In this thesis, I will present an experimental study of two single molecule magnets, Mn12-ac and Mn12-ac-MeOH. I will show that in both systems, the temperature dependence of the inverse susceptibility yields a positive intercept on the temperature axis (a positive Weiss temperature), implying the existence of a ferromagnetic phase at low temperature.
Applying a magnetic field in the transverse direction moves the Weiss temperature downward towards zero. This implies that the transverse field triggers mechanisms in the system that compete with the dipolar interaction and suppress the long-range ordering.
I will then show that the suppression in Mn12-ac is considerably stronger than that expected for a pure TFIFM (Transverse Field Ising Ferromagnetic) model system. By contrast, the behavior of Mn12-ac-MeOH is consistent with the model.
We attribute the difference between the two systems to the presence of randomness in Mn12-ac associated with isomer disorder. Thus, in addition to spin-canting and thermal fluctuations, which contribute to the suppression of long-range order in both materials in the same way, the random fields due to isomer disorder that exist in Mn12-ac and not in Mn12-ac-MeOH causes further suppression of ferromagnetism in Mn12-ac. The behavior observed for Mn12-ac is consistent with a random field model calculated for this system by Millis et al.
|Advisor:||Sarachik, Myriam P.|
|Commitee:||Chudnovsky, Eugene M., Cohen, Morrel H., Kent, Andrew D., Vitkalov, Sergey|
|School:||City University of New York|
|School Location:||United States -- New York|
|Source:||DAI-B 74/06(E), Dissertation Abstracts International|
|Subjects:||Condensed matter physics|
|Keywords:||Demagnetizing field, Long range magnetic order, Manganese acetate, Random field theory, Single molecule magnets|
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