What is a Hysteresis Curve?
The word ‘hysteresis’ simply means lagging. In electromagnetism, it is a very useful analysis which suggests the lag of magnetic flux density (B), when magnetic field strength (H) is applied. It is observed that the materials don’t respond immediately to the field but try to align in the previous magnetic state. However, the internal alignment of the materials shows a late response to the external field, forming a loop to gain the final state. This loop formed by some powerful magnetic substances in the external field is known as hysteresis curve.
This curve has a huge impact in electromagnetism and material science because it represents the intrinsic features of ferromagnetic materials, such as coercivity, retentivity, and energy loss due to the internal friction. The shape and size of the hysteresis loop gives some important facts about the activity of material inside an external magnetic field.
To further understand the idea, let’s suppose a ferromagnetic substance is placed inside a current-carrying conductor. This easily comes in the influence of the field and becomes magnetized. Similarly, if we change the direction of the current, the substance becomes demagnetized. This phenomenon represents hysteresis.
Systems having hysteresis are often nonlinear. This can be technically difficult for several hysteretic models, like the Preisach and Bouc-Wen models.
Understanding Hysteresis in Magnetic Materials
In magnetic substances, hysteresis is a point where the material still remains in the previous state of magnetization. When a ferromagnetic material is kept under an external magnetic field, its internal domain tends to align along the field. But this alignment cannot occur instantly and goes through some ups and downs. This is because the material keeps a memory of its previous magnetic state and hence gives a late response to the applied field. The loop thus appears in a cycle.
When the magnetic field is applied, initially the magnetization increases as the internal domains line with it. On reaching the saturation state, almost all the domains get aligned. Now, if the field is removed, the material still remains in some magnetized state which is called the retentivity. To totally demagnetize the material, an opposing magnetic field must be applied. This opposing field required to attain the non-magnetized state of the material is known as the coercivity. This entire cycle gives a loop called hysteresis loop.
Hysteresis is also of two types: Rate-dependent and Rate-independent hysteresis.
Rate-dependent hysteresis: In such a type of hysteresis, the lagging between the input and output field is of sinusoidal nature.
Rate-independent hysteresis: In this type of hysteresis, the memory of the magnetic field is long-lasting and still observed even after removing the changes.
Components of the Hysteresis Loop
There are several factors to be understood before studying a hysteresis curve. These following factors describes the magnetic properties of the materials:
- The magnetic flux density (B) initially increases when the magnetic field strength (H) is increased from 0, which is shown by a magnetization path up to a saturation point. This curve is known as magnetization curve.
- As the magnetic field increases, the value of magnetism also increases and reaches to maximum, until it reaches a specific point which is known as the saturation point. Here B remains constant
- The value of magnetism decreases as the magnetic field decreases. Even if the external field is removed, the substance remains in its previous state which is known as retentivity or residual magnetism.
- Magnetism reduces as the magnetic field is removed and falls to zero which is denoted by the demagnetization curve.
- The force or the opposing field necessary to destroy the material’s retentivity is known as coercive force (C) or simply Coercivity.
- By this way a loop known as the hysteresis loop, is completed as a result of the forward and reverse direction processes involved.
The above factors help in identifying materials for various purposes. For example, materials having a smaller loop region are useful in building transformer cores due to their minimal energy loss. Also, the materials with large loop areas are used to build permanent magnets.
Physical Origin of Magnetic Hysteresis
Magnetic hysteresis arises in the internal structures of ferromagnetic materials. The composition of ferromagnetic materials is such that the internal region is made up of magnetic domains. All these domains align in the same direction. There is a boundary between each domain which is known as domain walls.
When the substance is kept in an external magnetic field, the domain walls are shifted and the domains aligned with the field grow. This movement is not perfectly smooth due to some defects in the material such as dislocations, impurities etc. These defects can block the moment of domain wall and the material is restricted from the magnetization process which is shown by hysteresis.
A certain amount of energy is lost to overcome these restrictions which are released as heat. This heat loss is indicated by the region of the hysteresis loop. Furthermore, alignment of the domain is not fully reversible, which results in the pre-magnetization state when the external field is removed.
Applications of Hysteresis Curves
Hysteresis is commonly seen in Chemistry, Physics, Engineering, Economics, and Biology. In fields like magnetic hysteresis, ferroelectric hysteresis, superconducting hysteresis, mechanical hysteresis, optical hysteresis, electron beam hysteresis, adsorption hysteresis, economic hysteresis the hysteresis curve is employed. We’ll look at several interesting applications of hysteresis.
- Materials having smaller regions of the hysteresis loop are applied in transformers to minimize energy loss during magnetic cycling.
- Hysteresis is widely applied in Ferro magnets. Due to the retaining property, they are used to store data in memory devices, such as hard drives, magnetic tape, and credit cards. They are also used in computer algorithms.
- Hysteresis is used in many artificial networks like thermostats and Schmitt triggers, to avoid unwanted or rapid switching.
