---
title: "Magnetic moments; ferro vs ferrimagnetic materials"
subject: "Electrical Engineering Material"
chapter: 4
key: "semester_3/electrical_engineering_material:dipole-moment-ferro-ferri"
source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/dipole-moment-ferro-ferri
site: BCT Engineering
---

# Magnetic moments; ferro vs ferrimagnetic materials

Electrical Engineering Material · Chapter 4: Magnetic Materials

- Asked in: 2078 Kartik, 2+4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/dipole-moment-ferro-ferri
- Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4

## Question

Define magnetic dipole moment and atomic magnetic moment. Differentiate between ferromagnetic material and ferrimagnetic material.

## Short answer

Magnetic dipole moment m = IA (A·m²) measures a dipole’s strength; atomic magnetic moment from unpaired electron spin and orbital motion is measured in Bohr magnetons (μB). Ferromagnetic materials have all parallel spins, high saturation (e.g., Fe); ferrimagnetic materials have antiparallel unequal sublattices, lower saturation, high resistivity (e.g., ferrites).

## Answer

#### Magnetic dipole moment
A **magnetic dipole moment** $m$ is a measure of the strength of a magnetic dipole (current loop or pair of poles). For a current $I$ circulating in a loop of area $A$:
$$
m = IA \quad (\text{unit: A m}^2)
$$
It points normal to the loop by the right-hand rule. In a field $B$ the dipole experiences a torque $\tau = m \times B$.

#### Atomic magnetic moment
The **atomic magnetic moment** is the net magnetic moment of an atom, arising from (i) the **orbital motion** of electrons round the nucleus, (ii) the **spin** of electrons, and (iii) a very small nuclear spin contribution. Filled shells cancel, so only unpaired electrons contribute. Its natural unit is the **Bohr magneton**:
$$
\mu_B = \frac{e\hbar}{2m_e} = 9.27\times10^{-24}\ \text{A m}^2
$$
Example: an iron atom has about $2.2\,\mu_B$ in the metal.

#### Ferromagnetic vs ferrimagnetic materials

| Point | Ferromagnetic | Ferrimagnetic |
|---|---|---|
| Spin arrangement | All moments parallel | Two sublattices antiparallel, unequal |
| Net magnetization | Sum of all moments, very high | Difference of sublattices, moderate |
| Saturation $M_s$ | High (Fe ≈ 2.1 T) | Lower (ferrite ≈ 0.3–0.5 T) |
| Material type | Metals and alloys | Ceramic oxides (ferrites) |
| Resistivity | Low (~$10^{-7}$ Ω m) | Very high (1–$10^{6}$ Ω m) |
| Eddy-current loss | High at high frequency | Very low |
| Critical temperature | Curie temperature $T_C$ | Curie (Néel) temperature |
| Uses | Power transformer and motor cores, magnets | HF transformers, inductors, antennas, microwave devices |
| Examples | Fe, Co, Ni | Fe₃O₄, MnZn and NiZn ferrites |

```text
 Ferro:  ^  ^  ^  ^       Ferri:  ^  v  ^  v
                                   (large ^, small v)
```

## Check yourself

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

1. For a current I in a loop of area A, the magnetic dipole moment is:
   A. m = IA, in A m^2, pointing normal to the loop
   B. m = I/A, in A/m^2
   C. m = IA, in T
   D. m = I^2A, in A m
   Answer: A. Dipole moment is current times loop area, directed by the right-hand rule. (Key point: Dipole moment)

2. Which contributions give an atom its net magnetic moment?
   A. Only the charge of the nucleus
   B. Orbital motion of electrons and electron spin (plus a tiny nuclear spin part)
   C. Only filled-shell electrons
   D. Only the mass of the atom
   Answer: B. Orbital and spin motion of unpaired electrons give the atomic moment; filled shells cancel. (Key point: Atomic moment origin)

3. How do the spin arrangements differ in ferromagnetic and ferrimagnetic materials?
   A. Ferro: antiparallel equal; ferri: all parallel
   B. Both have all moments parallel
   C. Ferro: all moments parallel; ferri: two antiparallel sublattices of unequal strength
   D. Ferro: random; ferri: antiparallel equal
   Answer: C. Ferrimagnets have unequal antiparallel sublattices, so the net moment is the difference. (Key point: Ferro vs ferri spins)

4. Why are ferrimagnetic ferrites used at high frequency instead of ferromagnetic metals?
   A. They have higher saturation magnetization
   B. They have lower resistivity
   C. They have no critical temperature
   D. They have very high resistivity so eddy-current loss is very low
   Answer: D. Ceramic ferrites have resistivity of 1 to 10^6 Ω m, giving very low eddy loss. (Key point: Eddy loss)

## Related in Electrical Engineering Material

- [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
- [Ferrimagnetic versus antiferromagnetic materials](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aferri-vs-antiferromagnetic) — Electrical Engineering Material, chapter 4
- [Hard vs soft magnetic materials for relays](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Ahard-soft-relay) — Electrical Engineering Material, chapter 4
- [Ferromagnetic versus antiferromagnetic materials](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aferro-vs-antiferromagnetic) — Electrical Engineering Material, chapter 4
- [Hard vs soft magnetic materials by hysteresis](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Ahard-soft-hysteresis) — Electrical Engineering Material, chapter 4

## Related in other subjects

- [Circular coil field and magnetic dipole](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Acircular-coil-axis-dipole) — Engineering Physics, chapter 8, asked 3×
- [Soft versus hard magnetic materials](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Asoft-hard-magnetic-materials) — Electric Machine Design, chapter 1, asked 5×
- [Hysteresis loop energy, hard and soft magnetic materials](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ahysteresis-area-energy-hard-soft) — Electrical Machines I, chapter 1
- [Beam thrust and bending moment diagrams with max moments](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Abeam-thrust-bmd-2kn-5kn-1knm) — Strength of Materials, chapter 1