Molecular magnetism is a multidisciplinary field involving aspects of synthetic chemistry and solid state physics. Landee's research centers on the synthesis and experimental investigation of novel magnetic materials (usually insulating). Presently Landee is currently investigating three separate, but related, topics listed below. In all three he collaborates with Mark Turnbull.
1. Low Dimensional Quantum Antiferromagnets: Insulating Analogs of the High Tc Superconductors
The discovery of the high Tc superconductors brought renewed interest in the magnetic properties of two-dimensional S = 1/2 Heisenberg antiferromagnets, because the copper oxide planes interact magnetically according to this model. Investigation of the magnetic properties of the copper oxides has been rendered difficult, and in some cases impossible, by the large exchange strengths (1500 K) of these compounds. We have been developing a series of new compounds which are described by the same Hamiltonian, but which have much more convenient interactions strengths (7 K). This low value means that experiments such as the magnetization (M(H,T), the magnetic specific heat, and the EPR linewidth divergence can now be studied.
Related to this project are investigations of the behavior of a one-dimensional S = 1/2 Heisenberg antiferromagnet in an applied field, for which detailed theoretical predictions have been made. This will prove an important probe of the role of "spinons" in magnetic systems. Also under development are new spin-ladders and rectangular magnets which are intermediate between one and two dimensions.
2. Bimetallic Ferrimagnets in 2D and 3D: Synthesis and Magnetic Characterization
Our goal is to create transparent insulating magnets with room temperature spontaneous moments. Our procedure is to use selected organic molecules to link together different transition metal ions through strong superexchange interactions to create a high temperature ordered state. To raise the ordering temperature to room temperature, the networks must exist in two or three dimensions. The appropriate molecules do not always exist, so they are first synthesized by Turnbull's group, and then used to create the new materials.
3. Frustrated Magnetic Lattices: Synthesis of Kagomé Lattice Magnets
Spin-glasses have been studied for a quarter century and still continue to interest physicists. The number of unresolved problems remains high, due to the complexity of the systems. One particularly difficult problem is the frustration due to random antiferromagnetic bonds. Theorists prefer to study the effects of frustration upon critical behavior in non-random systems, such as triangular lattices. The most frustrated two-dimensional lattice is the Kagomé lattice, but unfortunately there are no physical systems that adopt this lattice structure.
We have prepared a family new magnets based on the BTCA ligand, which has the connectivity necessary to create a Kagomé lattice. The Co/BTCA complex shows very strange magnetic behavior, including resonance in the magnetic absorption chi'' and the non-linear susceptibilities chi3 and chi5. We are currently working to determine the structure of this material by high resolution X-ray and neutron diffraction.
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