Elucidation of the wave function of the ground doublet in a Tb complex using INS in a magnetic field
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Date
2020-11-11
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Australian Institute of Nuclear Science and Engineering (AINSE)
Abstract
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.
Description
Keywords
Molecules, Magnetization, Energy, Ground states, Crystal field, Neutrons, Scattering, Density
Citation
Calvello, S., Mole, R., Soncini, A., Atkin, A., & Boskovic, C. (2020). Elucidation of the wave function of the ground doublet in a Tb complex using INS in a magnetic field. Paper presented to the ANBUG-AINSE Neutron Scattering Symposium, AANSS 2020, Virtual Meeting, 11th - 13th November 2020. (pp. 66). Retrieved from: https://events01.synchrotron.org.au/event/125/attachments/725/1149/AANSS_Abstract_Booklet_Complete_-_1_Page_Reduced.pdf