Understanding Magnetization Curves (B-H Curves) and Magnetic Hysteresis

Introduction

In today's technologically advanced world, magnetization plays a fascinating role closely tied to the core principles of electromagnetism. It forms the foundation of many modern applications. Scientists and engineers must understand how magnetic materials behave under different conditions. In this blog, we explore the intriguing concept of Magnetic Hysteresis and dive into Magnetization Curves, commonly illustrated by the B-H Curve.

Graph of a magnetization curve on a brick background, showing key points: saturation, retentivity, coercivity, with magnetic flux density and field intensity axes.

Understanding Magnetization

Magnetization occurs when a material becomes magnetic in response to an external magnetic field. This phenomenon happens because the material’s atomic or molecular magnetic moments align with the field. In simpler terms, a material gets magnetized when its magnetic domains align under the influence of a magnetic field.

The Magnetization (B-H) Curve Demystified

The B-H curve, also called the magnetization curve or hysteresis loop, shows the relationship between a material's magnetic flux density (B) and magnetic field strength (H). To generate it, B is measured while varying H. Figure 1 illustrates the typical non-linear shape of a B–H curve, indicating that relative permeability `\left(\mu_r=\frac B{\mu_0H}\right)` is not constant but changes with flux density. As shown in Figure 2, cast steel follows a specific B–H curve.

Graph depicting the B-H curve of a magnetic material, showing magnetization. The curve rises steeply from point A to B, indicating saturation.

Figure 1

Graph on lined paper titled "μr-B curve." It shows a parabolic curve peaking at 1200 on the μr axis, with B values ranging from 0 to 1.6 Wb/m².

Figure 2

Here in the Fig.no 3 shows the B–H curves for several common magnetic materials.The B–H curve for a non-magnetic material appears in Figure 4, offering a clear contrast.Unlike magnetic materials, this curve forms a straight line because`B=\left(\mu_0\right)H` or

since the value of

`\mu_0`

remains constant.

B-H curve graph on grid paper, showing magnetization characteristics of sheet-steel, cast-steel, and cast-iron. Curves show increasing magnetization levels.

Figure 3

Graph titled "B-H curve for a non-magnetic material" shows a linear relationship between magnetic field strength (H) and magnetic flux density (B).

Figure 4

Key Components of the Magnetization (B-H) Curve

Magnetization Process

Consequently, the B-H curve has a starting point at (0,0) and it has a linear relationship in the early stage. This part of the curve is where the material reacts to a magnetic field which is progressively increasing. With the increase in the field strength, the same proportional response will occur in the magnetic material,leading to the increase of its magnetic flux density as well.

Saturation Point

But the B-H curve eventually levelling off at the saturation point. Upon reaching saturation, the material's longitudinal domains are in complete alignment, and causing the subsequent magnetic field strength increase does not result in a significant increase in magnetic flux density. At this point, the material reaches the best magnetic saturation given by its polarization.

Hysteresis Loop

The B-H curve forms a closed loop, indicating that once a material reaches magnetic saturation, reducing the magnetic field strength to zero does not return the material to its original state. Instead, the material retains some residual magnetization, which creates the characteristic hysteresis loop. This loop represents the region where energy loss occurs, revealing the material’s magnetic signature.

Applications and Significance of Magnetization (B-H) Curve

Material Selection in Electromagnetic Devices

Engineers and scientists use the magnetization curve (B-H) curve to selects materials for magnetic devices like transformers and inductor. Knowing whether materials behave differently or not in various magnetic fields allow us to design and build the most efficient ones.

Magnetic Storage Devices

Engineers also use the B-H curve to develop magnetic storage devices such as hard disk drives (HDDs). The characteristics of a material’s hysteresis loop directly influence how safely and stably it can store data.

Magnetic Sensors and Actuators

Even simple machines like magnetic sensors and actuators rely on the principles of magnetization and the insights provided by the B-H curve. By understanding the behavior of magnetic materials, designers can create sensors with high-precision readouts and actuators that deliver motion with low power consumption.

Magnetic Hysteresis

When a magnetic material undergoes one complete cycle of magnetization—first in one direction and then in the other—the magnetic flux density $B$ lags behind the applied magnetizing force $H$ . This lagging behavior is known as magnetic hysteresis.
Therefore, we refer to the lag of magnetic flux density $B$ behind the magnetizing force $H$ in a magnetic material as magnetic hysteresis.
The term hysteresis comes from the Greek word hysterein, which means “to lag behind.” To understand the full phenomenon of magnetic hysteresis, consider a ring made of magnetic material with a uniformly wound solenoid, as shown in Fig. 5. A DC source connects to the solenoid through a double-pole double-throw reversible switch, set initially to position ‘1’.
As the field intensity $H$ gradually increases—by raising the current in the solenoid through a reduction in resistance $R$ —the flux density $B$ also rises until it reaches the saturation point at ‘a’. This produces the curve segment oa. If you then reduce the magnetizing force by lowering the current to zero, the flux density does not drop to zero. Instead, it follows the curve ab, as shown in Fig. 6. Even when the magnetizing force $H$ becomes zero, the flux density retains a value of ob.

Diagram showing a circuit to trace a hysteresis loop. It includes a solenoid around a magnetic ring, D.P.D.T switch, resistor, ammeter, and DC source.

Figure 5

Graph showing a hysteresis loop with a curved, elongated, S-like shape on grid paper. Axes labeled B (vertical) and H (horizontal), indicating magnetic behavior.

Figure 6

The flux density ob retained by a magnetic material is called residual magnetism, and its ability to retain it is known as retentivity. To demagnetize the ring, reverse the magnetizing force H by switching the double-pole double-throw switch to position ‘2’. As H increases in the reverse direction, flux density decreases to zero along curve bc. Thus, applying magnetizing force oc removes the residual magnetism.
The magnetizing force oc needed to remove residual magnetism is called the coercive force. To complete the hysteresis loop, continue increasing H in the reverse direction until saturation at point ‘d’, following path cd. As H returns to zero, the curve follows de, where oe is the new residual magnetism in the reverse direction.
Next, increase H in the positive direction by switching back to position ‘1’ and raising the solenoid current. The curve follows path efa, completing the loop. Here, of is the coercive force needed to erase the residual magnetism oe.
Therefore, $c_f$ represents the total coercive force needed in a full magnetization cycle in order to remove residual magnetism. Since hysteresis means "lagging behind," with the magnetic flux density $B$ lagging behind the magnetizing force $H$ , the resulting loop $ab c d e f a$ is called the hysteresis loop. As such, this loop visually demonstrates energy loss in the form of heat during each cycle of magnetization. Consequently, understanding this behavior is essential in designing efficient magnetic materials and devices.

Conclusion

The B-H curve is a valuable tool for understanding the complex relationship between magnetic flux density and field strength. A key feature is the hysteresis loop, which highlights magnetic hysteresis—a phenomenon with wide technological applications. Thorough knowledge of magnetization curves and hysteresis is vital as we advance innovation and explore new magnetic technologies.