Research Description

EPR Studies of Single Molecule Magnets

Computing technology is constantly changing and as it evolves, the search for smaller units of memory and faster processing capabilities becomes exponentially more important. Single-Molecule Magnets (SMMs) appear to have great promise for revolutionizing the field of computing. At low temperatures the SMMs are blocked in a certain spin state (up or down) and therefore show potential as magnetic memory units. SMMs are also of great interest because they are small enough to exhibit quantum characteristics as well as classical ones and it has been suggested that they could eventually be used as the quantum bits in quantum computing.

These single molecule magnets usually are comprised of several transition-metal ions connected by various organic ligands. One of the most significant properties of these SMMs is the property that gives rise to the blocking within specific spin states, the uniaxial magnetic anisotropy. This anisotropy defines a specific energy barrier that must be overcome in order to change from up spin to down spin or vice versa. For single molecule magnets this magnetic anisotropy barrier, U, is relatively large under a specific blocking temperature and gives rise to magnetic hysteresis reminiscent of classical magnets. The difference, however, is that the magnetization dynamics of SMM crystals are heavily affected by quantum effects as well. The intrinsic quantum properties of these SMMs can lead to tunneling between spin states and subsequently give rise to an effective energy barrier, Ueff that is significantly less than the magnetic anisotropy barrier U.

In order to deduce a value for the uniaxial magnetic anisotropy, high frequency electron paramagnetic resonance (HFEPR) experiments are conducted on the SMMs. This spectroscopy is analogous to the visible spectroscopy used to observe the quantum states of atoms, but in this case the pertinent energy scale is relatively low (~10 Kelvin). This small energy scale requires radiation with a frequency in the microwave range. We then compare the values for the spectroscopic barriers found from HFEPR experiments to the values of Ueff obtained from thermodynamic measurements (AC susceptibility).