Modern Physics

Diamagnetism: materials, applications, examples

The diamagnetism is one of the answers is the subject in the presence of an external magnetic field. It is characterized by being opposite or opposite to this magnetic field, and generally, unless it is the material’s only magnetic response, its intensity is the weakest of all.

When the repulsive effect is the only one that a material has before a magnet, the material is considered diamagnetic. If other magnetic effects predominate, depending on which one it is, it will be considered paramagnetic or ferromagnetic.

A piece of bismuth, diamagnetic material. Source: Pixabay

Sebald Brugmans, in 1778, is credited with the first reference to the repulsion between any of the poles of a magnet and a piece of material, particularly evident in elements such as bismuth and antimony.

Later, in 1845, Michael Faraday studied this effect in more detail and concluded that it was an inherent property of all matter.

Diamagnetic materials and their response

The magnetic behavior of bismuth and antimony, and others such as gold, copper, helium, and substances such as water and wood, differ greatly from the well-known and powerful magnetic attraction that magnets exert on iron, nickel, or cobalt.

Although it is usually a low-intensity response, before a sufficiently strong external magnetic field, any diamagnetic material, even living organic matter, is capable of experiencing a very noticeable opposite magnetization.

By generating magnetic fields as intense as 16 Tesla (and one of 1 Tesla is considered quite intense), researchers at the Nijmegen High Field Magnet Laboratory in Amsterdam, Netherlands, were able to magnetically levitate strawberries, pizzas and frogs in the 1990s.

It is also possible to levitate a small magnet between a person’s fingers, thanks to diamagnetism and a sufficiently strong magnetic field. By itself, the magnetic field exerts a magnetic force capable of strongly attracting a small magnet and this force can be tried to compensate for the weight, however, the small magnet does not remain very stable.

As soon as it experiences minimal displacement, the force exerted by the large magnet quickly attracts it. However, when the human fingers are between the magnets, the small magnet stabilizes and levitates between the person’s thumb and index. The magic is due to the repulsion effect caused by the diamagnetism of the fingers.

What is the origin of the magnetic response in matter?

The origin of diamagnetism, which is the fundamental response of any substance to the action of an external magnetic field, lies in the fact that atoms are formed by subatomic particles that have an electrical charge.

These particles are not static and their movement is responsible for producing the magnetic field. Of course, matter is full of them and you can always expect some sort of magnetic response in any material, not just iron compounds.

The electron is primarily responsible for the magnetic properties of matter. In a very simple model, it can be assumed that this particle orbits the atomic nucleus in a uniform circular motion. This is enough for the electron to behave like a small current loop capable of generating a magnetic field.

The magnetization of this effect is called orbital magnetization . But the electron has an additional contribution to the atom’s magnetism: the intrinsic angular momentum.

An analogy for describing the origin of intrinsic angular momentum is to assume that the electron has a rotational motion about its axis, a property called rotation.

Being a motion and a charged particle, spin also contributes to the so-called spin magnetization .

Both contributions give rise to a net or resultant magnetization, but the most important is precisely that which occurs due to rotation. Protons in the nucleus, although they have an electrical charge and rotation, do not significantly contribute to the magnetization of the atom.

In diamagnetic materials, the resulting magnetization is null, as the orbital momentum and rotational momentum contributions are canceled out. The first because of Lenz’s law and the second because the electrons in the orbitals are set up in pairs with opposite rotation and the shells are filled with an even number of electrons.

Magnetism in matter

The diamagnetic effect arises when orbital magnetization is influenced by an external magnetic field. The magnetization thus obtained is designated M and is a vector.

Regardless of where the field is going, the diamagnetic response will always be repulsive thanks to Lenz’s law, which states that the induced current opposes any change in the magnetic flux passing through the loop.

But if the material contains some kind of permanent magnetization, the answer will be attractive, as is the case with paramagnetism and ferromagnetism.

To quantify the described effects, consider an external magnetic field H , applied to an isotropic material (its properties are the same at any point in space), within which a magnetization M originates . As a result, magnetic induction inside is created B , as a result of the interaction that occurs between H and H .

All these quantities are vector. B and M are proportional to H , the permeability of the material being μ and the magnetic susceptibility χ, the respective proportionality constants, which indicate the particular response of the substance to external magnetic influence:

B = μ H

The magnetization of the material will also be proportional to H :

M = χ H

The above equations are valid in the cgs system. Both B and H and M have the same dimensions, although the units are different. For B gauss is used in this system and for H oersted is used. The reason for doing this is to differentiate the externally applied field from the field generated within the material.

In the International System, which is commonly used, the first equation takes on a slightly different look:

B = μ or μ H

μ or is the magnetic permeability of empty space equal to 4n x 10-7 at Tm / A (Tesla / Ampere meters) and μ r is the relative permeability of the medium in vacuum reference, which is dimensionless.

In terms of the magnetic susceptibility χ, which is the most appropriate characteristic to describe the diamagnetic properties of a material, this equation is written as follows:

B = (1 + χ) μ or H

With μ r = 1 + χ

In the International System B, it comes in Tesla (T), while H is expressed in Ampere / meter, a unit that was thought to be called Lenz, but which until now has been left out in terms of the fundamental units.

Materials where χ is negative are considered diamagnetic. And it is a good parameter to characterize these substances, since χ in them can be considered a constant and independent temperature value. This is not the case for materials that have more magnetic responses.

Generally χ is on the order of -10 -6 to -10 -5 . Superconductors are characterized by having χ = -1 and therefore the internal magnetic field is completely canceled (Meisner effect).

These are the perfect diamagnetic materials, in which diamagnetism ceases to be a weak response and becomes intense enough to levitate objects, as described at the beginning.

Applications: magnetoencephalography and water treatment

Living things are made of water and organic matter, whose response to magnetism is generally weak. However, diamagnetism, as we said, is an intrinsic part of matter, including organic matter.

Small electrical currents circulate inside humans and animals that undoubtedly create a magnetic effect. At that very moment, as the reader continues to observe these words, small electrical currents circulate in his brain, allowing him to access and interpret the information.

The weak magnetization that occurs in the brain is detectable. The technique is known as magnetoencephalography , which uses detectors called SQUIDs ( Superconducting Quantum Interference Devices ) to detect very small magnetic fields, on the order of 10 to 15 T.

SQUIDs are able to locate sources of brain activity with great precision. Software is responsible for collecting the data obtained and transforming them into a detailed map of brain activity.

External magnetic fields can affect the brain in some way. How much? Some recent research has shown that a very intense magnetic field of around 1 T is capable of affecting the parietal lobe, interrupting some of the brain’s activity for a brief moment.

Others, on the other hand, in which the volunteers spent 40 hours inside a magnet that produces an intensity of 4 T, left without suffering noticeable negative effects. Ohio University, at least, has indicated that so far there is no risk of remaining in 8T fields.

Some organisms, such as bacteria, are able to incorporate small magnetite crystals and use them to orient themselves within the Earth’s magnetic field. Magnetite has also been found in more complex organisms, such as bees and birds, which would use it for the same purpose.

Are there magnetic minerals in the human body? Yes, magnetite was found in the human brain, although it is not known for what purpose it exists. One might speculate that it’s an abandoned skill.

As for water treatment, it is based on the fact that sediments are basically diamagnetic substances. It is possible to use strong magnetic fields and thus remove calcium carbonate sediments, gypsum, salt and other substances that cause hardness in water and accumulate in pipes and containers.

It is a system with many advantages to preserve the environment and keep the tubes in good working order for a long time and at low cost.

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