What Is Magnetic Anisotropy?
Magnetic anisotropy refers to the property of certain materials to have a preferred orientation of their magnetic moments, or “magnetization,” under the influence of an external magnetic field.
Materials with magnetic anisotropy tend to prefer one direction for their magnetization over others, and this direction is known as the easy axis. The strength of the magnetic anisotropy determines how strongly the material resists magnetization in directions other than the easy axis.
Causes of Magnetic Anisotropy
Several factors can contribute to the development of magnetic anisotropy in materials. Some of the main causes of magnetic anisotropy include:
- Crystal structure: A material crystal structure’s symmetry can influence its easy axis’s direction. For example, in cubic materials, the easy axis is often aligned with the body diagonal of the crystal, while in non-cubic materials, the easy axis may be aligned with a specific crystal axis.
- Spin-orbit coupling: The spin-orbit coupling of the electrons in a material can also contribute to the development of magnetic anisotropy. This interaction between the spin of the electrons and their orbital motion can lead to a preferred orientation of the magnetic moments.
- Shape: The shape of a material can also influence its magnetic anisotropy. For example, thin films or small particles often exhibit shape anisotropy due to the influence of surface or edge effects.
- Doping: Incorporating impurities or defects into a material can also affect its magnetic anisotropy. Doping can change a material’s electronic structure and magnetic properties, leading to changes in its magnetization behavior.
- Strain: Applying mechanical strain to a material can also affect its magnetic anisotropy. Strain can alter the crystal structure’s symmetry and influence the easy axis’s direction.
Different Types Of Magnetic Anisotropy
Several types of magnetic anisotropy can be observed in materials. Each type of anisotropy can influence a material’s magnetic properties and behavior differently.
- Crystalline anisotropy: This type of anisotropy arises from the symmetry of the material’s crystal structure. It can be either cubic or non-cubic. Cubic anisotropy is observed in materials with cubic crystal symmetry, such as cobalt, while non-cubic anisotropy is observed in materials with lower crystal symmetries, such as iron.
- Shape anisotropy: This type of anisotropy arises from the shape of the material, and it is often observed in thin films or small particles. In these materials, the easy axis may be aligned with the surface or edges of the film or particle rather than along a specific crystal axis.
- Exchange anisotropy: This type of anisotropy arises from the exchange of interactions between the magnetic moments of the material. It is often observed in materials with strong spin-orbit coupling, such as rare earth metals.
- Magnetostriction: This type of anisotropy arises from the interaction between the magnetic moments of the material and its lattice structure. It is often observed in ferromagnetic materials, which tend to expand or contract in response to an applied magnetic field.
- Magnetic field anisotropy: This type of anisotropy arises from the interaction between the magnetic moments of the material and an applied external magnetic field. It is often observed in materials with high magnetic susceptibility, such as iron.
Anisotropy in Hard Magnetic Materials
Hard magnetic materials have a high anisotropy, which means they strongly prefer a specific orientation of their magnetization and are resistant to demagnetization.
Neodymium magnets are the strongest commercially available hard magnetic materials, followed by samarium cobalt, ferrite, and Alnico. Each of these materials has its unique properties and is used in different applications depending on the application’s specific requirements.
Anisotropy in Soft Magnetic Materials
Soft magnetic materials can have either intrinsic anisotropy or extrinsic anisotropy. Intrinsic anisotropy is due to the material’s internal structure, while extrinsic anisotropy is due to external factors such as the way the material is processed or the presence of impurities.
Some examples of soft magnetic materials that can have anisotropy include iron, cobalt, and nickel. These materials can be magnetized in different directions, and the strength of their magnetic field can depend on the direction in which they are magnetized. Other soft magnetic materials include permalloy, mu-metal, and amorphous metals, which also have anisotropic properties.
How to Achieve Better Magnetic Anisotropy?
There are several factors that manufacturers should consider to achieve better magnetic anisotropy in neodymium magnets during the manufacturing process:
- Material selection: The type of material used can significantly impact the magnetic properties of the final magnet. Neodymium magnets are made from rare earth materials and have a high magnetic moment, which makes them well-suited for applications requiring strong magnetic properties.
- Orientation of the material: The orientation of the material during the manufacturing process can influence the magnetic anisotropy of the final magnet. By aligning the magnetic moments in a specific direction, manufacturers can create magnets with strong magnetic properties.
- Impurities and other elements: The presence of impurities or other elements in the magnet material can affect the magnetic properties of the final magnet. It is important to carefully control the purity of the material to achieve the desired magnetic properties.
