Hauptseminar "Kohlenstoffbasierte Materialien"
Universität Stuttgart
Written Report to the Talk
Molecular Magnets
by Kilian Dietrich
February 27, 2014
Contents
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Abstract
Motivation & Basic Knowledge
Single Molecule Magnets
Slow Relaxation of the Magnetization
Quantum Tunneling of the Magnetization
Outlook and Conclusion
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1 Abstract
“What are Molecular Magnets and why should anyone be interested in them?”
A short answer to that is: “Molecular Magnets are single molecules consisting of a magnetic
core and shielding organic ligands. They have magnetic anisotropy and show ferromagnetic behavior when cooled sufficiently. This makes them interesting not only for data storage purposes
but also for observing quantum effects in mesoscopic objects. The ferromagnetic behavior is
due to the high spin ground state and its (2S+1) secondary spin states. Because of the negative
Zero Field Splitting an energy barrier is created that causes the magnetization to relax slowly
over time because the thermal inversion of the magnetic moment is phonon induced with steps
of ∆MS = 1 or ∆MS = 2. Additionally Quantum Tunneling of Magnetization, appearing at degenerate states, enhances the relaxation process. The contribution of this phenomenon increases
with increasing temperature. To finally use Molecular Magnets as Bits in a binary System an
implementation to a medium is required. In this talk the implementation to Carbon Nanotubes
is addressed.”
2 Motivation & Basic Knowledge
The idea of storing information using the ferromagnetic property of a remanent field goes back
to 1898 where Valdemar Poulsen, a Danish Engineer, invented the Telegraphon. This device
was capable of storing a magnetic amplitude which was proportional to the recorded signal
on a moving wire. The principle had not changed when Fritz Pfleumer, a German-Austrian
engineer, upgraded the recording medium to a tape which carried a powder of iron oxide.
The obvious benefit of the new medium was the low weight and the increased capacity. He
granted his idea to the German company AEG which built the worlds first tape recorder named
Magnetophon K1. A real revolution had taken place when IBM 1956 invented the first magnetic
hard drive, the IBM 305, which could store the amount of 5 MB of data with a density of
2000 b/in2 . In the following years improvements have been made to increase the density of
hard drives exponentially (doubling every two years) to the current value of 300 Gb/in2 . This
increase will find its limit because of the superparamagnetic limit. This phenomenon refers to
thermally induced spin flipping at sufficiently low temperatures.
Figure 1 : The Picture shows the development of magnetic data storage devices, whereas a) is the Telegraphon, b) is the
Magnetophon, c) is the first hard drive built and d) is a conventional hard drive available today.
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3 Single Molecule Magnets
To find a way to still increase the data density, one has to think of new possibilities. Molecules
can provide the structure needed to steadily break the symmetry and therefore get magnetic
anisotropy. Thus, an easy axis for magnetization is given. The definition of Single Molecule
Magnets refers to one single molecule which contains paramagnetic ions in its center which is
surrounded by organic ligands. Due to the coupling of the paramagnetic ions a ground state
spin exists which gives the ferro-/ferrimagnet needed to store information magnetically.
Figure 2 : Model of the Mn12ac molecule. The green balls resemble Mn4+ -ions and the orange balls resemble Mn3+ ions. Due to the anti-ferromagnetic coupling of the Mangan ions the ground state spin S equals 10. Here the surrounding
organic ligands are missing to provide insight.
The most discussed Single Molecule Magnet ([Mn12 O12 (O2 CCH3 )16 (H2 O)4 ]) is abbreviated Mn12ac.
It contains 4 Mn4+ -ions which are anti-ferromagnetically coupled to 8 Mn3+ -ions giving a
ground state spin of S= 10.
4 Slow Relaxation of the Magnetization
Due to spin-orbit coupling and dipolar interactions the (2S+1) secondary spin states are split
up. This is called Zero Field Splitting. Since the Zero Field Splitting parameter D is negative
this results in an energy barrier which has to be overcome for relaxation. That is, the spin states
of a magnetically saturated molecule will need time to relax into equilibrium because phononinduced relaxation is only allowed with steps of ∆MS = 1 or ∆MS = 2 . The time strongly
depends on the temperature and increases exponentially for decreasing temperature. Below 2 K
a satisfying relaxation time of about 50 years and more can be achieved.
Figure 3 : Due to axial anisotropy the (2S+1) secondary spin states of the ground state spin split. The double-well
picture shows the energy barrier created by the lowest lying levels due to the negative Zero Field Splitting parameter D.
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5 Quantum Tunneling of the Magnetization
Without the presence of transverse anisotropy the secondary spin states are the distinct eigenstates of the system. Transverse anisotropy will change the character of the states to superpositions which allows the spin to oscillate between two degenerate levels (if quantum-mechanically
allowed) giving rise to faster relaxation. This can be observed in the hysteresis curve of the
molecule. Steps indicate a faster relaxation of the magnetization as expected. This phenomenon
is called quantum tunneling of the magnetization.
Figure 4 : The figure shows a classical hysteresis loop (red) and a hysteresis loop of the Mn12ac SMM. The steps in the
curve are indications for the tunneling process of the spin.
6 Outlook and Conclusion
Recently, Maria del Carmen Giménez-López et al. successfully integrated Mn12ac molecules
into the cavity of graphitized multiwalled carbon nanotubes (GMWNT). Due to non-covalent
interactions the molecules stay mobile within the nanotubes and align to applied magnetic
fields. The positive outcome of the integration is that the SMMs keep their magnetic properties
and show hysteresis at 1.5 K. If compared to a reference Mn12ac molecule the hysteresis is
reduced. This is due to the alternative pathway for relaxation provided by the host nanotubes.
Figure 5 : (left) Mn12ac molecules have been inserted in Multi Walled Carbon Nano Tubes to connect these nanoscopic
objects to the macroscopic world. (right) Among other indications, inserted Mn12ac molecules show the same maxima
in the derivative of the magnetization if compared to a free reference Mn12ac molecule. The magnetic properties are
therefore maintained.
So far, Single Molecule Magnets are a promising path to increase data density. The hurdle which
still has to be overcome is to find a way of connecting the molecules to the macroscopic world.
If this finally becomes possible, the benefit will be a thousandfold increase in density.
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References
[1] R. Sessoli and D. Gatteschi.Quantum Tunneling of Magnetization and Related Phenomena in
Molecular Materials. Wiley-VCH, (2003).
[2] S. J. Blundell and F. L. Pratt.Organic and Molecular Magnets. Journal of Physics: Condensed Matter 16, (2004).
[3] C. Schlegl.Quantum Coherence in Molecular Magnets. Dissertation, 1. Physikalisches Institut, Universität Stuttgart, (2009).
[4] www.molmag.de - Dr. J. v. Slageren.Introduction to Molecular Magnetism. 1. Physikalisches
Institut, Universität Stuttgart. - [Online; Accessed 27.01.2014]
[5] www.pi1.physik.uni-stuttgart.de
[6] O. Pieper et al.Inelastic neutron scattering and frequency domain magnetic resonance studies of
S = 4 and S = 12 Mn6 single-molecule magnets. Physical Review B81, 174420 (2010).
[7] www.tomshardware.com/reviews/hard-drive-magnetic-storage-hdd,3005-7.html
[8] Jeremy Micah North. Synthesis and Characterization of Single-Molecule Magnets: Mn12ac,Fe8Br8, and Analogs. Florida State University, (2004).
[9] Maria del Carmen Gimenez-Lopez. Encapsulation of single-molecule magnets in carbon
nanotubes. Nature Communications 1415, (2011).
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