Imagine a classroom setting at any school. Before the class begins, the students are engaged in various activity, some may be sitting oppositely in a bench to talk with someone or they might be running around etc. But as soon as the teacher arrives they occupy their benches facing same direction and even if teacher leaves they usually remain seated. This is what hysteresis magnetism is about. The teacher is like the external field and the students are the magnetic moments of materials and the benches are their domains.
Just as the students remain seated in a certain way even after the teacher leaves, magnetic materials are said to retain some magnetization even after the external magnetic field has changed or removed. This residual magnetization is because of hysteresis, where the material or the student(s) "remembers" its previous magnetization(orderly) state even after the external field has been changed or removed. This phenomenon is usually observed in ferromagnetic materials. (Also observed in Spin glasses). The way in which a piece of any magnetic material behaves when exposed to a changing magnetic field, tells a great deal about its properties.
What is Hysteresis Magnetism?
Hysteresis magnetism refers to the phenomenon where a ferromagnetic material's magnetization, like iron, nickel, cobalt, etc, lags behind the changes in any applied magnetic field. Or to put it in simple words, when any magnet is exposed to any external magnetic field, its internal atomic dipoles (tiny magnets) align with the field. And, even after the field is removed, some of these dipoles remain aligned, causing the material to retain some degree of magnetization in the absence of the field.
Behavior of Materials when H=0
Example, Hard Disc Drivers use hysteresis magnetism to store and retrieve data. We can explain the read and write head with respect to this phenomenon. Write head, uses magnetic field, to change the alignment of these domains to encode data in the binary form(0s or 1s). And Read-head, can detect the direction of the field from the domains, to retrieve stored data. It even ensures that the domains stay aligned even when the drive is off, to preserve the data, for further use.
Hysteresis is derived from a Greek word, husteros (ὕστερος) which means to lag or fall behind. It was first used in the early 1880s, in a paper by J. Alfred Ewing (1882), as per Oxford’s English Dictionary. So in a nutshell magnetic hysteresis describes how the state of something can depend on the presence of magnetic field applied previously.
Types of Magnetic Hysteresis
Magnetic hysteresis itself is not categorized into types. But we can describe its behavior based on its response to the rate of change in the applied field into two types, namely:
- Rate-Dependent Hysteresis
- Rate-Independent Hysteresis
Rate-Dependent Hysteresis:
- Lag between input (magnetic field) and output (magnetization) depends on the rate of change of the input field.
- Faster change in magnetic field results in a larger lag.
Rate-Independent Hysteresis:
- Depends on the history of the applied field, not necessarily the rate of change.
- The loop remains relatively the same regardless of the speed of the magnetic field change. Most magnetic materials exhibit rate-independent hysteresis.
What is a Hysteresis Loop?
The H-M relationship when plotted for all strengths of applied magnetic field, then the result is a hysteresis loop called the main loop. To sum it up,
- X-axis represents the strength of the applied magnetic field (H).
- Y-axis represents the magnetization within the material (B).
- The initial curve represents the magnetization process where the external field strength is increased from left to right, the material's magnetization also increases but upwards. But there's a delay, so the curve does not rise linearly as given in the figure below.
- And at a certain point, the material tends to be saturated with magnetic field, and further increases in H-look at the curve flattening out at the top.
- When the external field is reduced as it decreases towards the left, then magnetization follows a different curve that remains above the initial curve.
- The point where the loop intersects the y-axis at H = 0 represents the retentivity. And the point where the loop intersects the x-axis at B = 0 represents the coercivity.
- The area enclosed by the entire loop represents the energy lost as heat energy during this one cycle of magnetization & demagnetization.
Hysteresis Loop
Characteristics of Magnetic Hysteresis
Magnetic Hysteresis is said to be characterized in many ways like on the basis of measurement techniques and their configurations. To sum it up, we have the following points:
B-H Curve
We can characterize hysteresis by applying a varying magnetic field (H) to the material and measuring the resulting magnetic flux density (B). This data is plotted as a B-H curve. Importantly, the shape of this curve depends on the history of the applied magnetic field, reflecting the material's "memory" effect.
B-H curve
M-H Curve
We can also plot the magnetization (M) of the material instead of magnetic flux density (B), creating an M-H curve.These two curves are said to be directly related because of the following relation:
B = μ0(H+ M)
M-H curve
The way we measure hysteresis depends on how the material is placed within a magnetic circuit.
- In Open circuit configuration the magnetic material is suspended freely between the poles of an electromagnet to create an unwanted magnetic field within the material itself, which can differ from the applied field. But it needs to be mathematically corrected to obtain an accurate B-H curve.
- In Closed circuit configuration, the material is placed in direct contact with the electromagnet poles. This configuration minimizes any distortions within the material, ensuring the magnetic field experienced by the material (internal H field) closely matches the one being applied by the electromagnet.
Properties of Hysteresis Magnetism
- Ferromagnets are said to consist of some microscopic regions called magnetic domains, each having its own internal magnetic field.
- The strength of permanent magnets is determined by the residual magnetism after the external field is removed.
- The external magnetic field strength needed to completely demagnetize the material.
- Any material's ability to concentrate magnetic field lines can also be determined using the hysteresis loop.
Effects of Hysteresis Magnetism
It has both positive and negative effects. It is crucial to understand it before using it in a design and/or device.
- Magnetic memory: Hard drives and magnetic tapes use hysteresis to store information. The material tends to retain magnetism even after the field is gone, like a tiny magnet remembering a "north" or "south" pole.
- Electromagnet control: It even let us fine-tune the strength of electromagnets by just adjusting the magnetic field. It is important for critical devices like MRI machines where precise control is crucial.