- Manufacturing techniques: The processing techniques used during the manufacturing process can also impact the magnetic anisotropy of the final magnet. Techniques such as hot pressing or injection molding can help align the magnetic moments of the material and increase the anisotropy of the magnet.
- Grain size and shape: The grain size and shape of the powder used in the manufacturing process can impact the magnetic properties of the final magnet. By carefully controlling the grain size and shape, manufacturers can ensure that the final magnet has consistent and uniform magnetic properties.
- Oxygen content: The powder’s oxygen content can influence the material’s flowability and the magnetic anisotropy of the final magnet. Limiting the oxygen content of the powder can help improve the flowability and preserve the magnetic anisotropy in downstream processing steps.
- Perpendicular pressing under a magnetic field: This manufacturing technique involves pressing the material under a magnetic field to align the magnetic moments in the desired direction. This can help increase the magnetic anisotropy of the final magnet.
- Isostatic pressing: This technique involves pressing under high pressure, typically using oil, to densify the material and preserve the magnetic orientation achieved in earlier steps.
- Sintering: This is a high-temperature densifying process performed in a vacuum furnace that can help further increase the magnetic anisotropy of the magnet. It is essential to control the sintering process carefully to achieve the desired magnetic properties.
Sintered Neodymium Magnets Are Anisotropic
Sintered neodymium magnets are a type of anisotropic magnet, meaning they have different magnetic properties in different directions. Anisotropic magnets are produced in such a way that the magnetic moments within the material are aligned in a preferred direction, resulting in different magnetic properties depending on the orientation of the magnet.
Sintered neodymium magnets are made by pressing a mixture of neodymium, iron, and boron into the desired shape under high pressure and then sintering the material in a furnace to densify it. During the manufacturing process, the grains of the material are aligned in a preferred direction for magnetization. This alignment of the magnetic moments within the material is what gives sintered neodymium magnets their anisotropic properties.
The anisotropic nature of sintered neodymium magnets is important because it allows them to be magnetized in a specific direction when they are placed in a magnetic coil. This makes them useful in applications where the direction of the magnetic field is essential, such as motors or generators.
Bonded Neodymium Magnets Are More Isotropic
Bonded magnets are a type of magnet made by pressing a mixture of neodymium magnet material and a resin binder. They are generally prepared without a preferred orientation, which allows designers more flexibility in terms of orientation during the manufacturing process.
One advantage of bonded magnets over sintered magnets is that they can be produced in more complex shapes. Sintered magnets are brittle and difficult to machine, so they are usually limited to simple shapes. On the other hand, bonded magnets can be made in more complex geometries and can be magnetized more easily in multiple directions.
However, it is important to note that bonded magnets are generally less strong than sintered magnets. This is because they have a lower concentration of magnetic material and are not oriented in a specific direction like sintered magnets. As a result, they are more isotropic, and their magnetic properties are similar in all directions until they are magnetized.
Frequently Asked Questions
What Is Magnetic Isotropy?
Magnetic isotropy does not necessarily mean that a material or magnet has no magnetic properties. Instead, it refers to the property of a material or magnet to have the same magnetic properties in all directions. This means that the material’s magnetization will be the same regardless of its orientation.
Bonded magnets are a type of magnet that is magnetically isotropic before they are magnetized. This means that the magnetic grains in the material are randomly oriented, and the material does not have a preferred direction for its magnetic moments. Once the bonded magnet is magnetized, it becomes anisotropic and exhibits different magnetic properties in different directions.
What Are The Differences Between Isotropic and Anisotropic?
Isotropy refers to the property of a material or magnet to have the same properties in all directions, while anisotropy refers to the property of a material or magnet to have different properties in different directions.
It is common to study the directionality of properties in materials science, as this can significantly impact the performance of engineered materials. The crystal structure of a material can influence its isotropy or anisotropy, with materials that have a more evenly spaced crystal structure being more likely to be isotropic.
The manufacturing process can also influence the isotropy or anisotropy of a material. In the case of neodymium magnets, they are specially processed to maximize their anisotropy, which makes them magnetize very strongly in one direction. This can be useful in specific applications, such as motors or generators, where a strong and consistent magnetic field is needed in a specific direction.
Understanding the concept of magnetic anisotropy is important to make informed decisions when selecting a magnet or material for a specific application. Anisotropic materials have different magnetic properties in different directions, while isotropic materials have the same magnetic properties in all directions.
If you are interested in learning more about magnetic anisotropy and how it can impact the performance of magnets, speak with an expert from we JDA Magnet Factory. We would be pleased to help you understand magnets’ properties and how they can be used effectively in your application.