Inorganic chemistry; materials chemistry; single-molecule magnets; dynamics of transition metal complexes
University of California, Berkeley
University of California, Los Angeles
California Institute of Technology
Awards and Academic Honors
Alexander von Humboldt Foundation Research Award for Senior U.S. Scientists
Japan Society for the Promotion of Science Faculty Fellowship
Alfred P. Sloan Fellowship
Dreyfus Teacher-Scholar Fellowship
There is intense academic and technological interest in magnetic nanostructures. Miniaturization leads to nanomagnets (< 100 nm) and to previously unimportant quantum effects that dramatically affect the functionality of nanomagnetic memory devices. Our discovery in 1993 of molecular nanomagnets, called single-molecule magnets (SMM), has made it possible to study systematically several fundamental questions about nanomagnets. In the case of a SMM, each molecule is a nanomagnet where the molecule has a large spin and magnetoanisotropy. A benchmark observation in the area of nanomagnets was made on a SMM, where it was shown that such a molecule can change from "spin-up" to "spin-down" not only by thermalizing over a potential-energy barrier, but also by quantum tunneling of the direction of magnetization. The mechanism of magnetization tunneling is under study by making systematic changes in molecule nanomagnets. Several new SMMs are being prepared to increase the blocking temperature and to find applications in magnetic memory devices, including quantum computing.
Electron transfer processes in chemical and biological systems can change markedly under varying environmental conditions. The study of mixed-valence transition metal complexes in the solid state gives fundamental information about the environment control of the rate of electron transfer between two metal ions. We are studying mixed-valence biferrocenes, dinuclear CuIICuI complexes, and trinuclear oxo-centered metal carboxylates. The architecture of these complexes is modified to determine the factors controlling the rate of electron transfer. The electronic and molecular structures are probed by EPR, IR, Mössbauer spectroscopies as well as magnetometry and X-ray crystallography.
The factors controlling the rate at which FeII or FeIII spin-crossover complexes interconvert between low- and high-spin states are being studied. Transition metal complexes with catecholate and/or o-semiquinonate ligands exhibit valence tautomerism. Dramatic changes in color and number of unpaired electrons accompany an intramolecular electron transfer between the metal and the redox-active o-quinone ligand and metal spin-state change.
Primary Research Area
- Exchange Bias in Ni4 Single-Molecule Magnets. With E.-C. Yang, W. Wernsdorfer, S. O. Hill, R. S. Edwards, M. Nakano, S. Maccagvano, L. N. Zakharov, A. L. Rheingold, and G. Christou. Polyhedron. 22, 1727 (2003).
- High Frequency Electron Paramagnetic Resonance Investigations of Tetranuclear Nickel-Based Single-Molecule Magnets. With R. S. Edwards, S. Maccagnano, E-C. Yang, S. Hill, W. Wernsdorfer, and G. Christou, J. Appl. Phys. 93, 7807 (2003).
- Exchange-Biased Quantum Tunneling in a Supramolecular Dimer of Single-Molecule Magnets. With W. Wernsdorfer, N. Aliaga-Alcalde, D. N. Hendrickson and G. Christou, Nature. 416, 406 (2002).
- Magnetization Tunneling in Single-Molecule Magnets. With J. Yoo, E. M. Rumberger, A. Yamaguchi, H. Ishimoto, E. K. Brechin, and G. Christou, J. Appl. Phys. 91, 7155 (2002).
- Spin-Parity Dependent Tunneling of Magnetization in Single-Molecule Magnets. With W. Wernsdorfer, S. Bhaduri, C. Boskovic, and G. Christou. Phys. Rev. B. 65, 180403 (R)1-4 (2002).
- Effects of Paramagnetic Ferrocenium Cations on the Magnetic Properties of the Anionic Single-Molecule Magnet [Mn12O12(O2CC6F5)16(H2O)4]-. With T. Kuroda-Sowa, M. Lam, A. L. Rheingold, C. Frommen, W. M. Reiff, M. Nakano, J. Yoo, A. L. Maniero, L.-C. Brunel, and G. Christou. Inorg. Chem.40, 6469 (2001).