- Energy loss (Hysteresis loss): Remember the loop traced by a magnet? The B-H plot? That loop represents wasted energy usually in the form of heat. This can be a problem in applications where saving energy is thought to be of priority.
- AC inefficiency or core losses in AC: In devices that use constantly changing current (AC), hysteresis creates a bigger energy drain because the magnetism has to flip back and forth. Imagine a magnet with a workout routine it can't skip, but it gets tired every time and is unable to recharge itself.
- Magnetic mood swings or instability: In some situations, a sudden change in magnetism can cause a material's magnetic state to switch rapidly. This can be good or bad depending on what the material is supposed to do.
Working and Evolution of Hysteresis Magnetism
Hysteresis has either persistent or non-persistent memory. In order to know how it evolve over time, one needs to know how it got to that state (i.e. its history) and where it will go next. Or in simple words, we can say that what happens now depends not only on the current situation but also on what happened before.
How does it works?
The magnetic hysteresis loop shows the relationship between a material's magnetization and the applied magnetic field. It reveals how the material "remembers" the magnetic field that it was previously exposed to.
Hysteresis Loop
First of all, imagine tiny magnets:
- Inside a ferromagnetic material (like iron), numerous microscopic regions called "magnetic domains" exist. Each domain acts like a tiny magnet with its own north and south poles.
- Initially, these domains are randomly oriented, resulting in minimal overall magnetization of the material.
Now consider applying a magnetic field then we have:
- Initial Magnetization (Saturation)
- Magnetic Susceptibility
- Saturation
- Removing the field (Remanence)
- Demagnetization (Coercivity)
- Cycling the field- Loop
Initial Magnetization (Saturation)
- When a weak external magnetic field (H) is applied, some domains begin to align with the field.
- The magnetization (M) of the material increases proportionally with the field strength.
Magnetic Susceptibility
- As the field (H) strengthens, more domains align in the direction of the field.
- However, the rate of magnetization increase slows down. This reflects the material's "susceptibility" - its ease of further alignment.
- Some domains are already aligned, and aligning the remaining ones becomes progressively harder.
Saturation
- At high field strengths, nearly all domains are aligned with the applied field.
- The material reaches a maximum level of magnetization (saturation point).
- Further increase in the field have minimal impact on magnetization.
Removing the field (Remanence)
- When the external field (H) is completely removed (H = 0), some domains remain aligned due to internal interactions within the material.
- This residual magnetization is called remanence.
- The material exhibits a "memory" of the previously applied field.
Demagnetization (Coercivity)
- To completely demagnetize the material (M = 0), a magnetic field needs to be applied in the opposite direction (-H).
- The strength (absolute value) of this reverse field required to bring the magnetization to zero is called coercivity.
- It represents the material's resistance to demagnetization.
- Materials with high coercivity are difficult to demagnetize- make good permanent magnets.
Cycling the field- Loop
- If the applied field (H) is cycled back and forth from positive to negative and back, the magnetization (M) follows a closed loop.
- The shape and area of the loop depend on the material's magnetic properties.
- During each cycle, some energy is lost as heat due to friction within the material (hysteresis loss).
Also, interpreting the hysteresis loop allows engineers to choose appropriate materials for specific applications, like,
- Narrow loops means low energy loss- preferred for transformers and motors. (Soft materials)
- Wide loops means high retentivity and coercivity, desirable for permanent magnets. (Hard materials)
Difference between Magnetization and Demagnetization:
Parameters | Magnetization | Demagnetization |
|---|
Magnetism(M) | Increases M of any material | Decreases M of any material |
|---|
Alignment | Increases within domains | Decreases within domains |
|---|
Loop | Arrows point upwards in loop | Arrows point downwards in loop |
|---|
External Field(H) | Needs an increasing H | Needs a decreasing H |
|---|
Applications | Used in electromagnets | Used in degaussing tools |
|---|
Advantages and Disadvantages of Hysteresis Magnetism
Some of the Advantages and Disadvantages are
Advantages
- Memory: Retains magnetic state or stores information
- Stability: Resists changes if magnetized once
- Thresholding: Requires a certain level of H value to switch to magnetic state
- Transformers: Efficient energy transfer
- Signal Filtering: Can filter noises
Disadvantages
- Memory: Difficult to erase that stored information
- Stability: Requires a lot of energy to change state
- Thresholding: Limits sensitivity upto some level
- Transformers: Heat generated
- Signal Filtering: May limit usability of some applications
Applications of Hysteresis Magnetism
Some of the advantages and disadvantages of Hysteresis Magnetism are
- Materials with high coercivity (wide hysteresis loop) can retain magnetization (memory) even after the external field is removed. It is used in Hard disks, Magnetic tape, Credit cards (magnetic stripe)
- Materials with low coercivity (narrow loop) can easily change magnetization with minimal energy loss. It is used in Transformer cores to boost magnetic field for efficient energy transfer. Also, it is used in Electromagnets where the magnetic field needs to change frequently.
- Specific magnetic hysteresis materials can absorb vibrations and dampen the angular motion of satellites. It is used in Satellites to maintain stability and precise positioning.
Conclusions
To sum up, hysteresis magnetism is the term used to describe the process when a magnet is exposed to any external magnetic field, and its internal atomic dipoles (tiny magnets) align with the field. However, even after the field is removed, some of these dipoles remain aligned, causing the material to retain some degree of magnetization.
It finds applications in hard drives, transformers, and electromagnets, but it also results in energy dissipation and can induce instability in specific materials. The shape of the loop formed in the plot of hysteresis describes the material's properties too.