Single Molecular Magnets: A very Basic Approach to Understand

SINGLE MOLECULAR MAGNETS AN ANOTHER INDEX FOR THE SMART FUTURE OF MANKIND

Subhamoy SahaTSO-60th Batch

HBNI

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What causes to magnetism?

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Diamagnetism: Property of all

matter

Uncompensated orbital &

angular momentum

Permanent atomic

moments

Independent atomic

moments

Ideal paramagnetism

Co-operating atomic

moments

Ferromagnetism

Antiferromagnetism Ferrimagnetism

Electronic bands in metals

Pauli spin paramagnetism

Band anti-ferromagnetis

m

Band ferromagnetis

m

Hierarchy of types of magnetism

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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 curvesMagnetochemistry by Dutta, Shamal

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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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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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What is a Single Molecular Magnet (SMM)?

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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

Mn84

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Let us have a look of some properties of SMMs

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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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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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Some famous SMMs

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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 = 28Mn3+ = 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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Fe ions are linked by O2- and OH- bridges.

tacn ligands are bound on the outside of themolecule.

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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What is the origin of the slow relaxation of the magnetization?

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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.

Solid State Commun., 127, 131139, 2003

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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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What is QuantumTunneling of theMagnetization?

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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?

M. Soler, et al. J. Am. Chem. Soc., 2004, 126, 2156

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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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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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What is Quantum Tunneling of the Magnetization?

There are three ways that the magnetisation can invert.1. Thermal relaxation2. Thermally- (phonon-) assisted tunneling3. Ground state tunneling.

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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

Lapo Bogani & Wolfgang Wernsdorfer, “Molecular spintronics using single-molecule magnets”, Nature Materials, 7, 179 - 186 (2008)

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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

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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.

http://www.idema.org

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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 inChemistry

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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