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

Introduction

In our technologically advanced world, magnetization is a fascinating phenomenon that is intricately linked to the basic ideas of electromagnetism. It is the foundation of many applications. Both scientists and engineers must comprehend how magnetic materials behave under various circumstances. In this blog, we explore the fascinating idea of Magnetic Hysteresis and set out to understand Magnetization Curves, which are commonly represented by the B-H Curve.

Understanding Magnetization

The process by which materials become magnetic in response to an external magnetic field is known as magnetization.The alignment of the material's atomic or molecular magnetic moments is the cause of this phenomenon.Put more simply, a material gets magnetized when its magnetic domains align when it is exposed to a magnetic field.

The B-H Curve Demystified

The relationship between a material's magnetic flux density (B) and magnetic field strength (H) is depicted graphically by the B-H curve, also called the magnetization curve or hysteresis loop. By measuring the magnetic flux density as the magnetic field strength is changed, the curve is produced.The general shape of the B–H Curve of a magnetic material is shown in Fig. 1. The shape of the curve is non-linear This indicates that the relative permeability (μr= B/μ0 H) of a magnetic material is not constant but it varies. The value of μr largely depends upon the value of flux density.Its shape is shown in Fig. 2. (for cast steel).

Figure 1

Figure 2

The B–H curves of some of the common magnetic materials are shown in Fig. 3. The B–H curve for a non-magnetic material is shown in Fig. 4. It is a straight-line curve since B = P0 H or B v H as the value of P0 is constant.

Figure 3

Figure 4

Key Components of the B-H Curve:Key Components of the 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 is a closed loop which indicates that once the material has reached its saturation level then the magnetic strength is reduced to zero it does not take the material back to its initial state. Rather than a hysteresis loop, which resembles residual magnetization, is plotted. The loop is a zone where energy is lost and this gives away its magnetic signature.

Applications and Significance

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

The B-H curve is likewise applied to develop magnetic storage devices such as HDD (Hard Disk Drive). The feature of the hysteresis loop the materials have affect the how safe and stable the data can be stored.

Magnetic Sensors and Actuators

Machines as simple as magnetic sensors and actuators draw principles off magnetization and plots of B-H curve. Through magnetic material behavior expertise, a designer can develop sensors for high precision readouts and actuators for motions with low power consumption.

Magnetic Hysteresis

When a magnetic material is magnetized first in one direction and then in the other (i.e., one cycle of magnetization), it is found that flux density B in the material lags behind the applied magnetizing force H.This phenomenon is known as magnetic hysteresis.
Hence, the phenomenon of flux density B lagging behind the magnetizing force H in a magnetic material is called magnetic hysteresis.
‘Hysteresis’ is the term derived from the Greek word hysterein meaning to lag behind.To understand the complete phenomenon of magnetic hysteresis, consider a ring of magnetic material on which a solenoid is wound uniformly as shown in Fig. 5. The solenoid in connected to a DC source through a double pole double throw reversible switch (position ‘1’).
When the field intensity H is increased gradually by increasing current in the solenoid (by decreasing the value of R), the flux density B also increases until saturation point a is reached and curve so obtained is oa. If now the magnetizing force is gradually reduced to zero by decreasing current in the solenoid to zero. The flux density does not become zero and the curve so obtained is ab as shown in Fig. 6. When magnetizing force H is zero, the flux density still has value ob.

Figure 5

Figure 6

The value of flux density ‘ob’ retained by the magnetic material is called residual magnetism and the power of retaining this residual magnetism is called retentivity of the material.To demagnetize the magnetic ring, the magnetizing force H is reversed by reversing the direction of flow of current in the solenoid. This is achieved by changing the position of double pole, double throw switch (i.e.,position ‘2’). When H is increased in reverse direction, the flux density starts decreasing and becomes zero and curve follows the path bc.Thus, residual magnetism of the magnetic material is wiped off by applying magnetizing force oc in opposite direction.
The value of magnetizing force oc required to wipe off the residual magnetism is called coercive force.To complete the loop, the magnetizing force H is increased further in reverse direction till saturation reaches (point ‘d’) and the curve follows the path cd. Again, H is reduced to zero and the curve follows the path de. Where oe represents the residual magnetism. Then H is increased in the positive direction by changing the position of reversible switch to position ‘1’ and increasing the flow
of current in the solenoid. The curve follows the path of efa and the loop is completed. Again, of is the magnetizing force utilized to wipe off the residual magnetism oe.
Hence, cf is the total coercive force required in one cycle of magnetization to wipe off the residual magnetism.Since the meaning of hysteresis is lagging behind, and in this case flux density B always lags behind the magnetizing force, H, therefore, loop (abcdefa) so obtained is called hysteresis loop.

Conclusion

The B-H curve is a useful tool in the field of magnetization because it can be used to decipher the complex relationship between magnetic flux density and magnetic field strength. One interesting aspect of this curve is the hysteresis loop, which introduces us to the idea of magnetic hysteresis, a phenomenon that has significant applications across a range of technological domains. Understanding magnetization curves and hysteresis thoroughly is essential as we push the boundaries of innovation and discover new applications and technologies related to magnetism.