Residual magnetism refers to the remaining magnetization in a circuit after removing the external magnetic field. This article describes the residual magnetism and its types.
Table of contents
What is Residual Magnetism?
We will understand the residual magnetism with the help of the BH curve or hysteresis curve. A magnetic material’s magnetization curve( BH curve) is given below.
When an electric current passes through a coil of wire, it creates an electromagnetic field that results in the magnetization of a material. If the coil is wrapped around a magnetic core, like iron, the core becomes magnetized, and the magnetic moment in the material gets aligned.
We apply the current in one direction and increase the current to increase the flux density. Does the magnetic flux density keep increasing with an increase in current? The magnetic flux density increases up to a certain point till the material gets fully magnetized. Being fully magnetized means all the dipoles of magnetic material align, and if we increase the current, the magnetic flux density does not increase. We can say that the material is fully saturated and can not hold any more flux density. The point “a” on the graph is the saturation point.
After increasing the current to get flux density corresponding to point “a,” we decrease the current to demagnetize the material. The magnetic field intensity(H) decreases with a current decrease and follows path “abc.”
The above graph shows that when the current decreases and reaches zero, the magnetic field does not reach zero and has a magnetic field “ob.” The magnetic field “ob” after removing the current is called residual magnetism.
To demagnetize the magnetic material, you need to reverse the magnetizing force H by changing the direction of the current flow. The “oc” current causes demagnetization of the material.
From the above clarification, we can say that the magnetic material retains magnetization after removing the magnetizing force, and this retained magnetization is called residual magnetization.
The magnetic flux passes through the cores of transformers, motors, and generators. After power is cut, the core retains residual magnetism, also known as remanence. The terms residual magnetism and romance are often used interchangeably.
Types of Remanence or Residual Magnetism
Types of remanence are as follows.
- Saturation Remanence
- Isothermal Remanence
- Anhysteretic Remanence
- Thermal Remenance (TRM)
- Chemical Remanence (CRM)
- Viscous Remanence (VRM)
1. Saturation Remanence (SIRM)
The maximum remanence that a material can achieve is the saturation remanence. The saturation remanence in material occurs when the external magnetic field is applied to magnetize fully, and the external field is removed. Saturation remanence is denoted by the letter Mr; it is also called saturation Isothermal Remanence, denoted by Mrs.
2. Isothermal Residual Magnetism (IRM)
Isothermal remanence (IRM) is magnetic remanence that occurs during the magnetization of a material under constant temperature conditions. A strong magnetic field at a constant magnetic field is applied to the material, and the magnetic field is removed.
This method depends on the magnetic properties of the material. The first step involves magnetizing the material in an alternating magnetic field by applying magnetizing field H and then removing it. It is also known as Initial Remanence.
The Isothermal Remanence is of two types.
DC Demagnetization Remanence
This method is also known as DC Demagnetization residual magnetism and is denoted by Md(H). The process involves magnetizing the magnet in one direction by applying an electric current until it reaches saturation. Then, the current is applied in the opposite direction, and the magnetic field is removed.
AC Demagnetization Remanence
The material’s magnetization is achieved by applying AC until the saturation point, denoted by Ma(H).
3. Anhysteretic Residual Magnetism (ARM)
This method applies a large alternating magnetic field and a small DC bias field to the material. When the amplitude of the alternating field is gradually reduced to zero, remanence is obtained. Finally, the bias DC field is removed from the circuit to complete the process.
4. Thermal Remenance (TRM)
When the material is heated in the presence of the magnetic field and then cooled, it causes magnetic remanence.
5. Chemical Remanence (CRM):
The chemical changes such as oxidation and reduction cause magnetic remanence in the material.
6. Viscous Remanence (VRM)
When the material is kept under magnetization for a long time, the magnetic moments align with the magnetic field and may remain in this state after removing the magnetic field.
Reduction of Residual Magnetism
The following are the ways to reduce residual magnetism.
- Hot-rolling involves processing materials at elevated temperatures. Exposure to higher temperatures above their Curie point potentially reduces magnetic remanence. The magnetic remanence of hot rolled materials is reduced by up to 50%.
- The excessively high excitation current drives the material into saturation during the initial magnetization; in that case, the material has less magnetic remanence.
- A constant force (or magnetic field strength) should be applied when magnetizing a material to ensure a controlled and gradual buildup of magnetization. The force should be gradually increased until the material reaches a state of saturation. The applied force (or magnetic field) should be slowly reduced to demagnetize the material. Thus, we can reduce the magnetic remanence by Controlled Magnetization, Avoiding Sudden Changes, Saturation for Maximum Magnetization, and Controlled Demagnetization.
- The magnetic remanence decreases when the electric force or current for the magnetization and demagnetization process has the same magnitude.
- Soft magnetic materials, like silicon steel, have low remanence. They are used in transformers and inductors where minimizing residual magnetization is essential.
Types of Materials and their Residual Magnetism
The table below shows the remanence of different materials.
| Type of Materials | Remenance |
| Ferromagnetic Materials | High |
| Ferrimagnetic Materials | Moderate |
| Antiferromagnetic | Low |
| Paramagnetic Materials | Low |
| Soft Magnetic Materials | Low |
| Hard Magnetic Materials | High |
| Magnetic Alloys | Variable ( Low or High) |
| Non-Magnetic Materials | Zero |
Factors Affecting the Residual Magnetism
The following factors affect the residual magnetism of the materials.
Material Type: The remanence of magnetic materials depends on their type. Ferromagnetic materials have higher remanence than paramagnetic materials, while hard magnetic materials have higher remanence than soft magnetic materials.
Magnetic Structure: Materials with well-defined magnetic domains and a high degree of alignment have higher remanence.
Cyclic Magnetization: The cyclic magnetization or exposure to varying field strengths affects the remanent magnetization.
External Magnetic Field Strength: The strength of the external magnetic field during magnetization affects the level of remanence. A higher magnetic field strength can result in better alignment of magnetic domains, leading to increased remanence.
Temperature: Heating a material can cause changes in its magnetic structure and affect remanence.
The remanence of materials is also affected by factors such as stress, strain, coercivity, and external interference.