---
title: "Domain wall and domain wall motion"
subject: "Electrical Engineering Material"
chapter: 4
key: "semester_3/electrical_engineering_material:domain-wall-motion"
source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/domain-wall-motion
site: BCT Engineering
---

# Domain wall and domain wall motion

Electrical Engineering Material · Chapter 4: Magnetic Materials

- Asked in: 2074 Asoj, 6 marks
- Video: [Magnetic Domains {Texas A&M: Intro to Materials (MSEN 201)}](https://www.youtube.com/watch?v=koeDE-O18I8) — Patrick Shamberger
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/domain-wall-motion
- Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4

## Question

What is a domain wall? How does a domain wall motion occur?

## Short answer

A domain wall is a thin transition layer between domains where spins gradually rotate. Motion occurs when an external field makes favourably oriented domains grow: at low fields walls bulge reversibly; at higher fields they break free from defects in irreversible jumps (Barkhausen effect); finally domains rotate into the field direction for saturation.

## Answer

A **domain wall** (Bloch wall) is the thin boundary layer, typically a few hundred atomic spacings thick, between two magnetic domains magnetized in different directions. Inside the wall the atomic moments turn gradually, a little from atom to atom, from the direction of one domain to that of the next. A wall has energy (exchange + anisotropy), and its width is set by the balance between them: exchange energy favours a wide wall; anisotropy energy favours a narrow one.

```text
 domain A        wall           domain B
 ^ ^ ^ ^ |  ^  /  ->  \   v  | v v v v
         |<- spins rotate ->|
```

#### How domain wall motion occurs
When an external field $H$ is applied, the domains magnetized close to the direction of $H$ have lower energy ($-\mu_0 M\cdot H$) than the others. The material lowers its energy by enlarging these favourable domains. This happens by moving the walls: spins at the edge of the wall rotate one after another towards the favourable direction, so the wall shifts into the unfavourable domain.

```text
 H = 0         H ->              H -> (larger)
 [ ^ | v ]     [ ^ ^ | v ]       [ ^ ^ ^ |v]
  wall at       wall moves        favourable
  centre        right             domain grows
```

**Stages**
1. **Reversible wall motion (low field)**: walls bulge or shift slightly but stay held by defects. Removing $H$ returns them; the initial part of the B–H curve is gentle.
2. **Irreversible wall motion (medium field)**: walls break free from impurities, voids, dislocations and grain boundaries and jump suddenly to new positions. These jumps (**Barkhausen effect**) can be heard as clicks with a coil and amplifier. B rises steeply, and after $H$ is removed the walls stay in new positions, causing **remanence** and **hysteresis loss**.
3. **Domain rotation (high field)**: after the walls have swept through, the remaining domain rotates into the field direction, giving saturation.

**Importance**: easy wall motion (pure, annealed, large-grain material) gives soft magnetic behaviour; walls pinned by fine particles and defects give hard magnetic behaviour.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Inside a domain wall, the atomic moments:
   A. Turn gradually from atom to atom from one domain direction to the other
   B. All point in the field direction
   C. Are zero
   D. Reverse abruptly between adjacent atoms
   Answer: A. A Bloch wall is a gradual rotation of moments. (Key point: Wall structure)

2. What sets the width of a domain wall?
   A. Only the applied field
   B. Balance between exchange energy (favours wide) and anisotropy energy (favours narrow)
   C. Only the temperature
   D. Magnetostatic energy alone
   Answer: B. Exchange wants a wide wall, anisotropy a narrow one. (Key point: Wall width)

3. Why do walls move when a field H is applied?
   A. Walls are attracted to impurities
   B. Domains oppose the field and shrink
   C. Domains near the direction of H have lower energy, so they grow at the expense of others
   D. Moments lose their exchange coupling
   Answer: C. Lower energy -μ0 M·H in favourable domains drives their enlargement by wall motion. (Key point: Why walls move)

4. What happens in irreversible wall motion at medium fields?
   A. Walls return to original positions when H is removed
   B. Domains rotate to saturation
   C. No energy is lost
   D. Walls break free from defects and jump suddenly (Barkhausen effect), leaving remanence and hysteresis loss
   Answer: D. Sudden jumps past pinning sites are not reversible, causing remanence and loss. (Key point: Barkhausen jumps)

## Related in other subjects

- [Force for just motion of block B](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Ablock-b-just-motion-force-p-15deg) — Applied Mechanics, chapter 5
- [Force P to start motion of bar and block](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Abar-ab-pin-block-force-p-start-motion) — Applied Mechanics, chapter 5
- [Two oscillating masses: motion, centre of mass, energy](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_mechanics_dynamics%3Atwo-masses-springs-equations-energy) — Applied Mechanics (Dynamics), chapter 3
- [Velocities in plane motion by instantaneous centre](https://www.bctengineer.com/question.md?key=semester_6%2Ftheory_of_mechanism_and_machine_i%3Aladder-sliding-wheel-rolling-velocities) — Theory of Mechanism and Machine I, chapter 7