Browsing by Author "Calvello, S"

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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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    Elucidation of the wave function of the ground doublet in a Tb complex using INS in a magnetic field
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Calvello, S; Mole, RA; Soncini, A; Atkin, A; Boskovic, C
    Lanthanoid Single Molecule Magnets (SMMs) are molecular materials that exhibit slow relaxation of the magnetisation of molecular origin, thus showing promise for a wide range of technological applications, such as spintronic devices, qubits for quantum computers and molecular memories. The origin of the energy barrier to the reorientation of the magnetic moment in a lanthanoid SMM lies in the weak splitting of the atomic lowest-lying spin-orbit multiplet of the LnIII ion, induced by the electron density of the ligands, into a set of crystal field states. Since the crystal field splitting is significantly weaker than other energy contributions in lanthanoid complexes, the details of the ligand-Ln interaction are crucial for the development of SMMs which can operate at high temperatures. For this reason, it is crucial to achieve a detailed understanding on how the crystal field influences the relaxation of LnIII SMMs, and to this end computational and theoretical methods have been very useful to rationalise experimental results, and are routinely employed in the molecular magnetism literature. On the other hand, comparison of simulations with different experimental measurements can showcase the shortcomings of the theoretical or computational approach, providing a rationale on the improvements to introduce in order to improve their ability to successfully describe the properties of the system. Previously, we have performed Inelastic Neutron Scattering (INS) measurements on the complex [Tb(bpy)2(Cl4Cat)(Cl4CatH)(MeOH)using the PELICAN instrument, where we observed a shoulder to the elastic line which, after fitting, was established to arise from a single transition. While theoretical calculations of the low-lying energy spectrum of the complex rationalized the observed transition as arising from the quantum tunnelling of the ground Ising doublet, the predicted tunnelling gap is significantly smaller than what observed experimentally, highlighting the inability of such calculations to correctly predict the electronic properties of the ground state of the molecule. In this study, therefore, we have measured the INS spectra for the aforementioned complex, under the same experimental conditions of the previous experiment, employing the newly commissioned magnet on PELICAN, where the evolution of the observed transition in an increasing magnetic field would provide useful information on (i) the properties of the ground state and (ii) the nature of the magnetic transition observed in the zero-field spectrum. The measurements show that, with increasing magnetic field, the INS transition (i) becomes less intense, thus confirming our theoretical description of the observed peak as arising from the quantum tunnelling of the ground state, and (ii) splits into multiple peaks, both as a result of the usage of a powder sample and the complex spin-orbit composition of the wave function of the ground Ising doublet. Lanthanoid Single Molecule Magnets (SMMs) are molecular materials that exhibit slow relaxation of the magnetisation of molecular origin, thus showing promise for a wide range of technological applications, such as spintronic devices, qubits for quantum computers and molecular memories. The origin of the energy barrier to the reorientation of the magnetic moment in a lanthanoid SMM lies in the weak splitting of the atomic lowest-lying spin-orbit multiplet of the LnIII ion, induced by the electron density of the ligands, into a set of crystal field states. Since the crystal field splitting is significantly weaker than other energy contributions in lanthanoid complexes, the details of the ligand-Ln interaction are crucial for the development of SMMs which can operate at high temperatures. For this reason, it is crucial to achieve a detailed understanding on how the crystal field influences the relaxation of LnIII SMMs, and to this end computational and theoretical methods have been very useful to rationalise experimental results, and are routinely employed in the molecular magnetism literature. On the other hand, comparison of simulations with different experimental measurements can showcase the shortcomings of the theoretical or computational approach, providing a rationale on the improvements to introduce in order to improve their ability to successfully describe the properties of the system. Previously, we have performed Inelastic Neutron Scattering (INS) measurements on the complex [Tb(bpy)2(Cl4Cat)(Cl4CatH)(MeOH)using the PELICAN instrument, where we observed a shoulder to the elastic line which, after fitting, was established to arise from a single transition. While theoretical calculations of the low-lying energy spectrum of the complex rationalized the observed transition as arising from the quantum tunnelling of the ground Ising doublet, the predicted tunnelling gap is significantly smaller than what observed experimentally, highlighting the inability of such calculations to correctly predict the electronic properties of the ground state of the molecule. In this study, therefore, we have measured the INS spectra for the aforementioned complex, under the same experimental conditions of the previous experiment, employing the newly commissioned magnet on PELICAN, where the evolution of the observed transition in an increasing magnetic field would provide useful information on (i) the properties of the ground state and (ii) the nature of the magnetic transition observed in the zero-field spectrum. The measurements show that, with increasing magnetic field, the INS transition (i) becomes less intense, thus confirming our theoretical description of the observed peak as arising from the quantum tunnelling of the ground state, and (ii) splits into multiple peaks, both as a result of the usage of a powder sample and the complex spin-orbit composition of the wave function of the ground Ising doublet. © 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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    Theoretical study of a family of lanthanoid-dioxolene single- molecule magnets
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2018-11-19) Calvello, S; Soncini, A; Atkin, A; Mole, RA; Boskovic, C
    Lanthanoid Single-Molecule Magnets (SMMs) are molecular materials that exhibit slow relaxation of the magnetization of molecular origin, thus making them promising targets for the development of spintronic devices and molecular memories. Since the electronic and magnetic properties of lanthanoid-based SMMs are strongly dependent on the characteristics of the electrostatic crystal field induced by the ligands on the lanthanoid ion, a thorough understanding of such magnetostructural correlations is crucial to develop molecules displaying SMM behaviour at sufficiently high temperatures to warrant commercial applications. For this reason, ab initio calculations have proven to be valuable tools to elucidate the details of the electronic structure of SMMs and improve the understanding of their effect on magnetic properties and relaxation mechanisms. In this work, we have performed a set of ab initio calculations on the family of molecules [Ln(bpy)2(Cl4Cat)(Cl4CatH)(MeOH)] (Ln = Tb, Dy, Ho), employing the CASSCF/RASSI-SO method, and we have compared the predicted electronic and magnetic properties with the experimental data. These molecules, recently synthesized, are expected to display SMM behavior due of their structural similarity to other SMMs previously described in literature, with their low-lying energy spectrum determined with Inelastic Neutron Scattering (INS) for Ln = Tb, Ho. We show that there is a good agreement between computational and experimental results, thus confirming the validity of theoretical predictions of electronic and magnetic properties of lanthanoid-based SMMs. © The Authors.