Magnetic Anisotropy is the capacity of a magnet to maximize its magnetic orientation in the direction of its poles, and it is similar to general anisotropy of other material characteristics.
Strong permanent magnets – especially sintered Neodymium magnets- have had their magnetic domains oriented in the direction of the easy axis to maximize their strength.
Anisotropy is the property of any material that maximizes certain properties in a preferred direction. Anisotropy may refer to physical, mechanical, metallurgical –or in this case-magnetic properties.
Anisotropic materials properties are different across different axes, so the properties along the x axis will be different from the properties along the y and z axes.
Anisotropic magnets, for example, are prepared with a preferred direction of orientation along their “easy” axis.
Difference Between Isotropic and Anisotropic
Isotropic means “the same properties in all directions” while anisotropic means there is a preferred direction. In this case, the prefix “an” means “without” so “anisotropy” means “without isotropy.” “Iso” is a form of the Greek word isos, meaning “equal.” “Tropy” is a form of the Greek word tropos, meaning “to turn.” So isotropy means it’s the same if you turn it in any direction.
It is very common in materials science to study the directionality of properties (anisotropy) because these properties often matter a lot in engineered materials.
Materials may be (an)isotropic due to their crystal structure. A crystal structure with equal atomic spacing in all directions is more likely to have the same properties in all directions (isotropic), while a crystal structure with different spacing may exhibit different properties in different directions (anisotropic).
Materials may be (an)isotropic due to processing, because certain methods may be used to create either isotropy or anisotropy depending on the properties that are preferred for the material. Neodymium magnets are specially processed to maximize their anisotropy and the preference of magnetic field orientation. This makes them magnetize very strongly in one direction.
At first the concept of magnetic isotropy seems to be a misnomer. If a magnet is totally isotropic there would be no net magnetism at all because the whole idea of polarity depends on some sort of anisotropy.
But there actually is a use for isotropic magnets. Bonded magnets –before they are magnetized- are isotropic. The magnetic grains have a random orientation before magnetizing occurs. For more on this, see the section below on bonded magnets.
In ferromagnetism, Magnetocrystalline anisotropy means it takes more energy to magnetize a magnet in one direction than in other directions.
This is because the energy levels of the principle axes of the crystal lattice are different in each direction.
Magnetocrystalline anisotropy is commercially important because these materials have high Coercivity -meaning they are hard to demagnetize either by exposure to high temperatures or opposing magnetic fields.Rare earth magnets –like Neodymium- get such strong magnetic properties in part due to perpendicular pressing.
An essential step to achieving an anisotropic magnetic structure for sintered Neodymium magnets is the practice of perpendicular pressing.
At this critical step of the production process, Neodymium magnet powder is axially pressed in a vertical direction while it's under the effect of perpendicular magnetic field from horizontal direction.
This, in turn maximizes the magnetic strength in the preferred (easy) direction.
Simultaneously, a magnetic field is applied perpendicular to the pressing direction. The result of this process is that the magnetic dipoles align in an orientation that maximizes their magnetic anisotropy.
Ferromagnetism of Neodymium Magnets
Neodymium magnets are ferromagnetic. This means their magnetic moments are aligned. The maximum alignment of magnetic dipoles occurs when a magnet is aligned on its easy axis.
Then we say the magnet has achieved maximum anisotropy.
At the micro level, ferromagnetic materials are arranged in magnetic domains. They are arranged like many tiny magnets inside each grain of the magnet.
When the magnet is unmagnetized, the dipoles have a net magnetic field of zero due to the dipoles pointing in opposite directions and canceling their magnetic fields.Upon magnetization, the domains are all pointed the same direction, reaching maximum anisotropy.
The strongest commercially-available hard magnetic material available today is the Sintered Neodymium magnet. But Neodymium is not the only hard magnet material. Samarium Cobalt is the next strongest magnetic material, followed by ferrite and Alnico.
What these magnets all have in common is that they are hard magnetic materials. Hard magnetic materials all have similar characteristics:
- They retain a high amount of remaining (Remanent) magnetism (Br) after the external magnetizing field is removed.
- They have high Coercivity. That is, they are difficult to demagnetize.
- Their high Coercivity is visible with a gently-sloped BH curve
- They have a large area BH curve
- The slope of their demagnetization curve is shallower than that of a soft magnetic material
- They undergo high eddy-current losses under the influence of high opposing magnetic fields
Soft Magnetic Materials can have Anisotropy too
Grain-oriented electrical steels are used in electric motors. They are a good example of a soft magnetic material with some anisotropy.
Grain-oriented electrical steels are prepared with their grains oriented in a preferred direction. This gives them some degree of anisotropy and makes them favorable for unidirectional applications like transformers. This greatly reduces electrical losses in transformers.
Non-grain-oriented electrical steels are isotropic –at least in the x and y directions. They are better suited to electric motors, where magnetic fields can act on them in multiple directions as they rotate through the magnetic fields.
Hard Magnetic Materials
The hardest magnetic materials are anisotropic, but some hard magnets are isotropic. Bonded magnets are hard magnets, but they are isotropic.
So, just as soft magnetic materials can have some degree of anisotropy, hard magnets can have a degree of isotropy.
Bonded Neodymium Magnets
Bonded magnet materials (Primarily Neodymium) are generally prepared without a preferred orientation. This allows designers many options with the orientation at a later point in the process.
If a complex magnetic shape is required, it can be produced with a bonded magnet. This is a great advantage for bonded magnets over sintered magnets. Sintered magnets are brittle and difficult to machine, so they are restricted to simple shapes.
Bonded magnets are prepared by pressing a mixture of Neodymium magnet material with a resin binder. There is a lower amount of magnetic material –and it is not oriented (anisotropic) like sintered magnets.
Bonded magnets are more isotropic, so there’s no preferred direction. So when they are magnetized, they are not as strong.
Bonded magnet’s magnetic properties are similar in all directions until they become magnetized. But the strength of bonded magnets is that they can be prepared with much more complex geometries and they can be magnetized more easily in multiple directions than sintered magnets.
Anisotropy –Production Factors in Neodymium Magnets
If your interest is in anisotropic magnets, here are a few factors to keep in mind.
Current top-tier magnet manufacturers work to maximize magnetic anisotropy at every manufacturing step. For example,
-During powder-making, the grain size and shape are tightly controlled and kept in a narrow range of size. Particles that fall outside the size range are separated from the value stream.
-Oxygen content of powder is strictly limited to improve powder flowability. This helps create maximum anisotropy in downstream processing steps
-Perpendicular pressing under a magnetic field –is used to achieve maximum alignment of magnetic domains in the designed direction. This aligns the magnetic dipoles in accordance with the preferred direction.
-Isostatic pressing is performed under very high pressure under oil after perpendicular pressing. It greatly densifies the green blank, preserving the magnetic orientation from the previous steps.
-Sintering –a high-temperature densifying process performed in a vacuum furnace- is carried out in accordance with a carefully designed plan to maximize the magnetic anisotropy of the magnets. This process is carried out in controlled conditions to conserve the anisotropy built into the previous processes.
-Anisotropic permanent magnets have their magnetic axis aligned in the easy direction. They are designed to be magnetized in only one direction.
-Sintered Neodymium magnets are always anisotropic.
-Bonded magnets may be oriented in any direction, but since they are isotropic, they have lower strength than anisotropic magnets.
-Hard magnets (permanent) magnets are usually anisotropic, yet soft magnets can also exhibit varying degrees of (an)isotropy depending on various microstructural factors.