---
title: "Ferro vs antiferromagnetic; magnetostriction"
subject: "Electrical Engineering Material"
chapter: 4
key: "semester_3/electrical_engineering_material:antiferro-magnetostriction"
source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/antiferro-magnetostriction
site: BCT Engineering
---

# Ferro vs antiferromagnetic; magnetostriction

Electrical Engineering Material · Chapter 4: Magnetic Materials

- Asked in: 2082 Kartik (new course), 5 marks
- Video: [4  Ferro Antiferromagnetism 2](https://www.youtube.com/watch?v=TPRDHgXUt3o) — Muhammad Farooq Nasir
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/antiferro-magnetostriction
- Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4

## Question

Distinguish between ferromagnetic and anti-ferromagnetic materials with suitable examples of each. Explain magnetostriction with neat diagram.

## Short answer

Ferromagnetic materials have parallel spin alignment and spontaneous net magnetization (e.g., Fe, Co, Ni). Antiferromagnetic materials have antiparallel spins, zero net magnetization (e.g., MnO, FeO). Magnetostriction is the change in dimensions of a ferromagnet when magnetized, caused by domain rotation and spin-orbit coupling, with strain λ ≈ 10⁻⁶–10⁻⁵ (Ni contracts).

## Answer

#### Ferromagnetic vs antiferromagnetic

| Point | Ferromagnetic | Antiferromagnetic |
|---|---|---|
| Spin alignment | Parallel | Antiparallel, equal |
| Net magnetization | Large, spontaneous | Zero |
| $\chi$ | Very large | Small, positive |
| Critical temperature | Curie $T_C$ | Néel $T_N$ |
| Hysteresis | Yes | No |
| Example | Fe, Co, Ni | MnO, FeO, NiO, Cr |

```text
 Ferro: ^ ^ ^ ^     Antiferro: ^ v ^ v
```

#### Magnetostriction
**Magnetostriction** is the change in the dimensions (length) of a ferromagnetic material when it is magnetized. It is measured by the strain $\lambda = \Delta l/l$, typically $10^{-6}$–$10^{-5}$.
- Cause: as domains rotate towards the field, the spin–orbit coupling changes the spacing of atoms along the magnetization direction.
- Nickel contracts ($\lambda < 0$); iron expands at low field.

```text
  H = 0:   |<------- l ------->|
           +-------------------+
           |      Ni rod       |
           +-------------------+

  H on:    |<---- l - dl ---->|
           +------------------+
           |/ / / coil / / / /|   H along the rod
           +------------------+
           lambda = -dl/l  (Ni contracts)
```

**Effects/uses**: hum of transformers at 100 Hz; sonar and ultrasonic transducers; sensors (Terfenol-D).

## Check yourself

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

1. Antiferromagnets differ from ferromagnets in that they have:
   A. Equal antiparallel moments, zero net magnetization and no hysteresis
   B. Parallel moments
   C. Large spontaneous M
   D. Curie temperature
   Answer: A. Their critical temperature is the Néel temperature. (Key point: Antiferro difference)

2. Magnetostriction is:
   A. Change in resistivity when heated
   B. The change in dimensions of a ferromagnetic material when magnetized
   C. Magnetization from stress only
   D. Loss of magnetism above Tc
   Answer: B. Strain λ = Δl/l is about 10^-6 to 10^-5. (Key point: Magnetostriction)

3. Magnetostriction arises because:
   A. Eddy currents heat the rod
   B. Atoms disappear
   C. Domain rotation changes atomic spacing through spin-orbit coupling
   D. Exchange becomes negative
   Answer: C. Atom spacing along the magnetization direction changes. (Key point: Cause)

4. Which statement is correct?
   A. Nickel expands greatly
   B. It only occurs in diamagnets
   C. It is zero for all metals
   D. Nickel contracts when magnetized, and it causes transformer hum
   Answer: D. Magnetostriction produces the 100 Hz hum in transformers. (Key point: Behaviour)

## Related in Electrical Engineering Material

- [Ferromagnetic versus antiferromagnetic materials](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aferro-vs-antiferromagnetic) — Electrical Engineering Material, chapter 4
- [Ferrimagnetic versus antiferromagnetic materials](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aferri-vs-antiferromagnetic) — Electrical Engineering Material, chapter 4
- [Magnetic moments; ferro vs ferrimagnetic materials](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adipole-moment-ferro-ferri) — Electrical Engineering Material, chapter 4
- [Ferrimagnetic versus ferromagnetic materials](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aferri-vs-ferromagnetic) — Electrical Engineering Material, chapter 4
- [Classification by susceptibility; ferro vs ferrimagnetic](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aclassify-susceptibility-ferro-ferri) — Electrical Engineering Material, chapter 4