- Diamagnetic materials and their response
- What is the origin of the magnetic response in matter?
- Magnetism in matter
- Applications: magneto-encephalography and water treatment
- References
The diamagnetism is one of the answers is the matter in the presence of an external magnetic field. It is characterized by being opposite or opposite to this magnetic field and usually, unless it is the only magnetic response of the material, its intensity is the weakest of all.
When the repulsive effect is the only one that a material presents to a magnet, the material is considered diamagnetic. If other magnetic effects predominate, depending on what it is, it will be considered paramagnetic or ferromagnetic.
A piece of bismuth, a diamagnetic material. Source: Pixabay.
Sebald Brugmans is credited in 1778 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 more closely 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, differs greatly from the well-known powerful magnetic attraction that magnets exert on iron, nickel, or cobalt.
Despite being generally a low intensity response, in the face of a sufficiently intense external magnetic field, any diamagnetic material, even living organic matter, is capable of experiencing a very remarkable opposite magnetization.
By generating magnetic fields as strong as 16 Tesla (already 1 Tesla is considered quite strong), researchers at the Nijmegen High Field Magnet Laboratory in Amsterdam in the Netherlands managed 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 attracting a small magnet with force and you can try to make this force 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 attracts it rapidly. However, when human fingers come between the magnets, the small magnet stabilizes and levitates between the person's thumb and forefinger. 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 made up of subatomic particles that have an electric charge.
These particles are not static and their movement is responsible for producing a magnetic field. Of course, matter is full of them, and you can always expect some kind 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 with a uniform circular motion. This is enough for the electron to behave like a tiny loop of current capable of generating a magnetic field.
The magnetization from this effect is called orbital magnetization. But the electron has an additional contribution to the magnetism of the atom: the intrinsic angular momentum.
An analogy to describe the origin of the intrinsic angular momentum is to suppose that the electron has a rotational motion around its axis, a property that is called spin.
Being a movement and being a charged particle, spin also contributes to the so-called spin magnetization.
Both contributions give rise to a net or resultant magnetization, however the most important is precisely that due to spin. The protons in the nucleus, despite having electric charge and spin, do not contribute significantly to the magnetization of the atom.
In diamagnetic materials the resulting magnetization is zero, since the contributions of both the orbital moment and those of the spin moment cancel out. The first because of Lenz's law and the second, because the electrons in the orbitals are established in pairs with opposite spin 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 denoted M and is a vector.
Regardless of where the field is directed, 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 through the loop.
But if the material contains some kind of permanent magnetization, the response will be attraction, such is the case of paramagnetism and ferromagnetism.
To quantify the effects described, consider an external magnetic field H, applied on an isotropic material (its properties are the same at any point in space), within which a magnetization M originates. As a result, inside a magnetic induction created B, as a result of the interaction that occurs between H and M.
All these quantities are vector. B and M are proportional to H, being the permeability of the material μ and the magnetic susceptibility χ, the respective proportionality constants, which indicate what is 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 cgs system. Both B and H and M have the same dimensions, although different units. For B the gauss is used in this system and for H the oersted is used. The reason for doing this is to differentiate the field applied externally from the field generated inside the material.
In the International System, which is the one commonly used, the first equation takes on a somewhat different appearance:
B = μ or μ r H
μ o is the magnetic permeability of empty space which is equivalent to 4π x 10-7 Tm / A (Teslameter / Ampere) and μ r is the relative permeability of the medium in reference to vacuum, which is dimensionless.
In terms of the magnetic susceptibility χ, which is the most suitable characteristic to describe the diamagnetic properties of a material, this equation is written like this:
B = (1 + χ) μ or H
With μ r = 1 + χ
In the International System B comes in Tesla (T), while H is expressed in Ampere / meter, a unit that was once thought to be called Lenz, but which until now has been left in terms of fundamental units.
In those materials in which χ is negative, they are considered as diamagnetic. And it is a good parameter to characterize these substances, since χ in them can be considered a constant value independent of temperature. This is not the case for materials that have more magnetic responses.
Usually 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).
They are the perfect diamagnetic materials, in which diamagnetism stops being a weak response, and becomes strong enough to levitate objects, as described at the beginning.
Applications: magneto-encephalography and water treatment
Living things are made of water and organic matter, whose response to magnetism is generally weak. However, diamagnetism, as we have 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 this very moment, while the reader follows these words with his eyes, small electrical currents circulate in his brain that allow him to access and interpret the information.
The weak magnetization that occurs in the brain is detectable. The technique is known as magneto-encephalography, which uses detectors called SQUIDs (Superconducting Quantum Interference Devices) to detect very small magnetic fields, on the order of 10 -15 T.
SQUIDs are capable of locating sources of brain activity with great precision. A software is responsible for collecting the data obtained and transforming it 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 fairly intense magnetic field, around 1 T, is capable of affecting the parietal lobe, interrupting part of brain activity for brief moments.
Others, on the other hand, in which volunteers have spent 40 hours inside a magnet that produces 4 T of intensity, have left without suffering any observable negative effects. The University of Ohio at least, has indicated that so far there is no risk in staying within fields of 8 T.
Some organisms such as bacteria are able to incorporate small crystals of magnetite 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, who would use it for the same purpose.
Are there magnetic minerals in the human body? Yes, magnetite has been found in the human brain, although it is unknown for what purpose it is there. One could speculate that this is a deprecated skill.
Regarding water treatment, it is based on the fact that sediments are basically diamagnetic substances. Strong magnetic fields can be used to 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 conserve the environment and keep the pipes in good working order for a long time and at low cost.
References
- Eisberg, R. 1978. Quantum Physics. Limusa. 557 -577.
- Young, Hugh. 2016. Sears-Zemansky's University Physics with Modern Physics. 14th Ed. Pearson. 942
- Zapata, F. (2003). Study of mineralogies associated with the Guafita 8x oil well belonging to the Guafita field (Apure State) using Mossbauer Magnetic Susceptibility and Spectroscopy measurements. Degree thesis. Central University of Venezuela.