The hysteresis curve, also known as a hysteresis loop, is a fundamental graph that illustrates the relationship between the magnetic flux density (B) of a material and the applied magnetizing field strength (H). This loop is crucial for understanding the magnetic properties of ferromagnetic substances.
Understanding the Hysteresis Curve
The term "hysteresis" originates from a Greek word meaning "to lag behind," which perfectly describes the phenomenon where the magnetic state of a material depends not only on the current applied magnetic field but also on its magnetic history.
As explained by BYJU'S, "The hysteresis loop shows the relationship between the magnetic flux density and the magnetizing field strength." Furthermore, it clarifies that "The loop is generated by measuring the magnetic flux coming out from the ferromagnetic substance while changing the external magnetizing field."
This means that when an external magnetizing field (H) is applied to a ferromagnetic material and then removed, the magnetic flux density (B) within the material does not return to zero along the same path. Instead, it follows a distinct path, forming a closed loop when the field is cycled from positive to negative and back again.
Key Characteristics of the Hysteresis Loop
A typical hysteresis loop for a ferromagnetic material reveals several important magnetic properties:
- Magnetic Flux Density (B) vs. Magnetizing Field Strength (H):
- The vertical axis represents the magnetic flux density (B), measured in Teslas (T).
- The horizontal axis represents the magnetizing field strength (H), measured in Amperes per meter (A/m).
- Saturation Point: As the magnetizing field strength (H) increases, the magnetic flux density (B) also increases until it reaches a maximum point called the saturation point. At this point, all magnetic domains within the material are aligned, and further increases in H will not significantly increase B.
- Remanence (Residual Magnetism), B_r: When the applied magnetizing field (H) is reduced to zero, the magnetic flux density (B) does not drop to zero. The remaining magnetic flux density is known as the remanence or retentivity. This is why a material retains some magnetism even after the external field is removed.
- Coercivity (Coercive Force), H_c: To reduce the magnetic flux density (B) back to zero from the remanence point, a reverse magnetizing field must be applied. The strength of this reverse field required to demagnetize the material is called the coercivity.
Types of Hysteresis Curves: Soft vs. Hard Magnetic Materials
The shape and size of the hysteresis loop provide valuable insights into a material's magnetic behavior, distinguishing between soft and hard magnetic materials:
| Feature | Soft Magnetic Materials | Hard Magnetic Materials |
|---|---|---|
| Hysteresis Loop | Narrow and small area | Wide and large area |
| Remanence (Br) | Low to moderate | High |
| Coercivity (Hc) | Low (easy to magnetize and demagnetize) | High (difficult to magnetize and demagnetize) |
| Energy Loss | Low (due to small loop area) | High (due to large loop area) |
| Typical Use | Temporary magnets, cores for transformers, inductors, electromagnets | Permanent magnets, data storage (hard drives, magnetic tapes) |
Applications and Significance
The study of hysteresis curves is fundamental in various fields, enabling material scientists and engineers to:
- Characterize Magnetic Materials: The loop's parameters (remanence, coercivity, saturation) are critical for selecting the right material for specific applications.
- Design Magnetic Devices:
- Transformers and Inductors: Materials with narrow hysteresis loops (low hysteresis loss) are preferred to minimize energy waste during frequent magnetization and demagnetization cycles.
- Permanent Magnets: Materials with wide hysteresis loops (high remanence and coercivity) are essential for applications requiring a strong, stable magnetic field without an external power source.
- Magnetic Recording: High coercivity materials are used in hard drives and magnetic tapes to ensure data retention.
- Understand Energy Loss: The area enclosed by the hysteresis loop is directly proportional to the energy dissipated as heat during each cycle of magnetization and demagnetization. This energy loss is known as hysteresis loss.
In summary, the hysteresis curve is more than just a graph; it's a diagnostic tool that unveils the intricate magnetic memory and behavior of materials, guiding their application in countless technologies.
