1. SINGLE MOLECULAR MAGNETS
AN ANOTHER INDEX FOR THE SMART FUTURE OF MANKIND
Subhamoy Saha
TSO-60th Batch
HBNI

2. What causes to magnetism?
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3. Diamagnetism:
Property of all
matter
Uncompensate
d orbital &
angular
momentum
Permanent
atomic
moments
Independent
atomic
moments
Ideal
paramagnetism
Co-operating
atomic
moments
Ferromagnetis
m
Antiferromagne
tism
Ferrimagnetism
Electronic
bands in metals
Pauli spin
paramagnetism
Band anti-
ferromagnetism
Band
ferromagnetism
Hierarchy of types of magnetism
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4.  Diamagnetic: Paired, filled shell electrons.
 Paramagnetic: Unpaired electron spin and orbital motion
 Ferromagnetic: Neighbouring particles with unpaired spins interacting among themselves
 Antiferromagnetic: Neighbouring particles with unpaired spins interacting among themselves.
Fig: Magnetic susceptibility vs. temperature curves4
Magnetochemistry by Dutta, Shamal

5. What our smart life wants?
Smaller and smaller devices. The need of nanomagnets increases.
What is the problem??
The 'top-down' approach is reaching its limits as the ability to fabricate nanoscale
magnets that are of identical size becomes increasingly more difficult with
decreasing size.
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6. What can be a solution??
Follow up the alternate “bottom-up” route, i.e. magnetic functionating of single molecule.
From this point the role of “Single molecular magnets” begins.
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7. What is a
Single Molecular Magnet (SMM)?
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8. What is a single molecular magnet (SMM)?
SMMs are nanoscale magnetic molecules that exhibit slow relaxation of the
magnetization at low temperatures
1960 : Molecular based magnets first developed
1991 : the first single-molecule magnet was reported by Lis
1996 : the term "single-molecule magnet" was first coined
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Mn84

9. Let us have a look of some properties of SMMs
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10.  A single molecule magnet is a molecule that shows slow relaxation of the magnetization of purely molecular
origin.
 It is a molecule that can be magnetized in a magnetic field, and that will remain magnetized even after
switching off the magnetic field.
 This is a property of the molecule itself. No interaction between the molecules is necessary for this
phenomenon to occur.
 This makes single molecule magnets fundamentally different from traditional bulk magnets.
 You can dissolve a single molecule magnet in a solvent or put it in some other matrix, like a polymer, and it will
still show this property.
Properties of SMMs
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11.  Single molecule magnets belong to the more general class of compounds called exchange coupled clusters.
 A small number (~4 to 30) of paramagnetic transition metal ions are linked together by simple bridges such as O2-
, -OH, -OCH3, F-, Cl-, RCOO-.
 This bridge creates a superexchange pathway giving rise to an isotropic exchange coupling in the order of 1 to 100
cm-1.
 The organic ligands on the outside shield the exchange coupled cluster from the environment, esp. with respect
to exchange interactions.
 Often single molecule magnets are portions of a mineral lattice encapsulated by organic ligands
Properties of SMMs
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12. Some famous SMMs
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13. Mn(III) Mn(IV) O
Fig: [Mn12O12] core of
[Mn12O12(O2CR)16(H2O)8] complexes that
gives their S=10 ground states
Mn3+ : 1s2 2s2 2p6 3s2 3p6 3d4
S = 4/2 = 2
8Mn3+ = 8*2 = 16
Mn4+ : 1s2 2s2 2p6 3s2 3p6 3d3
S = 3/2
4Mn4+ = 4*3/2 = 6
Spin State of [Mn12O12(O2CR)16(H2O)8] complex = 16-6 = 10
 4 central MnIV atoms are weakly ferromagnetically
coupled
 MnIII MnIV antiferromagnetic coupling is stronger than
MnIII MnIII
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14.  Fe ions are linked by O2- and OH- bridges.
 tacn ligands are bound on the outside of the
molecule.
Fe8
Fe3+ ions (S = 5/2)
2 ions with spin down: S = 2* (5/2) = 5
6 ions with spin up: S = 6* (5/2) = 15
Total S = 10
[(tacn)6Fe8(O)2(OH)12(H2O)]
tacn = 1,4,7-Triazacyclononane
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15. What is the origin of the
slow relaxation of the magnetization?
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16. What is the origin of the slow relaxation of the magnetization in SMM?
E(ms) = D ms
2
 The large ground spin state S combined with a large and
negative Magneto-anisotropy as measured by the axial zero
field splitting parameter D
 There is an energy barrier of energy ∆E to magnetisation reversal
∆E = S2 │D│ when S is integer
= (S2 – 4) │D│ when S is half-integer
Where S is the large ground state spin
 The axial aniosotropy forces the magnetic moment to point
either parallel or antiparallel to the quantization axis.
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Solid State Commun., 127, 131139, 2003

