Lectures 11&12 (SPM, Stoner-Wohlfarth Particles & Microscopic Anisotropy Origins) Flashcards
What does uniaxial anisotropy mean?
Particle has a single easy axis
Energy of a particle with uniaxial anisotropy?
E = kVsin^2ø = µ0µHcosø
What is the super-paramagnetic limit/effect?
Need particles large enough to be stable (~100 years)
But this limits density at which they
As well as increasing temperature, how can SPM be induced?
Decreasing particle volume
Does SPM happen at large or small temperatures?
Large
Will an experiment faster than 1/f0 see a FM or SPM?
FM
How can the SPM limit be resolved?
Heat assisted magnetic recording
Bit patterned media
(Thought to increase K but then it’s hard to write the information to begin
Instead localised heating e.g., plasmons)
What is blocking temperature?
Temperature at which the time of the experiment ~ tau
What happens below blocking temperature?
Particles are ‘blocked’ and cannot switch between minima
How to find T_B?
- Measure M(T) in a fixed direction
- Field cool or Zero field cool
What is field cool (FC)?
Cool in magnetic field, so that M is frozen in the direction of H.
Measure nothing at blocking temp. since M switches a lot at this temp (reached SPM)
What does field cool graph look like?
M vs T
M diverges to 0 when it reaches blocking temperature
What is zero field cool?
Cool sample in H=0 field, then warm it in a small field
At the blocking temperature FM -> SPM - the random directions unfreeze at blocking temperature so there is a brief increase in M before thermal fluctuations take over again
What does zero field cool graph look like? Why?
M vs T
Spike in M at TB
Since the random magnetisation directions unfreeze at T_B, increasing M before thermal fluctuations take over again
Does SPM occur well below Tc?
It can do, yes
