Browsing by Author "Boskovic, C"
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- ItemElucidation 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, CSingle-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
- ItemElucidation 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, CLanthanoid 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.
- ItemInelastic neutron scattering of lanthanoid single molecule magnets(Australian Institute of Nuclear Science and Engineering, 2016-11-29) Vonci, M; Giansiracusa, MJ; Mole, RA; Soncini, A; Boskovic, CWith their signature energy barrier to magnetic relaxation and quantum tunnelling through this barrier, single-molecule magnets (SMMs) are important candidate molecules for future applications in molecular spintronics and quantum computation. Among the most promising SMMs are those based on trivalent lanthanoid ions (Ln-SMMs). One of the most powerful experimental techniques for elucidating the electronic structure of SMMs is inelastic neutron scattering (INS), which provides a direct probe of the relevant energy levels. INS is very sensitive to the electronic structure of the lowest lying energy levels of Ln(III) ions, which are dominated by crystal field (CF) splitting effects. Despite these advantages, relatively few INS spectra with well-defined magnetic scattering have been reported for Ln-SMMs. We have recently completed a study of two structural families of Ln(III)-polyoxometalates: Na9[Ln(W5O18)2] (Ln = Nd, Tb, Ho, Er) and Na11[{Ln(OH2)}3CO3(PW9O34)2], (Ln = Ho, Er). In both cases the INS measurements have been analysed using both a conventional crystal field and a more comprehensive ab initio approach. In the current contribution I will address the issues related to getting high quality neutron scattering data and summarise what has been learnt about the rational design of SMMs from this series of experiments.
- ItemInelastic 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, CSSingle-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.
- ItemNew family of ferric spin clusters incorporating redox-active ortho-dioxolene ligands.(American Chemical Society, 2009-08-17) Mulyana, Y; Nafady, A; Mukherjee, A; Bircher, R; Moubaraki, B; Murray, KS; Bond, AM; Abrahams, BF; Boskovic, CSeven new di-, tri-, tetra-, and hexanuclear iron complexes that incorporate a polydentate Schiff base and variously substituted catecholate ligands have been synthesized from the trinuclear precursor [Fe3(OAc)3(L)3] (1), where LH2 = 2-[[(2-hydroxyethyl)imino]phenylmethyl]-phenol. These were isolated as the compounds [Fe3(OAc)(Cat)(L)3] (2), [Fe6(OAc)2(Cat)4(L)4] (3), [Fe4(3,5-DBCat)2(L)4] (4), [Bu4N][Fe4(OAc)(3,5-DBCat)4(L)2] (5a, 5- is the complex monoanion [Fe4(OAc)(3,5-DBCat)4(L)2]-), [Fe4(OAc)(3,5-DBCat)3(3,5-DBSQ)(L)2] (6), [Fe2(Cl4Cat)2(L)(LH2)(H2O)] (7), and [Et3NH]2[Fe2(Cl4Cat)2(L)2] (8a, 8²- is the complex dianion [Fe2(Cl4Cat)2(L)2]2-), where CatH2 = catechol; 3,5- DBCatH2 = 3,5-di-tert-butyl-catechol; 3,5-DBSQH = 3,5-di-tert-butyl-semiquinone, and Cl4CatH2 = tetrachlorocatechol. While compounds 2-4, 5a, 7, and 8a were obtained by directly treating 1 with the appropriate catechol, compound 6 was synthesized by chemical oxidation of 5a. These compounds have been characterized by single crystal X-ray diffraction, infrared and UV-visible spectroscopy, voltammetry, UV-visible spectroelectrochemistry, andmagnetic susceptibility and magnetization measurements. An electrochemical study of the three tetranuclear complexes (4, 5-, and 6) reveals multiple reversible redox processes due to the o-dioxolene ligands, in addition to reductive processes corresponding to the reduction of the iron(III) centers to iron(II). A voltammetric study of the progress of the chemical oxidation of compound 5a, together with a spectroelectrochemical study of the analogous electrochemical oxidation, indicates that there are two isomeric forms of the one-electron oxidized product. A relatively short-lived neutral species (5) that possesses the same ligand arrangement as complex 5- is the kinetic product of both chemical and electrochemical oxidation. After several hours, this species undergoes a significant structural rearrangement to convert to complex 6, which appears to be largely driven