Understanding Spin-Spin and Spin-Lattice Interactions in Molecular Nanomagnetism

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Non-technical Abstract:This highly interdisciplinary project involves the study of magnetic molecules whose properties can be chemically engineered at the molecular level to give desired physical properties. These so-called molecular nanomagnets provide remarkable fundamental insights into magnetism at the nanoscale, while promising important advances in information technologies. In this regard, two distinct thrusts are under investigation: the first involves the potential use of magnetic molecules as the elementary memory units (bits) in classical computers, where the information is stored in the magnetic polarization state (up or down) of the molecule; the second explores the possibility of exploiting the quantum states of such molecules to implement quantum computing algorithms. This project employs unique magnetic resonance spectroscopic techniques that have been developed by the PI at the US National High Magnetic Field Laboratory to establish structure-property relations that chemists can then use to optimally design molecules for targeted applications. Although the design criteria for classical and quantum memories (qubits) are somewhat different, at a fundamental level, they typically involve tuning/optimizing the same interactions in molecular systems. The research team is exploring how the choice of magnetic element, molecular geometry, and surrounding host material influence the stability of both classical and quantum information encoded into magnetic molecules. The team is also researching possibilities for "wiring" magnetic molecules together using organic linkers. Results obtained from the project are of interest to other research communities, including materials science, inorganic and bioinorganic chemistry. The PI is strongly committed to diversity, both in terms of student recruitment, and through active participation in the American Physical Society's Masters-to-PhD Bridge Program and National Mentoring Community.Technical Abstract:The molecular nanomagnetism field has witnessed remarkable progress during the past few years, e.g.: a four-fold increase in the temperature below which a magnetic molecule can retain its magnetization has been achieved; and the coherent manipulation and readout of a single Tb nuclear spin has been demonstrated in a molecular device. Crucial to both of these results has been a focus on simple molecules comprised of magnetic ions with strong spin-orbit anisotropy. Recent activity has seen a bifurcation into distinct thrusts: the first continues to focus on single-molecule magnets (SMMs) that can function as classical memory elements; the 2nd thrust involves the potential use of molecular nanomagnets in quantum computing applications. This project addresses both areas. Work on SMMs focuses on: (i) anisotropic spin-orbit mediated exchange involving orbitally degenerate transition metals coupled to one or two additional high-spin centers; and (ii) direct exchange between anisotropic lanthanides using spin bearing (radical) ligands. The aim in both cases is to obtain simple, yet highly anisotropic molecules (dimers and trimers), which have sufficiently large spin ground states to shut down quantum tunneling and spin-lattice relaxation pathways that prevent magnetization blocking. Meanwhile, (iii) work on spin qubits focuses on anisotropic mononuclear species in which crystal field states can be engineered that protect against dipolar spin-spin decoherence. In this situation, spin-lattice relaxation can end up limiting phase memory times. High-field/frequency electron paramagnetic resonance (EPR) and high pressures are used to gain insights into the relevant static anisotropic interactions, while pulsed EPR is employed to probe dynamical properties, including spin-spin and spin-lattice relaxation. Instruments available at the National High Magnetic Field Laboratory enable studies spanning unprecedented field and frequency ranges, opening up this powerful technique to many materials of current interest within the molecular nanomagnetism community. Strong collaboration with chemists provides a much needed feedback loop, with the potential for major advances in molecule-based magnetic materials.