Magnetization Tunneling in Molecular Magnets
Why do atomic-sized objects obey the laws of quantum mechanics while "everyday-sized" objects behave classically? This question has intrigued physicists since the founding of quantum mechanics. It has garnered renewed attention recently because of growing interest in the field of quantum computing. In my research, I study systems that lie near the border of the quantum and classical realms and therefore exhibit behaviors typical of both worlds.
A magnetic particle or domain can reverse its direction by a classical thermal process or by quantum mechanical tunneling, if it is small enough. Molecular magnets, which are a few nanometers in size, represent the smallest possible bistable magnets. A few years ago, my collaborators and I discovered that the molecular magnet Mn12 Acetate reverses through a process of thermally assisted resonant tunneling. This process can be modeled in terms of a double-well potential (see figure), where one well corresponds to the magnetization pointing “up” and the other to it pointing “down.” A magnetic field tilts the potential and at specific values of field energy levels in opposite wells come into resonance, allowing tunneling to take place. Much of the thermally assisted tunneling process is generally understood. However, many details are still not known, including precisely which levels participate in the tunneling. I intend to determine the tunneling levels by using a transverse magnetic field to reduce the energy barrier as well as by using microwaves to induce “photon-assisted tunneling”.
A magnetic field applied perpendicular to the magnetization axis does not tilt the potential but, rather, reduces the height of the energy barrier. As the barrier is reduced the tunneling point should drop down the ladder of levels. An AC susceptibility technique will be developed to measure the height of the effective barrier as a function of transverse field. This effective barrier should reduce abruptly as the tunneling moves from one resonant pair of levels to the pair below it. A careful comparison of the data with theory should elucidate many features of the tunneling process and allow identification of which pair of levels participate in the tunneling for each value of transverse field.
Thermally assisted tunneling can be a rather messy process since
temperature induces transitions between many different levels. Using monochromatic microwave radiation, one can induce transitions between selected pairs of levels, helping the system climb the barrier in a controllable way. If the radiation induces transitions between levels below the tunneling levels, the system gets a boost in getting over the barrier and a faster magnetization reversal rate should be measured. On the other hand, inducing transitions between levels above the tunneling levels will not aid the reversal since the system does not need to access such high levels in order to traverse the barrier. Thus, by seeing how the radiation affects the transition rate, one can pin down where the tunneling takes place.
Both of these projects will primarily involve low-temperature
experiments, but there will also be opportunities to do some theoretical work like numerically solving the rate equations for the processes to be investigated.