- Biology also uses hysteresis analysis in cell biology and genetics, immunology, neuroscience, respiratory physiology, voice and speech physiology, ecology, and epidemiology.
- Materials with larger hysteresis regions and high retentivity are used to build permanent magnets.
- Modern devices use the concept of hysteresis in creative designing of refrigeration and other electronics applications.
Measuring and Modeling Hysteresis
Hysteresis loops are usually measured using devices like a vibrating sample magnetometer (VSM), a hysteresis graph, or a B-H curve tracer. These technologies sketch the loop by applying a periodic magnetic field to a sample and measuring its magnetic response.
Hysteresis behavior is difficult to model hypothetically because it is nonlinear and path dependent. There are various models to illustrate hysteresis. Some models are listed below:
- Preisach Model: It regards the material as an array of smaller hysteresis units (hysterons) and each of these units have unique shifting behavior.
- Jiles-Atherton Model: This theory is based on the consideration of energy. Domain theory is also used to provide a physical interpretation of hysteresis.
- Stoner-Wohlfarth Model: It describes the movement or alignment of single-domain particles in magnetic fields, which is useful in understanding magnetic anisotropy.
These models are used in simulation and modeling programs to build magnetic circuits and devices and estimate how they will behave under different magnetic situations.
Factors Influencing Hysteresis Behavior
The hysteresis behavior of a material is affected by certain factors like the characteristics of that material, temperature, impurities and defects in the material, stress and strain and also the external environment. By well analyzing these factors, researchers can study the phenomena of material hysteresis. Better performance of the materials can be obtained by studying its hysteresis behavior. Understanding these characteristics is important for improving efficiency and preventing any problems during the experiments. Some of the key factors to be kept in mind are listed below:
- Material Composition: The composition of a material highly affects its hysteresis behavior. Different materials have different internal structures and magnetic domains. This inherent property of a material makes it differ in the hysteresis behavior. For example, ferromagnetic materials like iron have magnetic domains which align with the applied magnetic field and hence can create a visible and powerful hysteresis loop. In contrast, rubber also exhibits a very small region of hysteresis, in response to strain-stress as a result of energy loss caused by its molecular composition. Hence, studying the composition of a material is important before using it in any applications.
- Temperature: Temperature plays a significant role on the hysteresis behavior of materials. As the temperature increases, the atoms or molecules of the material come in motion which brings change in the internal composition of the material. This change in composition can affect the hysteresis behavior of the material. For example, some alloys having special features like superelasticity and the shape memory effect can have some impacts on their hysteresis curve because of the change in temperature. Thus we can say, temperature can affect the retentivity and coercivity of the material.
- Impurities and Defects: Impurities present in the material can obstruct the domain walls which will affect the magnetic alignment of the domains. This causes change in the magnetic properties of the material and hence results in a mechanical failure.
- Loading rate:Loading rate means the rate at which a material is introduced to the external magnetic field. It also affects the hysteresis characteristics of the material as the faster loading rate may give different results than the slower rate.
- Mechanical stress history: A material’s past mechanical stress history can affect its hysteresis curve. This condition is frequently observed in those materials suffering repeated loading and unloading cycles. Repetition of this process for a long time can cause changes in the structure of the material like dislocations, corrosion etc. which can decrease the performance of the material.
Hence, examining these factors is important for constructing smoother systems with high efficiency and less chance of breakdowns.
Hysteresis in Other Systems
Hysteresis is not only a topic assigned to magnetism. It is widely used in other realms like engineering, chemistry, biology and economics. Some of the important systems employing hysteresis are as follows:
- Elastic and Plastic Deformation: In materials science and engineering it is important to study the elastic properties of a material which is made under various tests like tensile testing, compression testing etc. A stress-strain curve shows hysteresis during cyclic loading due to internal friction and microstructural changes.
- Thermal Systems: During a phase transition, hysteresis behavior of the substance can be observed.
- Control Systems: Some feedback systems also show the characteristics of hysteresis.
- Biological Systems: Neurons and hormones purely show hysteresis activity. Brain also acts according to the nerves. Thus biological systems also exhibit hysteresis.
- Economics: Employment rates and inflation can also show hysteresis like behavior, where past events can affect present market trends even after the remedies.
Conclusion
The concept of hysteresis has become a powerful analytical tool in the field of research and analogy. Magnetic behavior of a material has advanced usage in the field of physics and engineering. It also covers a wide range of applications like in the Artificial intelligence systems and computer algorithms for a memory backup. In modern systems like sensors and robotics, magnetism has a key role.
Beyond magnetism also hysteresis has become a major concept: from biology in studying the human brain to economics in market analysis. As our research in advancing to the complex systems, magnetism and hysteresis is opening doors to smarter technologies, so get ready for the changes! (Also read about Biot-savart law)
References
Hodgdon, M. L. (2002). Mathematical theory and calculations of magnetic hysteresis curves. IEEE Transactions on Magnetics, 24(6), 3120-3122.
Morris, K. A. (2011). What is hysteresis?. Applied Mechanics Reviews, 64(5), 050801.
Hysteresis
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