnetic field, but reduction to radical-bridged 3-Dy affords frequency-dependent peaks in the out-of-phase ac susceptibility in zero applied field. Exchange coupling between the Dy(III) ions and the radical bridging ligand thus reduces zero-field magnetisation quantum tunnelling and confers single-molecule magnet status on the complex. Comprehensive analysis of the magnetic relaxation data indicates that a combination of Orbach, Raman and direct relaxation processes are required to fit the data for both dysprosium bromanilate complexes. © Royal Society of Chemistry 2024.