Single Crystal High Frequency Cavity-based EPR Spectroscopy of Single Molecule Magnets
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Single Crystal High Frequency Cavity-based EPR Spectroscopy of Single Molecule Magnets S. Hill,1 R. S. Edwards,1 S. I. Jones,1 S. Maccagnano,1 J. M. North,2 N. Aliaga,3 E-C. Yang,4 N. S. Dalal,2 G. Christou,3 D. N. Hendrickson4 1 Department of Physics, University of Florida, Gainesville, FL 32611-8440, USA 2 Department of Chemistry, Florida State University, Tallahassee, FL 32310, USA 3 Department of Chemistry, University of Florida, Gainesville, FL 32611-8440, USA 4 Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093, USA ABSTRACT We report high frequency electron paramagnetic resonance (EPR) investigations of a series of high spin (total spin up to S = 10) manganese and nickel complexes which have been shown to exhibit single molecule magnetism, including low temperature (below ~ 1K) hysteresis loops and resonant magnetic quantum tunneling. A cavity perturbation technique enables high sensitivity oriented single crystal EPR measurements spanning a very wide frequency range (16 to 200+ GHz). Fitting of the frequency and field orientation dependence of EPR spectra allows direct determination of the effective spin Hamiltonian parameters. Studies on a range of materials with varying (approximately axial) site symmetries facilitates an assessment of the role of transverse anisotropy (terms in the Hamiltonian that do not commute with Sˆz ) in the magnetic quantum tunneling phenomenon. INTRODUCTION
Single molecule magnets (SMMs) have stimulated considerable experimental and theoretical research interest since the discovery of resonant quantum tunneling of the magnetization (QTM) in Mn12-acetate in 1996 [1]. Their main attraction is an intrinsic bistability which is realized via a large spin ground state (up to 26µB) and a significant axial (easy-axis) magnetocrystalline anisotropy [2,3]. This bistability has aroused great interest in terms of the use of SMMs in future molecular devices [4]. When grown as single crystals, the SMM unit is monodisperse—each molecule in the crystal has the same spin, orientation, magnetic anisotropy and structure. Thus, SMMs enable fundamental studies of properties intrinsic to magnetic nanostructures that have previously been inaccessible. All of the SMMs of interest possess a dominant uniaxial magnetocrystalline anisotropy and, to lowest order, the effective spin Hamiltonian may be written → ↔ Hˆ =D Sˆz2 + µB B · g · Sˆ + Hˆ ' ,
(1)
where D (< 0) is the uniaxial anisotropy constant, the second term represents the Zeeman interaction with an applied field B, and Hˆ ' includes higher order terms in the crystal field, as well as environmental couplings such as intermolecular dipolar and exchange interactions [1-7]. This Ising-type anisotropy is responsible for the energy barrier to magnetization reversal and the resulting magnetic bistability − factors which lead to magnetic hysteresis at sufficiently low temperatures [1-3]. Unlike bulk magnets, this hysteresis is intrinsic to each individual molecule − hence the term SMM. However, ess
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