Introduction to Molecular Magnets

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Introduction to Molecular Magnets

• Jason T. Haraldsen
• Advanced Solid State II
• 4/17/2007

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Basic Molecular Magnets
Mn12-acetate, S 10 Fe8Br8, S 10
V15, S 1/2
V15As6O42(H2O)K6
Mn12O12(CH3OO)16(H2O)4.2CH3COOH.4H2O
4 Spin-3/2 8 Spin-2
(C6H5N3)6Fe8(µ3-O)2(µ2-OH)12Br7(H2O)Br.8H2O
15 Spin-1/2
8 Spin-5/2
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Molecular MagnetsBy Definition
NaCuAsO4, S 0
Isolated clusters containing ionic spins ranging
from as few as two to up to several
dozen. Molecular magnets are not limited to
molecular solids. No long range order.
S. Nagler et. al. Unpublished
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Heisenberg Spin-Spin Exchange Hamiltonian
J gt 0 is antiferromagnetic
Anisotropic and Zeeman terms may be added to give
a more complete description of a given material.
J lt 0 is ferromagnetic
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Molecular MagnetsTheory and Model
Heisenberg Hamiltonian plus anisotropy and Zeeman
terms
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Molecular MagnetsTheory and Model
Ising and XY Variations
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Molecular MagnetsDimensionality
Hilbert Space
For mixed valance systems
For spin symmetric systems
Examples of how dimensionality increases in
systems
Spin-1/2 Dimer
Spin-1/2 Pentamer
Spin-3/2 Pentamer
Mn12O12(CH3OO)16(H2O)4.2CH3COOH.4H2O
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Experimental Techniques
Microscopic Quantities Inelastic Neutron
Scattering Raman Scattering Infrared
Spectroscopy

• Bulk Quantities
• Magnetization
• Magnetic Susceptibility
• Heat Capacity

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Magnetization

• Examines the overall magnetic moment.
• Steps denote magnetic spin flips in the material.

J. Schnack et. al. PRB (2006)
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Magnetic Susceptibility
Change in magnetization over change in
magnetic field.
J.T. Haraldsen et. al. PRB (2005)
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Heat Capacity
The ability of a material to store heat as a
function of temperature.
The peak corresponds to either a cooperative
phase transition or an anomaly.
J.T. Haraldsen et. al. PRB (2005)
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Effects of Dimensionality on Complexity
J.T. Haraldsen et. al. PRB (2005)
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Inelastic Neutron Scattering
Tennant et. al, PRL (2005)
Excitation Energy
Measures the dispersion relationship of energy
transfer and momentum transfer.
Structure Factor Intensity
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Infrared and Raman Spectroscopy
Not usually thought of when discussion magnetic
transitions. Activated when a magnetic system
has an anisotropic interaction. Usually coupled
to lattice excitations through the typically
selections rules of dipole moment (Infrared) and
polarizability (Raman).
? - NaVO
T. Room et. al. PRB (2002)
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Simplest Molecular MagnetSpin 1/2 Dimer
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The Textbook Dimer VO(DPO2)0.5H2O
Spin-1/2 vanadium system. Vanadium ions are
arranged structurally in a lattice. Magnetic
properties show the presence of an isolated dimer.
Tennant et. al, PRL (2005)
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Magnetic Susceptibility of VO(DPO2)0.5H2O
J. W. Johnston et. al., J. Am. Chem. Soc. (1984)
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Inelastic Neutron Scattering of
VO(DPO2)0.5D2O
Experiment
Predicted
Energy (meV)
Energy (meV)
q (Å-1)
q (Å-1)
D.A. Tennant et. al. PRL (1997)
?E J 7.86 meV
A 4.37 Å
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The Textbook Dimer VO(DPO2)0.5H2O
The use of magnetic measurements on the material
of VODPO4 ½ D2O clarified the magnetic
structure of the material.
V4
V4
V4
Tennant et. al, PRL (2005)
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Summary

• Molecular magnets are magnetically isolated
  clusters that exhibit no long range magnetic
  ordering
• Many techniques, both bulk and microscopic, can
  be used to determine the magnetic structure.
• Magnetic structure is not always obvious from an
  examination of the crystal structure.