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Electronic interactions between gold films and mn12-acetateMeans, Joel Lewis 15 May 2009 (has links)
Interactions between Mn12–acetate molecular magnets and thin gold films have been explored
in light of the theory of weak localization. Low-temperature measurements of the
magnetoresistance of gold films of varying thicknesses, with and without the presence of a
surface layer of Mn12–acetate, have been performed using a dilution refrigerator. Quantitative
fits to the data using the predictions of weak localization theory were performed using
a least-squares fit method in order to determine characteristic times for elastic, inelastic,
spin-orbit and spin scattering events within the gold. These data indicate that the presence
of Mn12–acetate on the surface of the gold film leads to a significant enhancement of the
spin scattering within the gold films.
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Electron tunneling studies of Mn12-AcetateMa, Lianxi 10 October 2008 (has links)
We used self-assembling tunnel junctions (SATJs) to study the electron transport
through films of the molecular magnets, Mn12-Acetate. Pulse laser deposition
(PLD) was used to deposit two monolayers of Mn12-Acetate on thin Pt wires (diameter
0.001 in). The electron tunneling current was measured with typical bias voltages
from -1 to 1 V at liquid helium temperature, 4.2 K. I, dI/dV , and d2I/dV 2 signals
were directly acquired with the aid of a current amplifier and two lock-in-amplifiers.
Results show that the differential conductance is approximately 10â 6 S for bias voltages
0.04 V < or =| V |< or = V and exhibits a strong voltage dependence. In the region
| V |< or = 0.04 V, we find a zero-bias feature (ZBF) in which the differential conductance
is suppressed. In some samples, we observe I -V staircases which we attribute to
electrons "hopping" between the electrodes and the molecules. The observed hysteresis
was attributed to the slow relaxation of molecules re-orienting within the junction.
Abrupt conductance jumps at a bias voltage of -0.12 V were also observed and may
indicate state transitions in the Mn12-Acetate molecules. Furthermore, we observed
that the zero bias feature (ZBF) can switch from an enhancement to a suppression
of the differential conductance.
A dip and dry (DAD) method was also used to form films of Mn12-Acetate on
Al and Pt wires. Although the conductances were similar to those obtained using the PLD method, there were some subtle differences. In particular, we did not observe
the I -V staircases and the state jumps were more ambiguous. The differential
conductance for the Mn12-Acetate films on Al wires were typically 10- 7 S, which we
attributed to the oxide layer on Al surfaces.
We have also found substantial changes in the I - V characteristics when the
Pt wires coated with the Mn12-Acetate films were stored in 10-2 Torr for 6 months.
In particular, we observed many new features such as peaks in the conductance as a
function as the bias voltage. We believe that these effects may be caused by the slow
oxidation of the Mn12-Acetate molecules. Read more
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Alignment of micro-crystals of Mn12-acetate and direct observation of single molecules thereofSeo, Dongmin 15 May 2009 (has links)
This dissertation focuses on three separate studies. First, magnetization of the Mn12-
acetate was studied by low temperature hysteresis loops and DC magnetization data on
magnetically aligned Mn12-acetate micro-crystals. Secondly, Mn12-acetate thin films
were fabricated and characterized by AFM and STM. Finally, magnetization of the film
material was also studied.
Enhanced alignment of Mn12-acetate micro-crystals as compared to prior studies was
verified by observation of several sharp steps in low temperature hysteresis loops. It was
found that ~ 0.5 T is sufficient to orient the micro-crystals in an organic solvent to a
degree comparable to a single crystal. The degree of the alignment was controlled by
varying the magnetic field at room temperature and during the cooling process.
Subsequently, low temperature hysteresis loops and DC magnetizations were measured
for each prepared orientation state of a sample. The high temperature magnetic
anisotropy responsible for the alignment could not be measured, possibly due to its small
magnitude. Mn12-acetate was deposited onto Si/SiO2 by a solution evaporation method. Atomic
force microscopy studies revealed that 2 nm thick films of molecular level smoothness
were formed. Mn12-acetate was also deposited onto a Highly Ordered Pyrolytic Graphite
(HOPG) surface for scanning tunneling microscopy (STM) studies. A self-assembled
triangular lattice was observed in the Mn12-acetate thin films by STM at room
temperature under ambient conditions. These STM images show typical center to center
intermolecular separations of about 6.3 nm and height corrugation of less than 0.5 nm.
Magnetization measurements were not successful in Mn12-acetate thin films due to
the small amount of material in the film and the large background signal from the
substrate. Therefore, a sample for the magnetization measurements, called “film
material”, was made by evaporating a dilute solution of Mn12-acetate powder in
acetonitrile. Significant changes in magnetic properties of the film material were
observed from magnetization measurements. The blocking temperature of the film
material was found to increase to TB > 10 K at low magnetic fields. Read more
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