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 Molecul

ar Materials.” Angew. Chem.Ed. 42(3), 2003,pp.268

V15:Müller,A., et.al. “Quantum Oscillation in a molecular magnet.” Nature lett. 453, 2008 pp.203Choirescu, et.al. “Environmental effects on big molecule with spin ½.” J.Appl.Phys.87(9) 20

00 pp.5496

Mn12:Friedman,J., Sarachik,M. “Mesoscopic Measurement of Resonant Magnetization Tunneling

in High-Spin Molecules.” PRL, 76(20),1996, pp.3830Barra, A., et.al. “High-frequency EPR spectra of a molecular nanomagnet: Understanding q

uantum 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.4742-4755Wernsdorfer,W., et.al. “X-ray Magnetic Circular Dichroism Picks out Single-Molecule Magnets Suitable

for Nanodevices.” Adv.Mater. 21, 2009, pp.167-171Sessoli,R., et.al. “Magnetic memory of a single-molecule quantum magnet wired to a gold surface.” Na

ture Mater. 8, 2009, pp.194