17. The isotropic Heisenberg exchange interaction is expressed as
Hisotropic = ∑ ∑ Jij Si . Sj
^ ^^
i j>i
 Ji,j is the magnitudes of the isotropic Heisenberg interaction between spin i and spin j (positive for antiferromagnetic coupling
and negative for ferromagnetic coupling).
• S represents the spin operator for an individual ion within the molecule.
Fig. : Ms quantum number corresponds to 2S+1 orientations (left) and energy spectrum of these Ms states
What is the origin of the slow relaxation of the magnetization in SMM?
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18. What is Quantum
Tunneling of the
Magnetization?
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19.  In 1995 it was discovered that there are steps at regular field intervals in the magnetic hysteresis curve of Mn12Ac.
 Apparently at certain fields, the magnetisation relaxes faster.
 This phenomenon became later known as Quantum Tunneling of the Magnetisation.
What is Quantum Tunneling of the Magnetization?
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M. Soler, et al. J. Am. Chem. Soc., 2004, 126, 2156

20. What is Quantum Tunneling of the Magnetization?
 In zero field, the levels on the left and right sides of the potential energy double well are degenerate.
 In the absence of transverse anisotropy, the energy eigenstates of the systems are the pure MS states.
 As soon as there is a transverse anisotropy, the MS states are no longer the energy eigenstates.
 Near the bottom, the eigenstates are then the superpositions of the MS states on the left and right of the potential energy
double well.
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21.  The eigenstates are now |MS>+|-MS> and |MS>-|MS>.
 The splitting between them is called tunnel splitting (Δ).
 In a static picture one can say that the system is located both on the left and right of the energy barrier.
 In a dynamic picture one can say that the system oscillates coherently between the two sides at a frequency Δ until coupling
to the environment destroys coherence.
What is Quantum Tunneling of the Magnetization?
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22. What is Quantum Tunneling of the Magnetization?
There are three ways that the magnetisation can invert.
1. Thermal relaxation
2. Thermally- (phonon-) assisted tunneling
3. Ground state tunneling.
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23. What is Quantum Tunneling of the Magnetization?
At certain levels the microstates cross.
The transverse anisotropy mixes the Ms levels at those
fields making tunneling of the magnetisation possible
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Lapo Bogani & Wolfgang Wernsdorfer, “Molecular spintronics using single-molecule magnets”, Nature Materials, 7, 179 - 186 (2008)

24. What is Quantum Tunneling of the Magnetization?
In the double well picture :
• At zero field, the Ms levels on the left and right are in resonance.
• Application of a magnetic field lifts this degeneracy.
• At a certain field levels on the left and right come into resonance again.
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What properties should we be trying to improve to make
future applications more feasible?
Raise the blocking temperature (TB)
Shut down quantum tunneling at zero field
Overcome the instability in water

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So, how are researches going on to raise TB?
The barrier to magnetization relaxation in SMMs is not due to intermolecular interactions but to zero-field splitting (ZFS).
Requirements for SMMs :
1. Large ground state spin (S)
2. Negative ZFS parameter (D)
But we have to keep in mind!
A) Keep the molecular and crystal symmetry high to minimize tunnelling through the barrier.
B) Ensure excited states are far above the ground state.
C) Control/eliminate those damn solvent molecules of crystallization!
So either increase S : -- increase the metal content (i.e. the molecular size)
-- ensure ferromagnetic coupling
and (or) increase D : -- incorporate lanthanides

27. Advantages of single molecule magnets?
Materials science:
• One molecule can be seen as one bit.
• This leads to unprecedented data densities.
• Conventional materials are reaching the superparamagnetic limit.
Physics:
• These systems are in between classical and quantum magnetic systems.
• They show distinct quantum properties.
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http://www.idema.org

28. One of the best future application of SMMs
Till the date the maximum data storage density a computer manufacturer can provide
3 billion bits (3 GB) / cm2
Our SMM can give
30 trillion bits ( 30 TB) / cm2
10000 times!
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On this day in
Chemistry
December 22nd
Swiss physicist Raoult-Pierre Pictet made
liquid oxygen for the first time on this day in
1877
The experiment was notified to the Academy of Sciences in Paris in a
telegram from Pictet in Geneva: “Oxygen liquefied to-day under 320
atm. And 140 degrees of cold by combined use of sulphurous and
carbonic acid”.

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