What is Coercivity?
When a magnetic material is in close contact with a magnetic field, the force of magnetism acting on the material compels it to become magnetized. We say, then, that the magnetic material has been coerced into magnetite. However, a magnetized material can be demagnetized. We are determining the material’s coercivity by measuring the field required to demagnetize a material.
Coercivity is a property that defines the resistance of a material to changes in its magnetic state. Though coercivity addresses the resistance to magnetization and demagnetization, coercivity is often used to address resistance to demagnetization. This is so because demagnetization reveals more about the magnetic properties of a material.
There are two kinds of coercivity in neodymium magnets. The first is the normal coercivity (Hcb), in which the external magnetic field has neutralized the material’s internal field. The other kind is intrinsic coercivity (Hcj), applied to materials that have lost all magnetism and must be re-magnetized to carry an internal magnetic field.
Magnetic Coercivity
For any material magnetized to fullness, the force required to cancel its magnetism is called magnetic coercivity. Coercivity is always carved out on the BH curve of a magnetic substance, where B represents the remanence and H represents the field intensity.
What is Magnetic Coercivity?
Magnetic coercivity is attained when the net flux is zero on the BH curve, also known as the hysteresis curve. At this point, there’s a balance between the magnetic field acting on a material and the force of the field present in the material. If the external field is removed at this point, the material will return to a full magnetic state. However, if the material is taken below the knee, at which point the external field exceeds the force of the material’s magnetic field, the material must be re-magnetized.
Understanding Magnetic Coercivity
In magnetism, coercivity and permeability are related. Permeability is concerned with the response of a material to magnetic influence. Understanding how permeability affects different materials simplifies the process of studying magnetic coercivity.
Remanence is another phenomenon that affects magnetic materials. While coercivity is a measure of the neutralizing field needed to demagnetize a material, remanence is the measure of the magnetization left in the material when the acting field is removed.
Materials are classified as either soft or hard magnetic materials based on coercivity. Soft magnetic materials are easy to magnetize and demagnetize. Once the applied magnetic field is removed, soft materials begin to demagnetize.
Hard magnetic materials are resistant to changes in magnetization. A hard magnetic material retains magnetism when the magnetic field acting upon it is removed. Rare earth materials, for instance, have a high coercivity and are difficult to magnetize.
The distinguishing behavior of soft magnetic materials determines their application. Given that their coercivity is much lower than hard magnetic materials, they fit for processes that require fluctuating magnetism and polarity. They can easily switch in-between cycles of magnetization and demagnetization.
Measuring Magnetic Coercivity
The unit for measuring magnetic coercivity is the Oersteds, Oe, with 1 Oe equaling 79.57747 A/m. The main methods of measuring a material’s coercivity are:
- Using a digital permeameter. This device measures several magnetic properties of materials, including magnetic coercivity.
- Using a vibrating sample magnetometer records electromotive force generated when magnetism is passed through a sample material, and the data obtained from its numerical analysis.
- Plotting a hysteresis loop. A hysteresis loop is formed when an alternating magnetic field is brought near a magnetic material. This loop can be constructed by reading the changes in the neodymium magnet using an oscilloscope.
- Using a hysteresis graph. This device measures a material’s remanence and coercivity; it is suitable for soft magnetic materials such as steel nails and hard magnetic materials such as Neodymium magnets.
Intrinsic Coercivity
Intrinsic coercivity applies to materials that are more plastic than elastic. The elastic material will return to its original shape when the magnetic field acting on it withdraws. If the material stays in the deformed shape, then we say that the magnetic material is experiencing intrinsic coercivity.
Difference between Intrinsic Coercivity and Magnetic Coercivity
Magnetic coercivity is related to materials that can resist the force of deformation. They do not retain a deformed stage when an external magnetic field is removed. Thus, they can be applied to processes that require materials with flexible reactions to magnetism.
Intrinsic coercivity is related to materials upon which a greater magnetic field is exerted. Once demagnetized, the material cannot return to its pre-magnetized state. In such materials, the force required to magnetize the material is equal to the intrinsic coercivity force of the material. This explains the high intrinsic coercivity of Neodymium magnets, given their high resistance to demagnetization, making Neodymium a suitable material for making permanent neodymium magnets.
