Scalable Magnetic Anisotropy from Molecular Lanthanide Building Units

University Of California-San Diego, La Jolla CA

Investigators

Abstract

Non-technical summary: One modern approach to the design of new functional materials is through the construction and connection of molecular building units. In this approach, chemists synthesize molecules with structures that, when linked together to form a bulk material, lead to a material with specific properties. New properties can be introduced and tuned simply through modification of the molecular building unit. By analogy, a limitless number of structures can be designed starting with bricks and mortar, as opposed to one structure from a single pre-formed slab of fired clay. An area of synthesis where the building block approach remains a challenge is magnetic materials. Magnetic materials are vital components in nearly every aspect of modern life, yet only a handful of materials meet the requirements for use in applications, and none of these are made through a molecular building unit approach. The complexity of interactions that determine what direction the magnetic moment wants to align (its magnetic anisotropy) contributes to the difficulties in making the materials. As molecular building units are connected, new interactions disrupt their individual magnetic anisotropy, drastically weakening it and destroying any rational connection with the anisotropy of the original building unit. Returning to the construction analogy, connecting molecular magnets to make a bulk magnet is often akin to using bricks that spontaneously become raw clay when stacked. To change this, with support from the Solid State and Materials Chemistry program and the Chemical Structure, Dynamics and Mechanisms B program at NSF, researchers at UCSD are developing molecular building units that retain their large anisotropy values when connected to form an extended magnetic material. This class of new materials, built from individual units containing a rare earth ion, Er3+, allows the researchers to create designer magnetic structures that mimic rare and difficult-to-study motifs from solid state chemistry or create entirely new magnetic structures by design. Additionally, this project educates graduate and undergraduate students through innovative approaches such as a "In the Lab" live-feed featuring lab tours and demonstrations to enhance classroom teaching. Technical summary: A molecular building unit approach to magnetic materials can be realized only if the preferred orientation of the electron spin (its magnetic anisotropy) can be controlled. With this project, supported by the Solid State and Materials Chemistry program and the Chemical Structure, Dynamics and Mechanisms B program at NSF, molecular building blocks are identified that are capable of controlling anisotropy in the presence of the intermolecular linkages necessary to construct magnetic materials. Generating and fixing the magnetic anisotropy at the single-ion level, shifts research efforts to the rational design of complex spin geometries, enhanced magnetic coupling, and expanded dimensionality. The design principles for creating strongly anisotropic magnetism by combining a lanthanide with its specific, suitable crystal-field environment are well understood. Maintaining this anisotropy while introducing coupling interactions, however, is a complex challenge. One approach is to determine a single ligand-lanthanide combination that can direct the anisotropy roughly independent of the rest of the coordination sphere. Through computational and experimental results researchers have previously shown that the interaction between Er(III) and the cyclooctatetraenide dianion fulfills the requirements of a magnetic building unit of roughly fixed anisotropy. In this project researchers at UCSD use the molecular magnetic building unit to probe the behavior of anisotropic spins in the presence of both exchange and magnetic dipolar coupling, which enables them to construct new magnets and to independently tune system parameters, unavailable in any other class of materials. Fixing the two-site anisotropy and enhancing the exchange interaction creates stronger and switchable magnetic interactions between ions. More complex 1- to 3-dimensional spin structures that explore the limits of magnetic strength are created from these molecular building blocks. Each aspect of the system, from the coupling anisotropy and strength to the connectivity and dimensionality, allows the controlled study of an array of fundamental magnetic phenomena - often in ways that are not tunable or practical in traditional oxide or intermetallic magnetic materials. While spintronic technology has become ubiquitous, it is almost exclusively based on top-down approaches. Molecule-based materials offer the promise of enhanced tunability, terminal scalability, self-assembly, and unique quantum-confinement properties that align well with the goals of the NSF Big Ideas category: Quantum Leap. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.