Transcript molecular_magnet - University of Tennessee

Single Molecular Magnets
Ge,Weihao
Introduction
What is Single Molecular Magnet?
Configuration: metal atom linked by oxygen, packed in ligands
Described as a total single spin; size and anisotropy has great effect
Why are they interesting?
Theoretical: quantum behavior at mesoscopic level
Application:
Quantum computer:
quantum interference and coherence
High-density storage device
Internal memory effect
high integration
Other applications
Two kinds of these clusters
Big integer spin
½ spin big molecule
Superparamagnetism
Superparamagnetism
paramagnetism below Curie’s temperature
large susceptibility
superparamagnetism limit
Origin of superparamagnetism
magnetism: result of spin alignment
thermal excitation, ferromagnetism <-> paramagnetism
small scale, below Tc:
thermal excitation destroys the ordering between the clusters
thermal excitation cannot upset alignment within the cluster
ferro~ inside & para~ outside => treated as a large spin as a whole
Experiment results
stepped hysteresis can be found below certain temperature.
frequency dependent AC susceptibility
Quantum Tunneling Magnetization
Experiment
steps found in hysteresis of Mn12 cluster
Model
two – well model
resonant tunneling
thermally assisted QTM & pure QTM
A commonly used form of Hamiltonian
axial anisotropic term
Zeeman splitting term
transverse anisotropic term
Integer Spin SMM: Mn12
Significance
archeological reason: first synthesized SMM & QTM first observed
most widely studied
Structure
Mn3+, external octagon; Mn4+, internal tetrahedron.
Ground state: S=10
Hamiltonian
symmetry:
lowest even power of transverse terms is 4
transition:
probably exist low-ordered odd-powered terms
Integer Spin SMM: Fe4
Structure
Fe3+: centered triangle, C2 symmetry
Ground state: S=5
Hamiltonian
general form
Advantages over Mn12 in application
More efficient tunneling
longer relaxation time
less affected when attached to a surface
stability
½ spin big molecule: V15
Structure
a center triangle between two hexagons
S=1/2, no large energy barrier, large zero field splitting
Experimental result
hysteresis observed
Rabi oscillation
coherence time: ~100 μs
Theoretical approaches
dissipative two-level system:
Landau - Zener transition
exchange interaction:
“spin rotation in a phonon bath”
Summary
General introduction to single molecular magnets
quantum behavior beyond the microscopic scale in these clusters
Origin of the magnetism of SMM
Quantum Tunneling Magnetization
A result of size and anisotropy
Integer spin clusters and ½ spin clusters
integer:
easy to interpreted by large-spin approximation
½ spin:
lack of barrier, tunneling caused by spin-phonon interaction
long-lived coherence
References
QTM:
Gatteschi,D.; Sessoli,R. “Quantum tunneling magnetization and related phenomena in
Molecular Materials.” Angew. Chem.Ed. 42(3), 2003,pp.268
Mn12:
Friedman,J., Sarachik,M. “Mesoscopic Measurement of Resonant Magnetization Tunneling
in High-Spin Molecules.” PRL, 76(20),1996, pp.3830
Barra, A., et.al. “High-frequency EPR spectra of a molecular nanomagnet: Understanding
quantum tunneling of the magnetization.” PRB. 56(13), 1997, pp.8192
Fe4:
Accorsi,S., et.al. “Tuning Anisotropy Barriers in a Family of Tetraion(III) Single-Molecule Magnets with
an S = 5 Ground State” JACS. 128(14), 2006, pp.4742-4755
Wernsdorfer,W., et.al. “X-ray Magnetic Circular Dichroism Picks out Single-Molecule Magnets Suitable
for Nanodevices.” Adv.Mater. 21, 2009, pp.167-171
Sessoli,R., et.al. “Magnetic memory of a single-molecule quantum magnet wired to a gold surface.”
Nature Mater. 8, 2009, pp.194
V15:
Müller,A., et.al. “Quantum Oscillation in a molecular magnet.” Nature lett. 453, 2008 pp.203
Choirescu, et.al. “Environmental effects on big molecule with spin ½.” J.Appl.Phys.87(9)
2000 pp.5496