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Defects, Tunneling, and EPR Spectra of Single-Molecule Magnets

Published online by Cambridge University Press:  10 February 2011

Kyungwha Park ,
M. A. Novotny ,
N. S. Dalal ,
S. Hill ,
P. A. Rikvold ,
S. Bhaduri ,
G. Christou  and
D. N. Hendrickson
Kyungwha Park
Affiliation:
School of Computational Science and Information Technology, Florida State University, Tallahassee, Florida 32306 Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306 Center for Computational Materials Science, Code 6390, Naval Research Laboratory, Washington DC 20375
M. A. Novotny
Affiliation:
Department of Physics and Astronomy and the Engineering Research Center, Mississippi State University, Mississippi State, Mississippi 39762
N. S. Dalal
Affiliation:
Center for Computational Materials Science, Code 6390, Naval Research Laboratory, Washington DC 20375
S. Hill
Affiliation:
Department of Physics, University of Florida, Gainesville, Florida 32611
P. A. Rikvold
Affiliation:
School of Computational Science and Information Technology, Florida State University, Tallahassee, Florida 32306 Center for Materials Research and Technology and Department of Physics, Florida State University, Tallahassee, Florida 32306
S. Bhaduri
Affiliation:
Department of Chemistry, University of Florida, Gainesville, Florida 32611
G. Christou
Affiliation:
Department of Chemistry, University of Florida, Gainesville, Florida 32611
D. N. Hendrickson
Affiliation:
Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, California 92093
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Abstract

We examine theoretically electron paramagnetic resonance (EPR) lineshapes as functions of resonance frequency, energy level, and temperature for single crystals of three different kinds of single-molecule nanomagnets (SMMs): Mn12 acetate, Fe8Br, and the S = 9/2 Mn4 compound. We use a density-matrix equation and consider distributions in the uniaxial (second-order) anisotropy parameter D and the g factor, caused by possible defects in the samples. Additionally, weak intermolecular exchange and electronic dipole interactions are included in a mean-field approximation. Our calculated linewidths are in good agreement with experiments. We find that the distribution in D is common to the three examined single-molecule magnets. This could provide a basis for a proposed tunneling mechanism due to lattice defects or imperfections. We also find that weak intermolecular exchange and dipolar interactions are mainly responsible for the temperature dependence of the lineshapes for all three SMMs, and that the intermolecular exchange interaction is more significant for Mn4 than for the other two SMMs. This finding is consistent with earlier experiments and suggests the role of spin-spin relaxation processes in the mechanism of magnetization tunneling.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 746: Symposia Q/R – Magnetoelectronics-Novel Magnetic Phenomena in Nanostructures – Advanced Characterization of Artificially Structured Magnetic Materials , 2002 , Q1.3
Copyright
Copyright © Materials Research Society 2003

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References

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