Lectures 15&16 (Magnetisation Dynamics & Reversal, Domain Nucleation and DW Propagation) Flashcards
How many domains do SW particles have?
1
How does magnetisation switch in SW particles?
Coherent rotation
What happens to Hc in samples with more than one domain? Why?
Hc much less than Hsat because M reverses by domain nucleation and DW motion
What point will be the first to reverse M in a multi-domain particle?
A defect, edge or point of low anisotropy
How does M reversal broadly happen in multi-domain particles?
A small reverse domain nucleates (at a defect, edge, other point of low K) at a nucleation field Hn<Hc
At slightly higher field the domain expands rapidly (DW propagation) Hp<Hc
In which materials can DWs move rapidly?
Hard, high coercivity
In which materials do DWs move slowly?
High coercivity, hard
Many grains and defects - microstructure critical to M reversal
What is total energy density made up of?
Exchange + Anisotropy + Demagnetising + Zeeman + Stress
Is a magnetic moment angular momentum?
Yes
What does the damping term in the LLG equation do?
Dissipates energy so precession spirals towards the energy minimum
What is the timescale for switching a macrospin?
Half a precessional period
How does magnetisation reversal occur in a single-domain particle with H applied 180º to M?
Thermal fluctuations enable a torque on M
Precession and damping about H until M spirals into equilibrium (i.e., M || H)
What happens to switching time in a thin film when H is applied in plane, 90º to M?
M precesses out of plane. There are positive charges on one surface and negative on the other.
This creates a large demagnetising field perpendicular to the plane
The magnetisation then precesses around this - decreasing switching time
How to find the critical size of the nucleation volume?
Free energy of droplet F(r)
∂F/∂r = 0
How to find the energy to sustain a nucleation volume?
Free energy of droplet F(r)
∂F/∂r = 0
Subs rc into F(r) = F(rc)