Experimental Studies in High-Spin Molecular Magnets

Cuny City College, New York NY

Investigators

Abstract

This project investigates the magnetic properties of molecules containing clusters of magnetic atoms that are exchange coupled to produce a lattice of identical "nanomagnets; in particular, Mn12-Acetate and Fe8. These materials have a number of interesting properties: resonant spin tunneling, quantum interference (Berry's phase) effects in the magnetic relaxation, quantum coherent oscillations of the spin, and evidence for an abrupt crossover between thermally assisted magnetic relaxation and relaxation by pure quantum tunneling. Although much progress has been made, a full understanding of these materials awaits the resolution of a number of important issues, including the strength of spin-phonon coupling, the roles of dipolar interactions and hyperfine fields, and the nature of the crossover between thermally-assisted and pure quantum tunneling. Through measurements at very low temperatures using micron size Hall bars, this project will study the effect of externally applied transverse magnetic fields, the effect of microwave radiation, and to search for possible ferromagnetic or dipolar ordering. Molecular magnets are interesting from a fundamental point of view because they represent the borderline between classical and quantum magnetism. They are also interesting for their possible use for very high-density storage, and as potential candidates for elements in quantum computation. Graduate students involved in the project receive training in fundamental experimental techniques with cutting edge technology. This training will prepare them for a range of careers in academe, industry or government. Mn12-acetate and Fe8 are organic molecular crystals containing a very large number of regularly spaced, magnetically identical spin-10 clusters of a size that is borderline between the quantum and classical regimes. Interest in these materials has been sparked by a remarkable series of experimental observations of quantum-mechanical effects, including quantum tunneling of the spin magnetic moment, quantum-mechanical phase interference and the superposition of quantum states. Although much progress has been made, a full understanding of these materials awaits the resolution of a number of important issues, including the strength of the coupling of the spin magnetic moment to lattice vibrations, the roles of dipolar interactions and nuclear fields, and the nature of the crossover between the quantum and classical regimes. Through measurements at very low temperatures using micron size Hall bars, we propose to investigate the effect of externally applied transverse magnetic fields, the effect of microwave radiation, and to search for possible ferromagnetic or dipolar ordering. In addition to their interest for fundamental reasons, these materials are important for their possible use in very high-density storage, and as potential candidates for elements in quantum computation. Graduate students involved in the project receive training in fundamental experimental techniques with cutting edge technology. This training will prepare them for a range of careers in academe, industry or government.