Relationship between Intrinsic Coercivity and Temperature
Coercivity is a temperature-dependent property of magnets. Some magnets excel at high temperatures. Samarium Cobalt, a Rare Earth magnet, performs efficiently at temperatures between 200C and 500C. Neodymium magnets, meanwhile, are known for low temperatures between 25C and 150C.
Different curves are plotted to show the neodymium magnet’s behavior at different temperatures when plotting the hysteresis loop of a magnetic material such as Neodymium magnets. This informs a buyer or user of the material’s potential performance at expected temperature ranges.
Neodymium magnets have high coercivity but tend to demagnetize at temperatures beyond their normal range. Estimated, a Neodymium magnet will begin to demagnetize above 80C and lose 0.1% of its magnetic property with each degree Celsius rise in temperature.
Relationship between Coercivity and Hysteresis Loop in Neodymium Magnets
The hysteresis loop of soft magnetic material is similar on the x-axis to the loop of a hard magnetic material like Neodymium magnets. In many cases, the amplitudes of both material types reach similar ranges of 1.5 – 2.0 Teslas. But on the horizontal axis, the difference is gaping.
Neodymium magnets reach up to 15 Teslas in amplitude on the horizontal axis. Compared to soft magnetic materials, their loops are about 10,000 times wider. This is easily read when represented on a chat. The coercivity of Neodymium magnets, being high, triggers the distinct hysteresis loop of Neodymium magnets.
Coercivity Characteristics
Coercivity, being a characteristic of magnetic behavior, can be understood through the following phenomenon:
- Remanence: The ability of a magnetic material to retain magnetization after the external magnetic field has been pried away. It is equivalent to the magnetic flux density of the material.
- Ferromagnetism: Materials that are liable to form permanent neodymium magnets are called ferromagnetic materials. They exhibit a permanent magnetic field without an external magnetic force.
- Intrinsic and Normal Coercivities are used to determine the magnetic field and magnetic flux reduction of magnetic materials, respectively.
Influencing Factors of Coercivity
Several factors can affect the coercivity of a magnetic substance. They include:
- Temperature: Hard magnetic materials operate at temperatures lower than soft magnetic materials. This affects the functionality of such materials.
- The geometry of Material: A neodymium magnet’s geometry can impose a behavior on it. This is known as the permeance effect, and the value of a permeance effect is considered the Permeance Coefficient (Pc). On a BH curve, the Pc is calculated as the slope of B over H.
- The hysteresis loop of a neodymium magnet will suggest the kind of coercivity a magnetic material exhibits.
- The Maximum Energy Product of a neodymiummagnet affects the coercivity of the neodymium magnet.
The coercivity of Different Materials
At room temperature (20C), the following neodymium magnets exhibit these intrinsic and normal coercive forces:
- Neodymium Magnet has an intrinsic coercivity of 199000 A/m, a normal coercivity of 938000 A/m, and a remanence of 1.26 – 1.32 T.
- Samarium Cobalt Alloy has an intrinsic coercivity of 1430000 A/m, a normal coercivity of 676000 A/m, and a remanence of 0.93 T.
- Aluminum Nickel Cobalt Iron Alloy has an intrinsic coercivity of 51000 A/m, a normal coercivity of 51000 A/m, and a remanence of 1.25 T.
- Iron Matrix Composite has a normal coercivity of 9800 A/m and a remanence of 0.11 T.
- Steel has a normal coercivity of 5 A/m.
We can classify these neodymium magnets as either soft or hard magnetic materials based on the ranges of their coercivity. Steel, with a coercivity of 5 A/m, is incapable of resisting magnetic attraction. Neodymium, meanwhile, has sufficient coercivity to withstand such.
Conclusion
Magnetic coercivity is a pivotal aspect of magnetism. One cannot displace its importance in understanding suitable neodymium magnets for various field applications. The knowledge of coercivity also improves other relevant fields, including magnetic refrigeration and data storage.
