Lectures 15&16 (Magnetisation Dynamics & Reversal, Domain Nucleation and DW Propagation) Flashcards

1
Q

How many domains do SW particles have?

A

1

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2
Q

How does magnetisation switch in SW particles?

A

Coherent rotation

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3
Q

What happens to Hc in samples with more than one domain? Why?

A

Hc much less than Hsat because M reverses by domain nucleation and DW motion

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4
Q

What point will be the first to reverse M in a multi-domain particle?

A

A defect, edge or point of low anisotropy

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5
Q

How does M reversal broadly happen in multi-domain particles?

A

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

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6
Q

In which materials can DWs move rapidly?

A

Hard, high coercivity

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7
Q

In which materials do DWs move slowly?

A

High coercivity, hard

Many grains and defects - microstructure critical to M reversal

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8
Q

What is total energy density made up of?

A

Exchange + Anisotropy + Demagnetising + Zeeman + Stress

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9
Q

Is a magnetic moment angular momentum?

A

Yes

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10
Q

What does the damping term in the LLG equation do?

A

Dissipates energy so precession spirals towards the energy minimum

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11
Q

What is the timescale for switching a macrospin?

A

Half a precessional period

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12
Q

How does magnetisation reversal occur in a single-domain particle with H applied 180º to M?

A

Thermal fluctuations enable a torque on M

Precession and damping about H until M spirals into equilibrium (i.e., M || H)

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13
Q

What happens to switching time in a thin film when H is applied in plane, 90º to M?

A

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

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14
Q

How to find the critical size of the nucleation volume?

A

Free energy of droplet F(r)

∂F/∂r = 0

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15
Q

How to find the energy to sustain a nucleation volume?

A

Free energy of droplet F(r)

∂F/∂r = 0

Subs rc into F(r) = F(rc)

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16
Q

What is F(rc) of a nucleation volume equal to?

A

KV

17
Q

How to find nucleation field?

A

Free energy of droplet F(r)

∂F/∂r = 0

Subs rc into F(r) = F(rc)

Rearrange for H

18
Q

What are the two terms in the free energy of the nucleation dropet?

A

Surface energy of the droplet, and volume energy

19
Q

How to measure M, K, and A?

A

M - Magnetometer that sense stray field from sample - trace back to M

K - measure hysteresis loops in different directions

A - harder. M dynamics experiment (M perturbed and see response as a function of time)

(from this can find DW width and E)

20
Q

How to get steady DW motion?

A

Manage to switch H off before complete precessional period

21
Q

How does the motion before and after the Walker breakdown differ?

A

Before - initially slower motion, but reaches a higher peak in DW velocity

After - too high a field / field to long - losing E through precessing spin rather than driving the wall, so velocity suffers

22
Q

What can drive DW motion?

A

Current or field

23
Q

What torques are produced in current-induced DW motion?

A

Adiabatic spin transfer torque

Non-adiabatic transfer torque

24
Q

What is the nonadiabatic transfer torque?

A

Acts like a field

(Acts in the direction of the torque we normally get)

25
Q

What is the adiabatic transfer torque?

A

Perpendicular to the non-adiabatic (same way as the damping), it distorts the velocity curve