---
title: "Magnetic domains in an external field"
subject: "Electrical Engineering Material"
chapter: 4
key: "semester_3/electrical_engineering_material:domains-in-external-field"
source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/domains-in-external-field
site: BCT Engineering
---

# Magnetic domains in an external field

Electrical Engineering Material · Chapter 4: Magnetic Materials

- Asked in: 2079 Bhadra, 8 marks; 2078 Kartik, 4 marks; 2071 Chaitra, 1+3 marks (asked 3 times)
- Video: [Ferromagnetism & curie temperature | Magnetism & matter | Physics | Khan Academy](https://www.youtube.com/watch?v=8a2IZZdIeXw) — Khan Academy India - English
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/domains-in-external-field
- Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4

## Question

What is a magnetic domain? Explain the behavior of magnetic domains in presence of external field.

## Short answer

In an external field, domains with magnetization aligned to the field grow by reversible wall motion at low fields, then irreversible wall jumps cause large magnetization increase, and finally domain rotation aligns moments to saturation, leaving remanence when field is removed.

## Answer

A **magnetic domain** is a small region of a ferromagnetic (or ferrimagnetic) material in which all atomic magnetic moments are aligned in the same direction because of the exchange interaction, so the region is magnetized to saturation. Neighbouring domains are separated by **domain walls**. In an unmagnetized body the domains point in random directions and cancel.

#### Behaviour of domains in an external field
As the applied field $H$ increases, the magnetization changes in stages that match the parts of the initial B–H curve.

```text
  B
  ^                 ______ saturation
  |            ___/  (3) rotation
  |          /
  |         / (2) irreversible wall
  |        /      motion (Barkhausen)
  |      _/
  |   _/  (1) reversible wall motion
  +-------------------------------> H
```

1. **Zero field**: domains are randomly oriented; net $M = 0$.
2. **Weak field: reversible domain wall movement**
   - Domains whose magnetization is close to $H$ have lower energy, so they grow by small shifts of their walls; unfavourable domains shrink.
   - If $H$ is removed, walls return to their original places. The curve is gentle and reversible.
3. **Medium field: irreversible domain wall movement**
   - Walls break free from impurities, voids and grain boundaries and move in sudden jumps (**Barkhausen effect**).
   - Favourable domains grow rapidly by eating unfavourable ones; $B$ rises steeply.
   - On removing $H$ the walls do not return completely; this gives **remanence** and **hysteresis**.
4. **Strong field: domain rotation**
   - Now most of the crystal is one domain, magnetized along an easy axis close to $H$.
   - Further increase in $H$ rotates the magnetization away from the easy axis into the exact direction of $H$, against anisotropy energy.
   - When all moments are parallel to $H$, the material reaches **saturation** $M_s$.

```text
 H = 0        weak H ->     strong H ->   very strong
 [^][>]       [^ ][>>>]     [>>>>>>]      [-->-->-->]
 [<][v]       [< ][>>>]     [>>>>>>]      aligned with H
 random       walls move    one domain    rotated (Ms)
```

If the field is then reduced to zero, the domain structure does not fully go back, leaving residual magnetism $B_r$.

## Check yourself

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

1. A magnetic domain is:
   A. A region with all moments aligned, saturated
   B. A region with random moments
   C. A region with no moments
   D. An air gap
   Answer: A. It is separated from neighbours by walls. (Key point: Magnetic domain)

2. In a weak field:
   A. Domains rotate
   B. Favourable domains grow by small reversible wall shifts
   C. Walls jump irreversibly
   D. Material saturates
   Answer: B. Removing H returns walls. (Key point: Weak field)

3. The Barkhausen effect refers to:
   A. Sudden jumps of walls past impurities, causing steep B rise and hysteresis
   B. Slow reversible wall movement
   C. Domain rotation
   D. Loss of magnetism at Tc
   Answer: A. Walls break free from impurities and jump. (Key point: Medium field)

4. Strong-field stage involves:
   A. Domain nucleation
   B. Random reorientation
   C. Rotation of magnetization from easy axis to H against anisotropy
   D. Wall pinning
   Answer: C. This leads to saturation Ms. (Key point: Strong field)

## Related in Electrical Engineering Material

- [Domains, domain walls and magnetic losses](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adomain-walls-magnetic-losses) — Electrical Engineering Material, chapter 4
- [Domain walls and demagnetization process](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adomain-walls-demagnetization) — Electrical Engineering Material, chapter 4
- [Closure domains and demagnetization](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aclosure-domains-demagnetization) — Electrical Engineering Material, chapter 4
- [Critical magnetic field and critical current](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Acritical-field-current-relation) — Electrical Engineering Material, chapter 5
- [Strong magnetic field and critical current](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Acritical-current-strong-field) — Electrical Engineering Material, chapter 5, asked 5×
- [Demagnetization by domains and deperming](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Ademagnetization-deperming) — Electrical Engineering Material, chapter 4

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