8a: Single molecule magnets

Question 1

Magnetism general

Answer 1

-a bulk property- each metal has a spin/magnetic moment - the property is dependant of the alignment of this RELATIVE to its environments
-Paramagnetic materials-a series of metal ions with a magnetic moment caused by unpaired electrons
-Ferromagnetic materials- the magnetic moments align parallel to each other under the curie temperature
-Antiferromagnetic materials - the magnetic moments align Antiparallel to each other under the Neel temperature
lead to rapid increase or decrease in magnetic susceptibility 

Question 2

Define a single molecule magnet

Answer 2

1)A molecule that can be magnetized in a magnetic field and will remain magnetized even after the magnetic field is switched off
2) Magnetism is a property of the molecule itself. No interaction between molecules are required for the phenomenon.
3) Slow relaxation of the magnetisation is of molecular origin
4) The molecule can be dissolved in a solvent or matrix and will still show this property


Question 3

Theory behind the SMM

Answer 3

Total spin of the molecule
=number of unpaired spins*0.5 consider if some ion spin opposes another
This total spin, gives rise to 2S+1 microstates in a double potential energy well
The ground states of the double energy well are degenerate and comprise of orientation of the magnetic anisotropy in opposite directions
S=+10 or S=-10 which can be thought of as the molecule being spin up or spin down
Picture from slide

Question 4

Example

Answer 4

Mn4+ S= 3/2 and Mn3+ =2
4xMn4+ and Mn(3+)
The spinds of the two sets of Mn ions are opposed 
S=16-6=10
number of microstates =21

Question 5

What happens upon applying a magnetic field?

Answer 5

-system can be peturbed to remove the degeneracy of the ground states and populate a single groundstate (S=10 OR S=-10)
-once the external field is removed the molecule will retain its magnetism as long as the potential well is deep enough
(so that a very large amount of activation energy is required to reform an equilibrium mix of ground states

Question 6

Why (currently) can SMM’s only exist at cold temperatures

Answer 6

The current activation energies are very small and so SMM behaviour can only be observed at low temperature
activation barrier U_eff 100 cm^-1
blocking temperature T_B- the temperature at which magnetic relaxation takes 100s - usually 50K

Question 7

How to maximise the activation barrier

Answer 7

• Maximise S- the total spin of the molecule
• Maximise D- the zero-field splitting parameter which is the energy difference between the ground states - due to the effect of the local electronic environment 

Question 8

What are the other mechanisms for the equilibration of the + and - states

Answer 8

• Thermal activation
• Ground State tunnelling can occur from one well from another
• Excited State tunnelling can occur from one well to another 

Question 9

Lanthanide and actinide examples

Answer 9

Ln and An are beginning to be studied in detail
producing large activation energies for magnetic relaxation Ueff 250 cm-1 cf. 100 ]
eg. [TbPc2][NBu4]

Question 10

Why study SMM?

Answer 10

• they could be extremely useful as switches and hence contribute to novel data storage techniques
• challenges current temperature requirements and inclusion into a material with single molecules aligned