by the preference for the 3,5-DBSQ- ligand to bind in a non-bridging mode. Variable temperature magnetic susceptibilitymeasurements for compounds 3, 4, 5a, 6, 7, and8a reveal behavior dominated by pairwise antiferromagnetic exchange interactions, giving rise to a poorly isolated S = 0 ground state spin for compound 3, well-isolated S = 0 ground state spins for complexes 4, 5-, 7 and 8²-, and a well-isolated S = 1/2 ground state spin for complex 6. The ground state spin values were confirmed by low temperature variable field magnetization measurements. The thermal variation of the magnetic susceptibility for compounds 3, 4, 5a, 6, 7, and 8a were fitted and/or simulated using the appropriate Hamiltonians to derive J values that are consistent with magnetostructural correlations that have been reported previously for alkoxobridged ferric complexes. © 2009, American Chemical Society
- ItemSingle-ion anisotropy and exchange coupling in cobalt( ii )-radical complexes: insights from magnetic and ab initio studies(Royal Society of Chemistry, 2019-10-07) Gransbury, GK; Boulon, ME; Mole, RA; Gable, RW; Moubaraki, B; Murray, KS; Sorace, L; Soncini, A; Boskovic, CThe concurrent effects of single-ion anisotropy and exchange interactions on the electronic structure and magnetization dynamics have been analyzed for a cobalt(II)-semiquinonate complex. Analogs containing diamagnetic catecholate and tropolonate ligands were employed for comparison of the magnetic behavior and zinc congeners assisted with the spectroscopic characterization and assessment of intermolecular interactions in the cobalt(II) compounds. Low temperature X-band (ν ≈ 9.4 GHz) and W-Band (ν ≈ 94 GHz) electron paramagnetic resonance spectroscopy and static and dynamic magnetic measurements have been used to elucidate the electronic structure of the high spin cobalt(II) ion in [Co(Me3tpa)(Br4cat)] (1; Me3tpa = tris[(6-methyl-2-pyridyl)methyl]amine, Br4cat2− = tetrabromocatecholate) and [Co(Me3tpa)(trop)](PF6) (2(PF6); trop− = tropolonate), which show slow relaxation of the magnetization in applied field. The cobalt(II)-semiquinonate exchange interaction in [Co(Me3tpa)(dbsq)](PF6)·tol (3(PF6)·tol; dbsq− = 3,5-di-tert-butylsemiquinonate, tol = toluene) has been determined using an anisotropic exchange Hamiltonian in conjunction with multistate restricted active space self-consistent field ab initio modeling and wavefunction analysis, with comparison to magnetic and inelastic neutron scattering data. Our results demonstrate dominant ferromagnetic exchange for 3+ that is of similar magnitude to the anisotropy parameters of the cobalt(II) ion and contains a significant contribution from spin–orbit coupling. The nature of the exchange coupling between octahedral high spin cobalt(II) and semiquinonate ligands is a longstanding question; answering this question for the specific case of 3+ has confirmed the considerable sensitivity of the exchange to the molecular structure. The methodology employed will be generally applicable for elucidating exchange coupling between orbitally-degenerate metal ions and radical ligands and relevant to the development of bistable molecules and their integration into devices. © The Royal Society of Chemistry 2019. Open Access CC-NC
- ItemSynthesis, structure and magnetic properties of a novel Tb4 spin cluster and synthesis of a Tb chain(Elsevier, 2007-08-06) Bircher, R; Abrahams, BF; Gudel, HU; Boskovic, CThe syntheses of two new polynuclear TbIII compounds are reported. A tetranuclear complex [Tb4(H2L)2(H4L)2(OAc)8] (1), with H4L=2-((2-hydroxy-benzylidene)-amino)-2-hydroxy-methyl-propane-1,3-diol, has a butterfly-type structure, while [Tb(OAc)3MeOH]∞ (2) is a linear chain. A novel binding mode for H2L2− is observed in 1, with two alkoxides each bridging three TbIII ions. The magnetic properties of 1 are determined by the superposition of the ligand field split 7F6 states of TbIII single-ions located on two crystallographically distinct sites. Weak antiferromagnetic exchange interactions possibly contribute to the lowering of the magnetic moment at the lowest temperatures. © 2007, Elsevier Ltd.
- ItemTheoretical 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, CLanthanoid 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.