## What is a demagnetization curve?

Ferromagnetic materials such as iron, cobalt, nickel, and many alloys, including permanent magnets, show a **magnetic hysteresis** phenomenon. The hysteresis loop is generated by measuring the magnetization **M**, while changing the applied external magnetic field **H** (see green curve in the figure to the left).

The magnetization in a ferromagnetic material increases from 0 or ‘virgin’ state (point ⓪ as show in the figure to the left) due to an increasing applied magnetic field, to a maximum--the point where the magnetization does not increase anymore, called magnetic saturation (point ①). Upon reducing the magnetic field back to 0, permanent magnets still retain a high value of magnetization (point ②) also known as **B _{r}**. The magnetization will eventually decrease to 0 (point ③) by applying a magnetic field in the opposite direction. The magnetic saturation in the opposite direction will occur (point ④) with increasing magnetic field in that direction.

## Why is the demagnetization curve important?

Important magnetic properties of permanent magnets, such as residual induction **B _{r}**, coercivity

**H**, intrinsic coercivity

_{c}**H**, and maximum energy product

_{ci}**(BH)**, can be obtained from demagnetization curves.

_{max}**B _{r} **is the residual induction or flux density, which represents the magnetic induction corresponding to zero magnetizing field in a magnetic material after full saturation when measured in a closed magnetic circuit. The unit for magnetic induction

**B**and residual induction

**B**is

_{r}**G**, Gauss (CGS unit) or

**T**, Tesla (SI unit).

**M _{r}** is the remanent magnetization or remanence, which represents the retained magnetization when the external magnetic field is reduced to zero after full saturation when measured in a closed magnetic circuit.

The unit for magnetization **4πM** and remanent magnetization **4πM _{r}** is

**G**, Gauss (CGS unit) or

**A/m**, Ampere per meter (SI unit)...