Half-Integer Spin Molecular Nanomagnets
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Half-Integer Spin Molecular Nanomagnets
David N. Hendrickson,1* George Christou,
2*
Wolfgang Wernsdorfer,3 Stephen O. Hill,
4
Núria Aliaga-Alcade,2 Sumit Bhaduri,2 Rachel S. Edwards,4 Sheila M. J. Aubin,1 and Ziming Sun1
1Department of Chemistry and Biochemistry-0358, University of California at San Diego, La Jolla, California 92093-0358, U.S.A. 2Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, U.S.A. 3L. Néel-CNRS, BP 166, 25 Avenue des Martyrs, 38042 Grenoble, Cedex 9, France. 4Department of Physics, University of Florida, Gainesville, Florida 32611, U.S.A.
ABSTRACT Single-molecule magnets (SMM) are molecules that function as single-domain nanomagnets. SMMs have been characterized with a ground-state spin ranging from S = 4 to S = 13. A few SMMs have been identified that have half-integer spin ground states. [Cation][Mn12O12(O2CR)16(H2O)4] complexes, where R is some substituent, are SMMs that have either a S = 19/2 or 21/2 ground state. Quantum tunneling of magnetization (QTM) is observed for these half-integer-spin Kramers [Mn12] degenerate SMMs in zero external magnetic field, as well as for a class of S = 9/2 Mn4 SMMs. The presence of QTM in zero external field is attributed to a transverse component of a nuclear spin field, dipolar interactions and intermolecular exchange interactions. The Landau-Zener method is used to measure the tunnel splitting as a function of transverse magnetic field for a single crystal of the S = 9/2 SMM [Mn4O3(OSiMe3)(OAc)3(dbm)3]. Spin parity dependent QTM is established. The effect of a magnetic exchange interaction between two S = 9/2 Mn4 SMMs upon QTM was studied for another compound. The hydrogen bonding and Cl…Cl contacts within a supramolecularly linked [Mn4]2 dimer lead to a weak antiferromagnetic exchange interaction between the two S = 9/2 SMMs. This interaction causes a shift (exchange bias) from zero field for the magnetic field at which QTM occurs. INTRODUCTION Single-molecule magnets (SMMs) are molecules that function as single-domain magnetic particles which, below their blocking temperature, exhibit the classical macroscale property of a
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magnet, namely magnetization hysteresis [1,2]. SMMs owe their properties to a combination of a large ground state spin and an easy-axis-type anisotropy, which give a significant barrier to magnetization relaxation. SMMs thus represent a molecular approach to new nanoscale magnetic materials, offering all the advantages of molecular chemistry (room temperature synthesis, purity, solubility in many solvents, a well defined periphery of organic groups, and a crystalline ensemble of monodisperse units) as well as displaying the superparamagnetism of a mesoscale magnetic particle. They also display quantum tunneling of magnetization (QTM), indicating that they straddle the interface between the classical and quantum regimes [3,4]. SMMs have many potential applications, but these require that their properties be both understood and controlled, particularly QTM. The Mn12 SMMs have been
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