Alignment of micro-crystals of Mn12-acetate and direct observation of single molecules thereof

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Title: Alignment of micro-crystals of Mn12-acetate and direct observation of single molecules thereof
Author: Seo, Dongmin
Abstract: 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 .
URI: http : / /hdl .handle .net /1969 .1 /ETD -TAMU -2574
Date: 2009-05-15

Citation

Alignment of micro-crystals of Mn12-acetate and direct observation of single molecules thereof. Available electronically from http : / /hdl .handle .net /1969 .1 /ETD -TAMU -2574 .

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