Defects, Tunneling, and EPR Spectra of Single-Molecule Magnets
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Defects, Tunneling, and EPR Spectra of Single-Molecule Magnets Kyungwha Park1,2,3 , M. A. Novotny4 , N. S. Dalal3 , S. Hill5 , P. A. Rikvold1,6 , S. Bhaduri7 , G. Christou7 , and D. N. Hendrickson8 1 School of Computational Science and Information Technology, Florida State University, Tallahassee, Florida 32306 2 Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306 3 Center for Computational Materials Science, Code 6390, Naval Research Laboratory, Washington DC 20375 4 Department of Physics and Astronomy and the Engineering Research Center, Mississippi State University, Mississippi State, Mississippi 39762 5 Department of Physics, University of Florida, Gainesville, Florida 32611 6 Center for Materials Research and Technology and Department of Physics, Florida State University, Tallahassee, Florida 32306 7 Department of Chemistry, University of Florida, Gainesville, Florida 32611 8 Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, California 92093 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, Fe8 Br, 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. INTRODUCTION Single-molecule magnets (SMMs) have recently been the focus of much attention because of the possibility of macroscopic quantum tunneling of their magnetizations [1, 2] and possible applications in magnetic storage devices and quantum computers
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[3]. SMMs are composed of identical single-domain nanoscale molecules, comprised of a core of several transition-metal ions surrounded by many different species of atoms, and they have a large effective spin. The characteristics of SMMs are relatively weak exchange and dipolar interactions between molecules, a large zero-field energy barrier against magnetization reversal, and magnetization steps in their hysteresis loops, which indicate quantum tunneling despite the large spin
