Magnetism is the closest thing to magic that we can find in nature. Magnets capable of attracting themselves thanks to an invisible force have fascinated and intrigued mankind from ancient Greece to the present day.
But despite knowing its existence for millennia, we still have certain doubts about its operation. Magnetism has received different historical explanations, and today we know that it is a phenomenon closely related to quantum chemistry. The only problem with the current explanation is that it is really theoretical, if we want to prove it with an experiment we would have to build a magnet considered impossible … until now.
What (little) we know about magnets
There are different types of magnetic materials, but the most common is the ferromagnetic group, which includes iron or cobalt. Among the ferromagnetic compounds there are both magnets and metals that can be easily magnetized if an electric current is applied or another more powerful magnet is approached. It is the most frequent group of magnets due to the abundance of these materials, and you can usually see them in children’s toys or fridge magnets.
Ferromagnetic metals have a magnetic field thanks to electrons, small particles of negative charge that move around the nucleus of atoms like a swarm of furious bees.
Normally the electrons remain close to the nucleus and only some of them have some freedom to generate chemical bonds with other nearby atoms. But in metals, atoms form a community, and yield a proportion of electrons so that they are free between the nuclei of atoms, creating a sea of electrons shared by all nuclei, but without actually belonging to one in particular.
These community electrons have much more freedom of movement than when they are confined around a single atom, and that explains some of the properties of metals. For example, metals conduct electricity because electric current is literally the movement of electrons along the metal, which advance by dodging the nuclei as in a small obstacle course.
In addition to moving through the metal, electrons have a property called spin, usually interpreted as their rotation, and that explains the magnetic field. The electron always forms a small magnet, and the spin indicates its orientation. In this way, an electron with a positive spin and an electron with a negative spin form opposite magnets and cancel each other out.
In non-magnetic metals, free electrons change spin with some freedom, but in ferroimans all electrons are synchronized and have the same spin. This causes the magnetic fields of each electron to add up to a field so strong as to hold the magnet in the refrigerator door.
This phenomenon explains why some magnets lose their magnetism if subjected to extreme temperatures and pressures, since these conditions affect free electrons and can break their synchronization. It also explains the magnetization process itself, because the electric current or the external magnetic field is able to force free electrons to synchronize.
But there is a doubt, why are there natural magnets? We have found deposits of minerals such as magnetite, which already include their own magnetic field without the need for electricity or other magnets. How have they achieved their properties?
In 1966, the Japanese physicist Yosuke Nagaoka had the same doubt, and began experimenting with mathematical models that describe the behavior of free electrons in metal. He verified that electrons influenced each other, and that the distance between them was key to predicting their behavior. If the atoms that make up the metal give up many free electrons, they get closer and change spin more easily, causing them not to synchronize or form a magnet. On the other hand, if there are fewer free electrons, they will be further away and are more likely to synchronize on their own.
If the Nagaoka equations were correct, a magnet with any type of metal should be achieved, even non-magnetizable ones such as gold or silver. All we would have to do is remove free electrons until the remaining ones are distributed at the correct distance to synchronize and generate a magnetic field. An interesting idea, but impossible to achieve in 1966. And it is almost impossible to start free electrons from a metal one by one with the precision necessary to form this magnet.
For that reason, although Nagaoka’s theory was quite accepted in the scientific community for its theoretical and mathematical basis, the idea of creating his magnet was kept in a drawer, discarded as impossible until someone developed the technology needed to create it, which It hasn’t happened until more than fifty years later.
The magnet that shouldn’t exist
The impossible magnet of Nagaoka was created in 2020 thanks to a collaboration between different nanophysics laboratories, taking advantage of a new technology increasingly popular among scientists: quantum dots.
A quantum dot is an electron trap, a small electrical circuit made up of a few atoms. When we lower the temperature of the circuit below -200 degrees Celsius, the functioning of the atoms slows down and the circuit acts like a cage in which we can put electrons one by one and see how they interact with each other.
In the experiment, the scientists created a quantum dot of four atoms and threw three electrons inside. In nature this is impossible to see, since four atoms should yield four electrons, not three. But at that temperature and conditions, the three electrons remain enclosed and cannot take any more. It is exactly as if we had removed one, imitating Nagaoka’s magnet.
When they saw the dance of those three electrons enclosed, they verified that the spins synchronized and generated a small magnetic field that they could measure. With an electron of less a magnet had formed spontaneously, just as Nagaoka predicted.
This experiment has been sounded but not so much for the result of the experiment, but for the possible applications of the quantum dot. The Nagaoka magnet is the first of many experiments that were considered impossible and can now be done, thanks to the precise manipulation of atoms and electrons. A new era arrives for chemists to carry out those forbidden experiments that were kept in a drawer just because they were impossible.
DON’T KEEP IT UP:
- The spin is usually interpreted as the rotation of electrons, but it is an inaccurate definition. It is better to refer to it as a quantum parameter that defines and differentiates each electron and that is related to the magnetic field they generate.
- There are more types of magnetism generated by different processes. The Nagaoka magnet and what is described in the article focuses exclusively on ferromagnetic materials.